Maverick22

I have a 72 cutlass with a 455, turbo hydra-matic 400 (M40) transmission, and 2.73 rear axel ration (no anti spin).

BFG Radial TA 225/70/14 **tire size added after original post**

At 60 mph i rev at 3400 rpm, 70 mph - 3600 rpm. This seems/feels/sounds high. Any thoughts?

The car is still getting back to street quality and i am wondering if something else isn't working that may cause this.

Thanks

BeenThere

You either have a 4:10 rear end or your tach is off. My money is on a bad tach reading.

oldcutlass

X2 - tach not right

Fun71

x3 on the tach.

My car has 3.23 rear gears and when I had short 245-60-14 tires the engine turned ~3400 RPM @ 70-75 MPH.

You can calculate your engine RPM with an online calculator such as this one:

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

Koda

I think you're stuck in second, dude.

ziff396

That sounds possible with 2.73's. My 3.42 with 275/60/15's ( 27.5 tall not 28 as claimed) only run around 3k or so.

Fun71

You may be onto something here. I ran my RPM calculator with 2.73 rear gears, 1.48 trans ratio (TH400 second gear ratio), guessed at 25" tall tires, and got 3258 RPM @ 60 MPH and 3801 RPM @ 70 MPH.

Well, now that I look at it, the calculated RPM @ 70 MPH is higher than what he's seeing. I'm back to thinking tach issue.

Koda

Ok, simple solution is to drop it from D to 2 and see what happens.

Fun71

Yep, that is a good idea.

MDchanic

Why not just check your tire diameter and do the math?

Asking us, without even telling us your tire diameter, is a fairly pointless exercise, while doing some simple arithmetic will give you the answer in about a minute.

- Eric

Maverick22

Confirmed i am not in second gear (counted shifts) and used dual gate 3rd and dropped down.

This morning drove with a gps and tach says 3150 at 60 and 3600 at 70.

Sorry never even though about tire size, been away from this for too long.

BFG Radial T/A - 225/70/14

Just used the online calculator (suggested above) and it said should be 2141 rpm. Another site 2081 rpm.

Based on this, it looks like bad tach, however it does sound like it is reving very high.

hookem horns

Thanks for the clarifications and I agree you can usually tell 3300rpm from 2300rpm by ear. If the tach is close, seems like you have 2 possibilities:
1. Significant slippage in the transmission or torque converter, but this is usually more noticeable pulling away from a stop than at cruising speed. Check for low or burnt trans fluid.
2. Someone swapped the rear end before you got it. Do a quick search on how to check the real ratio. You jack up the back end and count drive shaft revolutions as your turn the rear wheels.

Maverick22

1) I haven't changed the transmission fluid yet, but level is good.
2) I changed the diferential fluid and confirmed the gear ration by counting teeth.

I don't trust my knowledge of transmissions, so i will take to a shop for that maintenance.

MDchanic

I just did the math on this.

Your 255/70R14 tire should have a diameter of 28.06", which gives a circumference of 88.15", which gives 718.76 rotations per mile (and 718.76 RPM at 60mph).

A 2.73:1 rear in a 1:1 top gear should give 1,962 engine RPMs (let's say 2,000 RPM with converter slippage) at 60mph.

You would need a 4.38:1 rear end to give you 3,150 RPM at 60mph in top gear.

I'd get a new tach and then go from there.

- Eric

oldsmobiledave

He has a 225 70 14 not 255 70 14. No one makes a tire in that size.


225 70 14 is 26.4" tall not the 28.1" that you used for your calculation

MDchanic

Guess I'm dyslexic.

Circumference is 82.94", with 863.45 revolutions per mile, and 2,357 RPM at 60mph.

3,150 RPM at 60mph would require a 3.65:1 rear end ratio.

- Eric

Koda

Recommend cheapie autozone tach hooked up to engine bay and compare it to your tach at 1k, 2k, and 3k out of gear.

Fun71

So what tach do you have in the car? Many years ago my Sun Super Tach II started reading erroneously (like that word?) and I discovered the 4-6-8 switch had a loose connection. Simple solder job to make a permanent 8 connection as this tach will never be used on a 4 or 6 engine and all has been well for many years now.

Maverick22

Factory olds tic toc tach

I will get a tach and hook it up and compare.

oldsmobiledave

So what did you discover when you installed another tach?

GINCSC

I'm curious as well of the outcome. I have the same problem but put the issue on hold to get it painted. I'll be working on it in the next couple of weeks.

joe_padavano

Look, this is simple math. With a transmission that has a 1:1 top gear, the RPM at a certain speed is controlled by three things:

1) diameter of the tire (or, more accurately, the revolutions per mile of the tire since the tire deflects under load, changing the radius)
2) the rear axle ratio
3) the amount of slippage in the torque converter.

If you do the math (as shown above) and the calculated RPMs are wildly different from what you are seeing, there are only three possible causes:

1) the tach is wrong
2) the rear axle ratio is not what you think
3) there is a problem in the trans (excessive slippage or it's not in high gear).

That's it, there are no other options. Of course, you can always use the "count the turns" method to find out exactly what rear axle ratio you have.

Maverick22

Unfortunately i have been away for work longer than expected and have not yet purchased a tach. I am back for a few days and hope to get that tested.