need help on RPM, MPH, and gearing

[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