I recently bought a 1970 Cutlass Supreme with a factory 350 motor and factory 3 speed automatic transmission. The speedometer is running 10 MPH higher when running at 70 MPH (80 on speedometer). The person that had it before put 15 inch Craiger SS wheels with P235/60R15 tires. I have a 1970 Chassis manual but I did not see 15 tires. Also I could not find tag on the differential, but think I have 2.78 gears in rear. I did take speedometer drive gear out and it was a 35 tooth.

Any ideas? Wish I could figure out gears in rear end to be sure. Understand that 1970 Cutlass probably did not have 15 wheel option or I am not reading chart correctly.

 

The WHEEL diameter is irrelevant. What matters is the TIRE outside diameter. Your 235/60-15s are only about 26.10" in diameter. This is about an inch SMALLER than the outside diameter of the original G78-14 tires , which is why your speedo reads higher than the actual speed. You need to know your actual rear end ratio and the actual tooth count of the speedo drive gear - which is the gear on the output shaft of the trans. The one you pulled out is the speedo driven gear. Alternately, you can get an accurate calibration of the speedo error using GPS or mile markers on a freeway. Hold a constant indicated speed (say 60) for a few miles and determine your actual speed over that distance by timing or using the GPS.

Say as an example that for an indicated 60, your actual speed was 55. That means you need a speedo driven gear that is 60/55 larger. If your current gear is 35 tooth, you'd want to try 35 * 60/55, or 38 tooth. You may not be able to get the exact ratio you need without also changing the drive gear on the output shaft.

 

Thanks for the help. Just received the seal kit and a new 39 tooth gear. All good now.

 

The 35 tooth driven gear only came with the 2.56 ratio or with a .619 adapter on a dealer installed 4.33 ratio.

My guess is your car has a 2.56 gear in it.

 

Similar issue for me. I've got a 2:73 rear, TH 350 trans. But, my speedo is consistently reading 4 mph higher than what I am actually traveling. So, would I need a speedo driven gear that has 2 teeth more, or 2 teeth less than what's in there now (I haven't pulled it yet to see what's in there now.

thx

 

How did you come up with "2 teeth"? Without knowing what the drive gear ratio is, you can't know what you need to replace the driven gear with to correct your issue.

Also, if the 4 MPH over doesn't change as you go faster, then there might be something going on in the speedo itself. If it has to do with the speedo gears in the trans, it will ALWAYS be off by a percentage, not a specific MPH.

To get the calibration correct you need the axle ratio (you have that) and the tire diameter. Then you need to know the drive gear tooth count so you can select the proper driven gear.

 

If you know either the drive or driven gear count, and the tire diameter, use this chart to calculate the gears needed. https://www.tciauto.com/speedometer-gear-calculator

You can play around with different gears, try to find a combination that with get you as close to whole number. For example, if you combination needs a gear with 34.739 teeth, you would have to use either a 34, or 35 tooth. The 34 would make the speedo read faster, the 35 would slow it slightly. In this example the 35 would read a little closer. The ratio adapters were used to “fine tune” the gear combination.

I used this chart to figure gears on my car. At the track, the speedo reads 119, my time slip reads 115. A 4mph discrepancy at well over any posted limit (in this country anyway) is plenty close enough.

 

Do any of you know what the drive gear is on 2.73? I'm interested in knowing the number of teeth. I'm thinking it should be the same as many previous years but can't find that information. The page Randy submitted doesn't help me.

 

This is the speedo gear combination that would most accurate. Use a 20 tooth drive gear, with a 43 tooth driven. Should be very close

These are the speedo gears that are available from TCI. I have no idea if that is a complete list of ever gear that was used. Maybe you could find others at transmission shops or core transmission suppliers?

There are 2 different driven gear housings. To use the above gear combination I provided, your going to need a gear housing also. However, I wouldn’t buy anything until your positive what gear ratio is in the rearend. Pull the diff cover, count the gears, do some simple division, and be sure before wasting money on a guess only to find out you have a 2.56 ratio.

