KevinG

I have no clue what size/color speedo gear to order for my trans. I have 3.08 rear end, 275/60/15 rear tire size and a 4spd trans in my '69.

Thanks.

wombvette

Look up the rolling diameter for the tires you have. Look up the rolling diameter for the tires that came on the vehicle. Figure the distance that would be traveled by one revolution of the tires. Use the ratio between those to determine what gear you need. Mutiply the ratio by the number of teeth on your old gear.

KevinG

I have no clue what my old gear was b/c the car didn't come with an old gear (project car). All I know is:

stock tire size was: 235/70/15
current tire size: 275/60/15
rear end gears: 3.08

Mike Ward

Your AIM has a chart showing the correct and original gears.

anips

i came up with trans dr. gr. 8 teeth, speedo cable dr. Gr. 18 teeth, go to tri5chevyrides.com/speedo.php

KevinG

Thanks!

69 Chevy

Once you get everything together and head for the highway, take along a GPS. More accurate than having a buddy drive along side with his cell phone telling you his speed.

I have found that, on average, changing the plastic driven gear 1 tooth will change your speedo 3 mph.

larrywalk

Once you've found the speed correction, divide the indicated speed by the actual speed, and then multiply that times the number of teeth in the driven speedo gear. That will be the new desired number of teeth.

For example, 70 indicated mph / 63 actual = 1.111;
1.111 x 18 teeth = 20 teeth needed for the driven gear in the trans.

69 Chevy

Another thing I found when comparing my speedo with the GPS is that my speedo output is not linear. I have installed a combination of drive and driven gears to make it accurate at 70 mph. It is off by as much as 3-4 mph when going much slower or faster. I chose to make mine accurate at a speed where I think I'd be likely to get a speeding ticket. If I get caught doing 135 in a 65 zone, a difference of 3-4 mph won't matter much.

freddy4099

So am I correct to believe that if I have a worm gear that has 22 teeth and my speedometer reads 10 miles per hour over I should put a gear in with 25 teeth 1 tooth for each 3 miles per hour over to bring it down? or would I need a lot more info to make a good assessment of what gear I need? this is for a TH350 transmission, Thanks.

C3 Stroker

This link will help....scroll to select your application..

http://www.transmissioncenter.org/GM...eter_Gears.htm