need help on RPM, MPH, and gearing

[lukedukem]

so i got all my electrical working, (meaning that i replaced it all and bought all new electronic ign. parts from the orange box to coil to balast res, blah blah). Now since all is working good i decided to wire up the new factory tach i have in the dash, so i ran a wire from neg. coil to tach and a 12v supply. and as soon as i fired it up it went to about 1k RPMs. good right? then i went to drive it and it seemed off at first . it was reading about 70mph and 3k rpms, but my gps and my wife behind me said i was going 60mph, but i know that the speedo was off because the tranny needs the different gear in it. but its the tach that threw me off, if i'm going 60mPH at 3k RPMs then by the tables calculations i have a 410 geared rear? i used the link below and i put in my info: Automatic tranny, 275/60 r15, took the tach till speedo hit 60MPH and the tach said 2300, so i clicked on each gear till it said 3k and my result was 410 gears. does that sound right??? here's the link. i assumed i had 323's cause of fender tag, but not much is original to car.

http://vexer.com/automotive-tools/speed-rpm-calculator

luke

[elacruze]

You've answered your own questions. My car with 26" tires and 4.10 gears IIRC was about an indicated 3600rpm @ 70mph.

[lukedukem]

You've answered your own questions. My car with 26" tires and 4.10 gears IIRC was about an indicated 3600rpm @ 70mph.

Thanks elacruze, I thought that.

Luke

[lukedukem]

Is there a way I can tell what gears I have, besides what I did earlier. Is there markings on the 3rd member. Or would I have to open up the rear end to see?

Luke

[Cooter]

Best way is to pull chunk and look. Unless the little tag is still on one of the nuts holding chunk in, you COULD try the old "Turn and count" one revolution of tire, and see how many times the driveshaft turns, but this only works with proper size tire. Many gear ratios were figured using 26" tall tires.

[cudaken]

Best way is to pull chunk and look. Unless the little tag is still on one of the nuts holding chunk in, you COULD try the old "Turn and count" one revolution of tire, and see how many times the drive-shaft turns, but this only works with proper size tire. Many gear ratios were figured using 26" tall tires.

Cooter, tire sizes does not matter on the turn and count trick. Same with the rear ratio. Now when calculating what gears to make for the car, I will agree they probable used a 26" tall tire as a stranded to work with.

Cuda Ken 

[lukedukem]

i did try that method before i replaced all the wireing (and hooked up the tach). it came out to 3.23 gears (which is what the car came with from the factory). thats whats so confusing about this. could my tach be hooked up wrong? i will try again when i get home today.

just so i get this right. jack up rear end, the mark the tire a drive shaft, turn tire one revolution and count the driveshaft revollutions. correct?
maybe i miss counted last time but i'm pretty sure it was a bit past the third turn. i'll post my results later today

luke

[elacruze]

Luke,

The turn and count method is troublesome if you don't have a posi rear end-the tire on the opposite side can rotate in reverse, or follow along at part speed. You can block one wheel in this case and divide your driveshaft rotations by 2.

[lukedukem]

Luke,

The turn and count method is troublesome if you don't have a posi rear end-the tire on the opposite side can rotate in reverse, or follow along at part speed. You can block one wheel in this case and divide your driveshaft rotations by 2.

i do know it has a sure grip becuase i did try this method before and both turned the same way when i turned one. plus when i put the pedal down they both spin annd the ass end slides around

luke

[lukedukem]

Ok, just jacked up the rear end and tried the turn the wheel method and the driveshaft turned 3.25 or so times to one back wheel turn. What now. My tach is not reading right? Please advise

Luke

[Cooter]

Best way is to pull chunk and look. Unless the little tag is still on one of the nuts holding chunk in, you COULD try the old "Turn and count" one revolution of tire, and see how many times the drive-shaft turns, but this only works with proper size tire. Many gear ratios were figured using 26" tall tires.

Cooter, tire sizes does not matter on the turn and count trick. Same with the rear ratio. Now when calculating what gears to make for the car, I will agree they probable used a 26" tall tire as a stranded to work with.

