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.