 

That's handy if you know the number of teeth on the drive gear. I know my rear end (3.42) is not original but I want to know what the drive gear was for 2.73 so I can either change the driven gear, or change both.

 

Allan, isn't the information you're seeking provided in the CSM in the Speedometer Gear Chart "A"-Body- Auto.Trans. (my '71 CSM has it located on page 4B-31, Fig. 4B-82).
I believe (based upon the chart) you need to reference the type Trans: TURBO - 350 CODES JE-LA-MA, TURBO - 400 CODES OR-OK-OA, TURBO - 400 CODES OD-OG, or TURBO - 400 CODE OW.
I might be missing something in your question, perhaps.

 

You have to remember that even though the same gearing was available over many model years, the tire sizes changed and that would effect the speedo gears in the trans. So there is not going to be one set of gears for any given diff ratio. You're going to have to do the calculations specific to your setup to get the correct speedo gears.

 

 

 

Very nice. Mine is out of a 1970 service manual. I wasn't aware that they changed the drive gear so much in 1971.
I had a 20 tooth drive gear in the TH400 that was in my 70 when I got it. There is a trans shop close by that let me trade him driven gears and he gave me an 18 tooth drive gear.

 

My post wasn't meant to challenge you or argue a point; but, simply to demonstrate (as you've noted) the delta between the earlier (e.g. pre-1970) & later (e.g. post-1970) years and add additional information for CO users who might search threads relative to transmissions, speedometers & drive gears.

 

Can the speedo drive gear be changed on a Muncie trans without dropping the trans?

 

Maybe. You have to remove the rear casting. That involves dropping the drive shaft, removing the crossmember, removing the shifter, rotating the casting to unlatch the reverse shifter internally, etc... It is probably easier to pull the trans and do it on the bench.

 

That chart helps. My CSM doesn't provide any information for TH350 with 3.42 gears as it wasn't an option for the TH350 in 72. Looks like I'll need an adaptor .7333 for G78 (225/70/14). I guess I'll need to visit a transmission shop. Since I'm not great at math, I wonder if a 40 tooth driven gear would come close.

 

 

OK, so no 16 tooth was made for a TH350, BUT.....you can use the same drive gears as the 700R4. So you can use a 15 tooth drive and a 39 tooth driven and you should be good to go.

 

Thanks Randy! I do have a new 39 tooth driven, so I'll just need to find the 15 tooth drive.

 

https://transmissioncenter.net/shop/...er-drive-gear/

 

Nice video but this is not the drive gear, it is the driven gear. Easier to replace as the video shows.

 

Is the output shaft on a 700 4R the same size as the TH350?

 

I'm confused by this statement. The G78/14 is identical in diameter to the 225/70/14 IIRC. The 225/70/14 is 26.4" in diameter. The OP's 235/60/15's are 26.1". Is my math that bad, or how does this equal 1" difference in diameter???

 

Allan, I'm going from the dimensions in this table. It shows the OD for a G78-14 as 27.06", which is not quite an inch taller than the 26.1" 235/60-15s. Perhaps you are thinking of G70-14s, which are shown to be 26.8" OD in the table?

 

I just performed a rather exhaustive search on G-78/14 & every listing demonstrates 27.10" or 27.06" (which is rounded to 27.10").
225/70/14 demonstrates a diameter of 26.40"
235/60/15 demonstrates a diameter of 26.10"

My math demonstrates (27.10") - (26.10") = 1.00"

 

Tire diameter can affect the speedometer but I don't think your's would make that much difference. I have 14" wheels on my car and they are 26" in diameter. I think more likely someone has put the wrong speedometer gear on the car sometime.

 

I spent a couple minutes noodling about this site which has a speedometer difference calculator which is rather nice (if it is accurate). Someone obviously put some back-end embedded equations to get it to work. Pretty nice though.

https://www.tacomaworld.com/tirecalc...0r14-235-60r15

 

Thank you, that does clarify. I was thinking of G70/14's. Makes perfect sense now. Apologies.

 

Yes. 700R4

 

Thanks Randy, 15 tooth gear ordered, and the reseal kit for the tail shaft.