Cuda Ken  

Ken, oh yes it does...Otherwise, one could run a 33" tall tire with 4.10 gears and still turn the same RPM as with 26" tall tire, and NOT have to swap out speedo gear cause speedo is about 20 MPH off....Tire height most certainly does counter gearing. How do you think OD trans' work?  Ever tipped over a paper cup and rolled it? Big end always goes round the little end as it has further to travel.

http://www.dodgecharger.com/forum/index.php/topic,103537.0.html

[elacruze]

To check the gear ratio, you don't need tires at all, actually. You only need tire circumference to compare RPM with *true* MPH, which you'll need a stopwatch or GPS to know.

[cudaken]

Best way is to pull chunk and look. Unless the little tag is still on one of the nuts holding chunk in, you COULD try the old "Turn and count" one revolution of tire, and see how many times the drive-shaft turns, but this only works with proper size tire. Many gear ratios were figured using 26" tall tires.

Cooter, tire sizes does not matter on the turn and count trick. Same with the rear ratio. Now when calculating what gears to make for the car, I will agree they probable used a 26" tall tire as a stranded to work with.

Cuda Ken  

Ken, oh yes it does...Otherwise, one could run a 33" tall tire with 4.10 gears and still turn the same RPM as with 26" tall tire, and NOT have to swap out speedo gear cause speedo is about 20 MPH off....Tire height most certainly does counter gearing. How do you think OD trans' work?  Ever tipped over a paper cup and rolled it? Big end always goes round the little end as it has further to travel.

http://www.dodgecharger.com/forum/index.php/topic,103537.0.html

Cooter my friend. If I pull the wheel and brake drum off my car and just spin the axle the axle will move to the same ratio to the drive shaft wheel on or off. We are talking the Mechanical Ratio and not the speed that car will go at the same RPM.

Remember we are just trying to find out what gears are in the car, not the speed at this point!

Cuda Ken

[lukedukem]

Correct, what gears are in the rear end. So I've got two different methods giving me two different numbers.
Which do I believe ?

Luke

[firefighter3931]

I'd say the tach is off based on your observations. 

The axle ratio is 3.23 


Ron

[lukedukem]

Thanks Ron. So how can I fix the tach to read right. At idle in park it seems to read correct and in gear too. And I think I have it hooked up right.

Luke

[firefighter3931]

I can't remember off hand what ignition your running....is it a MSD by chance ?  What tach are you using ? Part number ?


Ron

[Cooter]

 

Ken, oh yes it does...Otherwise, one could run a 33" tall tire with 4.10 gears and still turn the same RPM as with 26" tall tire, and NOT have to swap out speedo gear cause speedo is about 20 MPH off....Tire height most certainly does counter gearing. How do you think OD trans' work?  Ever tipped over a paper cup and rolled it? Big end always goes round the little end as it has further to travel.

http://www.dodgecharger.com/forum/index.php/topic,103537.0.html

Cooter my friend. If I pull the wheel and brake drum off my car and just spin the axle the axle will move to the same ratio to the drive shaft wheel on or off. We are talking the Mechanical Ratio and not the speed that car will go at the same RPM.

Remember we are just trying to find out what gears are in the car, not the speed at this point!

Cuda Ken

As am I. I think we have a breakdown in understanding what I'm saying here. RPM aside, The taller the tire, the more turns of the driveshaft it will take to return it to the original Starting place used for counting one turn of wheel. Granted, it may not be much, but I've seen where it's as much as a half turn difference when doing the "turn and pray" method. Just didn't want the guy coming up with some weird sh*t and wondering about how the hell he got the only Charger with a 3.69:1 ratio.

[lukedukem]

I can't remember off hand what ignition your running....is it a MSD by chance ?  What tach are you using ? Part number ?


Ron

I'm running the orange box. And a flame thrower 2 coil if that helps. The tach came with the car and kinda worked. It would drop out at about 3k rpms. Then come back after letting off the gas. Weird. After replacing all wiring it works but maybe not good.

Luke

[Cooter]

Luke, are you sure that tach is compatible with higher energy ignition? I know many tachs have to have a converter/adapter in order to run with say MSD.

[lukedukem]

Luke, are you sure that tach is compatible with higher energy ignition? I know many tachs have to have a converter/adapter in order to run with say MSD.

