Thread: speedometer gear for a 700r trans.

Tweet

I installed a 700r in my 57 Chey. When i'm going 60 the speedo.
say 80. When I go to GM parts they want to know the year the
trans. is and what it is out of. It was made by Tpi and I have no idea what they used for a core. Has anyone had this problem
and can tell me what to ask for?
Thanks,
Balaam

I had a similar problem with my TH350 when I went to 3.42's. I ended up going to a local tranny shop, and talked to the owner, and expereniced trans guy, told him my problem, And he got me a gear with more teeth to slow it down. It wasn't new, but it was in good shape and it worked. my speedo was 40mph over, and now it's 10, so it's and improvement. seeing how yours is only 20 over to begin with you might be able to get it dead nuts with the right gear.

Also the parts guy at your local GM dealer can be helpful, as they deal with that stuff more than most. try bringing them your old gear, they can probably get you the right one.

worst case scenerio is you live with a 10 over speedo, as long as you know how much over it is, you'll be alright.

Oh and try this link, it will help you determine what RPM= what speed.....

http://www.richmondgear.com/101032.html

No need to guess. Go to this site. It will calculate the gears for you. Steve Holmes knows Chevy overdrive automatics inside out.

All you need to know is tire diameter - or the P-metric tire size (e.g. P 295/50R15).

http://www.700r4.com/speedoCalc/speedocalc.shtml

You may need to change both the drive gear and the driven gear. To replace the drive gear, you need to pull the tailshaft.

Thanks, that worked great. 275x60x15 with a 3.90 works
out to a 15 tooth drive gear and a 42 tooth driven gear. I'll
take it apart and see what I got.
Thanks again,
Balaam

3.90's should really be a blast with that 700R4. It's the equivalent of 5.40's behind a M-21 Muncie. That's my frame of reference for drive ratios.

My '34 coupe has 3.73's behind an R4 and a Fastburn 385 w/ the "Hot" service cam kit. It's not ready to fire yet - mucho blocking to do on the glass body - but I'm anxious to see what happens.

Yes it's wicked taking off. I need more rubber but I just clear the frame and the wheelwell. I don't want to cut it up. You will probably get more rubber under your rod. I have a 383 with lots of torque, but you shold be able to take advantage of yours.

Yep, Mine's a lowboy, so I can put as much rubber back there as I want. Current tires are P295/50R15.