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I can't figure this out
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Posted by: Troy Dennis

[SIZE=5]I have a 1971 Nova with a 350 and a 350 turbo trans. It also has 308 gears in the rear end. the tries are 25''. when i am running down the road at 65 mph the tach is reading 4 grand. i changed the tach and the new one reads the same, it also sounds like it is at 4 thousand rpm. I have asked everyone i know and we can't figure it out. thank you so much.

Posted by: John

OK, if your numbers are correct, your actual MPH at 4,000 RPM on your 25 inch tall tires with 3.08 gears is about 96 MPH. If you are travelling at 65 MPH and you aren't passing your buddy who is also doing an indicated 65 MPH then you have a transmission slippage problem.

For futher info, your RPMs for 65 MPH on 25 inch tall tires with 3.08 gears should be about 2700 RPMs. If you are doing 65 at 4,000 RPMs then again, you likely have a transmission problem.

Lastly, your overall gear ratio from tire size is closer to a 4.57:1 ratio and not a 3.08:1. So what I'm thinking is that you need to find out what the gear ratios are in your Turbo 350 and you also need to find out what your gear ratio is in your pumpkin. I think you might have something more along the lines of a 4.56 gear back there.

One more thing, according to the math and your numbers, for the RPM and speed you are running, your tires should be 16.8 inches tall. This points again to slippage somewhere. Or a blown torque converter.

You need to get the trans code off the side of your Turbo 350. Then look it up somewhere. There are resources online that you can use. Then you need to pull your diff cover and count the number of teeth on the ring gear and the pinion gear and divide the number of teeth on the ring gear by the number of teeth on the pinion gear. That will be your final drive ratio. Check your numbers first, make sure they are correct. Then redo the math. You can find it here at this site:

http://modernmusclecars.net/forums/articles.php?do=math

Then see if the numbers match up. If they don't, you have a transmission problem and likely should just junk the one you have and buy a whole new one. It'll end up being cheaper than the rebuild it would take to repair a transmission with enough problems to be over 2,000 RPM in slippage.

Posted by: Grape Ape

The first thing I'd do unless I could feel transmission slip is get a couple quarts of differential gear lube along with a new gasket or tube of gasket maker and then lift the car, pull the cover off the diff., and read the numbers on the ring gear. It sounds like you actually have 4.56 gears to me. If so...

You may want to keep those if you're into drag racing. Otherwise, maybe get a higher set to save fuel and extend the engine life. Gear changes aren't the most difficult thing in the world, but if you choose to replace them and cannot set lash, get help to do that. And you should change the seals and bearings at the same time just to avoid having to go back into the rear anytime soon.

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