That, cooter, I do not know. I'd have to take it out of my cluster and I guess take it apart to see if I can find any thing that will tell me who made it and what not. I do remember it had blue wrighting on the back labeling ing. Wire and clock wire. If that tells us anything. It was brand new looking too. No blemishes.

Luke

[firefighter3931]

I can't remember off hand what ignition your running....is it a MSD by chance ?  What tach are you using ? Part number ?


Ron

I'm running the orange box. And a flame thrower 2 coil if that helps. The tach came with the car and kinda worked. It would drop out at about 3k rpms. Then come back after letting off the gas. Weird. After replacing all wiring it works but maybe not good.

Luke

Sounds like the tach is suspect.    On some tachs there is a setting for 4-6-8 cylinder and possibly this one isn't set up for a V8 ? 

Based on the history I wouldn't count on it being very accurate. 


Ron

[lukedukem]

ok so the tach has issues, can i leave it like it is till winter? i just got everything back together and i'm really injoying the car alot now. i just wouldn't really rely on it.
if i decide to pull it, where is this setting for the cyliders, keep in mind this is a factory tach with clock. it looks ligit.
another thing, do you thihnk it could be my tranny and that its taking more RPMs to make the car go. would the tranny do this if it were going out... just a thought, not sure if anyone has heard of this

luke

[bakerhillpins]

 

Ken, oh yes it does...Otherwise, one could run a 33" tall tire with 4.10 gears and still turn the same RPM as with 26" tall tire, and NOT have to swap out speedo gear cause speedo is about 20 MPH off....Tire height most certainly does counter gearing. How do you think OD trans' work?  Ever tipped over a paper cup and rolled it? Big end always goes round the little end as it has further to travel.

http://www.dodgecharger.com/forum/index.php/topic,103537.0.html

Cooter my friend. If I pull the wheel and brake drum off my car and just spin the axle the axle will move to the same ratio to the drive shaft wheel on or off. We are talking the Mechanical Ratio and not the speed that car will go at the same RPM.

Remember we are just trying to find out what gears are in the car, not the speed at this point!

Cuda Ken

As am I. I think we have a breakdown in understanding what I'm saying here. RPM aside, The taller the tire, the more turns of the driveshaft it will take to return it to the original Starting place used for counting one turn of wheel. Granted, it may not be much, but I've seen where it's as much as a half turn difference when doing the "turn and pray" method. Just didn't want the guy coming up with some weird sh*t and wondering about how the hell he got the only Charger with a 3.69:1 ratio.

Not to start a pissing contest but Cuda Ken is correct.

[Mebsuta]

Not to start a pissing contest but Cuda Ken is correct.

I agree. 

[ws23rt]

Not to start a pissing contest but Cuda Ken is correct.

I agree.  

I agree as well. Having a senior moment doesn't require being senior  One turn on the axle shaft = how many turns on the drive shaft?---Gear ratio.

[ws23rt]

The question about RPM vs MPH ----What I get from reading this is a problem with the tac/electrical.
If the transmission was slipping some or the torque converter was not locking up I would expect to be able to feel it and it would make heat in the fluid.

[lukedukem]

Thanks for helping getting this back on track ws23rt. 
So I know the tach is the issue now, does anyone know how I can tell if I have the right one that will work with my electrical stuff. Or has anyone delt with talking them apart that can help. Not sure what my next step is

Luke

[Cooter]

Not to start a pissing contest but Cuda Ken is correct.

I agree.  

I agree as well. Having a senior moment doesn't require being senior One turn on the axle shaft = how many turns on the drive shaft?---Gear ratio.

Oh man, you know what.....You guys can have this one... A larger tire will throw off your counts if mounted on the axle when your trying to do the old turn and count trick.

Tire height affects gear ratio whether your going down the highway, or trying to turn the driveshaft to figure out ratio with tire mounted. Nobody removes the damn wheel to do the count and turn trick that I know of. I hope you figure this one out, but too many do not understand this concept for me...Next Step? Buy another tach that works....

[68r/t]

On a axel stand one turn is one turn doesn't matter what diameter tire you have, it can be 13" or 30", but put that tire on the ground  that's when rolling diameter comes in to play and makes a huge difference.  

[cudaken]

 Luke, is it factory a Tic Toc Tach? Back in the day you could order just about anything you wanted from Maw Mopar. While unlikely, could have they made a Tic Toc Tach for a 225 slant 6?  Boy that would be rare!

