I own a 1973 Challenger with a 340 engine and a Torqueflite 727 3-speed transmission. The differential ratio is 3.23:1. Rear tires are 225/70R14's. Here's the problem. It does not cruise the highways comfortably. By that, I mean the rpm's are too high for the speed. 3000 rpm and I am doing 60 mph instead of 72 mph. 3500 rpm and I am doing 70 mph instead of 84 mph. All of the speed/rpm calculators I have tried show the differential to be a 3.91:1 (almost exactly by the numbers) instead of 3.23:1. I have checked the speedometer (it's accurate), the tachometer (it's accurate), the speedometer gear in the transmission (it's the right one for the differential and the tire size), and verified the gear (it's stamped 3.23, and the tire spin vs. the drive shaft spin verifies this also). Can I have that much slip in the drive train somewhere? It seems excessive to me. Thoughts on what to try next?

 

U -

Welcome aboard.

I have a similar set up (except for a 440 and larger rear tires - 28" diameter), and yes, it does seem high.

Could someone have changed/modified the gear ratio in the tranny? I'm only saying that, because you said you checked everything else.

Gut reaction is that you may indeed have a 3.91 rear.

Archer

 

I'd say you do NOT have a 3.23. How did you determine this? Have you actually counted turns, or looked at the ring/ pinion?

Easy to do. IF you do NOT have limited slip rear (sure grip) block the front tires, jack up ONE rear wheel and put the car in neutral. Attempt to turn the wheel. IF you can, you do NOT have limited slip. Turn the tire TWO full turns and count driveshaft revolutions. The shaft will turn to whatever the ratio is, IE 3.23, the shaft will turn 3 and about 1/4 turns, etc.

IF the one wheel will NOT turn, make sure it's in neutral, brake released. If not, you have limited slip. You will have to jack up BOTH rear tires. Now turn one of them ONE full turn, while counting the shaft turns as above.

I'm running a Ford 3.89 rear and 225/ 14's and those RPM figures sound more like my car. I use a GPS for a speedo.

 

Could it have a very high stall racing converter ?? Power brake it and see what RPM the converter flashes too. MAKE sure there is nothing in front of you.. Trans final drive is not changeable. There is available a low gear set for 1th gear. Have you physically removed the differential and checked the gear ratio stamped on the ring gear..

 

To all - thanks for responding. Yes, I have checked the differential ratio. Two ways actually. First, I did the tire spin (twice around with the other tire on the ground), and the ratio checked out to be 3.23. Then, I checked the differential itself, and it is stamped 3.23. It is DEFINITELY a 3.23 gear.

I feel like if there was slip in the drivetrain somewhere, it would not have lasted the 15+ years tht I have had the car and driven it.

I have heard of people changing out the gearing in the trnsmission, but I cannot for the life of me figure out why they would do this.

Am I missing anything else? It is driving me crazy.

 

Geez, this does not make sense unless you have about a 3500 rpm stall converter

What happens if you play with the throttle, can you get the rpm up/ down much at the same speed?

Lock the brakes/ block the tires, and power up the car at a stop, see what the tach reads, that should give you some idea of the stall.

I just took a quickie at a calculator. Your tires, 3.23, should be 3000RPM about 73 mph

 

I was going to say it sounds like either the gears have been changed in the differential or the the torque converter is a high stall. I noticed on my 68 Plymouth Satellite 383 3000 rpm stall converter and 3.9 to 1 gear ratio. My tach sits about 2900 at 55 mph. around 3500 or so at 65. Check your stall speed.

Also if you have an open carrier in your diff, it is not accurate to check your gear ratio by turning the drive shaft and counting the turns of the wheel while 1 wheel is on the ground.

 

This is not correct. All you must do is turn the wheel TWO turns and count the shaft.

3.23 will be about 3 1/4 turns. 3.55 just a tiny bit past 3 1/2. 3.90, just under 4 turns, and 4.10 just a bit past 4.

You can EASILY figure out what ratio you have this way.

 

I checked the stall rpm of the torque converter. It appears the flash stall is about 3000 rpm. Could this account for the way the car drives?