Did a search on the internet and came across this.

if the tach is dead we can install a new circuit board and calibrate for $175. We often find working tachs to be 1,000 rpm and sometimes 2,000 rpm off! We can rebalance and rehang the meter plus calibrate for $75. You will probably need one of these services.

The Mopar factory tachs will not work with modern electronic ignition systems. We can modify your tach and replace the meter and board with modern components for $275. The tach will then work with any modern electronic ignition system. 

Here is there link.

http://www.autoinstruments.com/mopar.htm

More than likely they will be able to help you.

On a side note, when I was a kid I would set the tach in my Road Runner to 6 cylinder setting so the needle moved more! 

Cuda Ken 

[HeavyFuel]

Luke,

I've got the same thing going on with my original Tic Toc Tac......it shows about 1K more rpm than actual at 70 mph.  The amount of error is nonlinear, so it is less at slower speeds.  I have converted to electronic ignition as well.

So I think Ken hit the nail on the head.  Factory tacs have a compatibility problem with electronic ignitions.

[lukedukem]

Mine had Jacobs ignition before I changed to orange box.

[elacruze]

Not to start a pissing contest but Cuda Ken is correct.

I agree.  

I agree as well. Having a senior moment doesn't require being senior One turn on the axle shaft = how many turns on the drive shaft?---Gear ratio.

Oh man, you know what.....You guys can have this one... A larger tire will throw off your counts if mounted on the axle when your trying to do the old turn and count trick.

Tire height affects gear ratio whether your going down the highway, or trying to turn the driveshaft to figure out ratio with tire mounted. Nobody removes the damn wheel to do the count and turn trick that I know of. I hope you figure this one out, but too many do not understand this concept for me...Next Step? Buy another tach that works....

Cooter;

You're Wrong.

Sincerely.

[cudaken]

 Lets stop beating on old Cooter I am sure he has figured it out by now!  Bet he is jacking up his car right now to test the idea. 

Luke, have you called or E-Mailed yet? 

On my 70 Cuda, I run MSD 6-AL and cannot use the factory tach.

Cuda Ken

[A383Wing]

my one cents worth here....both my factory tachs in my 66's work fine with Mopars electronic ignition. The aftermarket tach in the Daytona worked with Mopars electronic set up, but not with MSD Ignition, I had to buy the "tach adapter" to get it working again

Bryan

[ws23rt]

Lets stop beating on old Cooter I am sure he has figured it out by now!   Bet he is jacking up his car right now to test the idea.  

Luke, have you called or E-Mailed yet?  

On my 70 Cuda, I run MSD 6-AL and cannot use the factory tach.

Cuda Ken

I am with Cuda Ken.---We all have had brain farts like this and I suspect we don't go around telling everyone about it. The light will click on for Cooter as we are all thinking about our own glitches.

I have a factory tac. in my Coronet with mopar electronic ignition and It seems ok or close. (never checked it). Back in the day before we got used to things being exact I think few even wondered how close the tac or speedometer were. It says X it must be X. Many of us had after marked tacs as well.
I used to tell the blue meenies that my spedo was off and I was guessing (that never helped a bit)
Those that needed to know RPM for shifting in real race conditions had a good tac for that. Not sure what the accuracy of the factory tac was but +- 500 rpm would not suprise me.

[lukedukem]

So maybe this happened. PO had Jacobs ignition and the tach matched it. I changed to orange box and mopar ignition and I need to change it back?   

[Cooter]

Cooter;

You're Wrong.

Sincerely.

Ok, then Since I'm obviously the only one who thinks this way, prove it. Tell me how a taller tire will NEVER effect final drive ratio if marked near the tire tread for reference in doing the "Turn and count" method, yet will effect final drive ratio if driven down the road.
If you marked the axle, then counted turns, your gonna tell me that the mark on the edge of a 30" tire is gonna come back to the same place a mark on the edge of a 5" axle flange did??? Please explain...



Sincerely.

[Cooter]

Lets stop beating on old Cooter I am sure he has figured it out by now!   Bet he is jacking up his car right now to test the idea.  

Instead of any type of explanation of why a taller tire effects final drive ratio on the highway, yet it somehow mysteriously doesn't on jackstands, all I get is some BS replies trying to be cute.

If I'm incorrect, then ok, whatever, but to make this into a BS reply?, "Lube your eye sockets" as Big chief would say.
See if I try and help your ass again...



Sincerely.

[lukedukem]

Lets stop beating on old Cooter I am sure he has figured it out by now!  Bet he is jacking up his car right now to test the idea. 

Luke, have you called or E-Mailed yet? 

On my 70 Cuda, I run MSD 6-AL and cannot use the factory tach.

Cuda Ken

Not sure who I need to call or email. I didn't buy this tach. It's a factory one that looks like the PO had rebuilt or that he bought from a company. Who idk. But I'm think like I said earlier. He had jacobs ignition and it sorta worked. I have orange box and its now off


Luke

[Cooter]

On a axel stand one turn is one turn doesn't matter what diameter tire you have, it can be 13" or 30", but put that tire on the ground  that's when rolling diameter comes in to play and makes a huge difference.  

This is the confusing part. If the rollout is increased on the ground, why isn't it increased in the air when marks are made? Tire is still moving the same distance when pinion/ring is still turned the same amount.

[ws23rt]

On a axel stand one turn is one turn doesn't matter what diameter tire you have, it can be 13" or 30", but put that tire on the ground  that's when rolling diameter comes in to play and makes a huge difference.  

This is the confusing part. If the rollout is increased on the ground, why isn't it increased in the air when marks are made? Tire is still moving the same distance when pinion/ring is still turned the same amount.

The confusing part comes from thinking about two things as if they were one.
Final drive ratio is the gear ratio in the diff. Put a mark on  the tire at six o clock/ on the bottom. turn the tire one turn (back to the bottom) and the drive shaft will have turned 3.23 times (in this case).
I believe you are thinking of the final drive ratio as the whole picture which includes the trans./od output and tire circumference as well as the diff. ratio. Those things all together will tell how far the car moves for every revolution of the engine. (The topic started with RPM,MPH and gearing)

[A383Wing]

This is the confusing part. If the rollout is increased on the ground, why isn't it increased in the air when marks are made? Tire is still moving the same distance when pinion/ring is still turned the same amount.

rotate tire one turn and it will move the driveshaft the same amount...does not make any difference how big diameter tire is, it's still one turn 360*....take tire off, turn drum 360* and driveshaft will still turn the same amount.

Now, a larger tire will make less revolutions per mile and a smaller tire will turn more times per mile measured.

It's 2 completely different items that you are thinking off

[ws23rt]

This is the confusing part. If the rollout is increased on the ground, why isn't it increased in the air when marks are made? Tire is still moving the same distance when pinion/ring is still turned the same amount.

rotate tire one turn and it will move the driveshaft the same amount...does not make any difference how big diameter tire is, it's still one turn 360*....take tire off, turn drum 360* and driveshaft will still turn the same amount.

Now, a larger tire will make less revolutions per mile and a smaller tire will turn more times per mile measured.

It's 2 completely different items that you are thinking off

We type at the same time and think alike.

[elacruze]

Cooter, as I said before, we need to separate final drive ratio away from rear axle ratio. The gears in the axle are set, the ratio from the tire to the tach is not. I believe you understand both, but the language isn't clear. He wants to know the rear axle ratio. If his tach is off, can't be known without knowing the axle ratio. Then you can math the tire circumference for final drive calculation.

[Cooter]

Now, a larger tire will make less revolutions per mile and a smaller tire will turn more times per mile measured.

It's 2 completely different items that you are thinking off

There we go....Now it's clear as mud. Bryan, Thanks for pointing out I was only half wrong. 

[cudaken]

Not sure who I need to call or email. I didn't buy this tach. It's a factory one that looks like the PO had rebuilt or that he bought from a company. Who idk. But I'm think like I said earlier. He had jacobs ignition and it sorta worked. I have orange box and its now off

Luke, they do conversions to the factory tach's, so more than likely they will know what the problem is. More than likely they will be able to fix yours when you are ready.

Cooter, so you understand now!  Guess we will let you hang around a while longer! 

Cuda Ken

[lukedukem]

Thanks ken

Luke