mr_stoop

The speedo in my 78 over-reads by a significant amount (35%) at 60mph. It has 27" tires and 3.08s in the back. Any idea why, and how I can go about fixing it? Does it just need a rebuild? I imagined if anything or something inside would slip it would cause an under rather than over-read condition. Your insight is appreciated.

Kris Tunetso

Verify that you have the right speedometer driven gear for your transmission and gear ratio. You'll have to disconnect the speedometer cable from the side of the transmission to remove the driven gear fitting, and then make sure the gear has the right number of teeth for your rear-end ratio. Fortunately, the gears are color-coded for the rear gear they're supposed to work with.

If you have the a stock THM350 auto, you should have a natural (white-ish) 19-tooth driven gear for your 3.08 rear gear ratio.
If you have a stock 4-speed, it'll be either a brown 18-tooh gear or a silver 22-tooth gear.

Best I can do for you right now. Hopefully others will chime in with better info.

mr_stoop

Thanks, Kris. I'll have a look at the gear and post back with my findings.

mr_stoop

Ok, looks like the driven (attached to the speedometer cable) is red and the drive (in the tail housing) is brown. I am starting to think maybe this transmission is from some other vehicle.

From my research the red driven has 37 teeth, and the brown drive has 20 teeth, and using a calculator the closest match for the driven would be light blue with 45 teeth.

Does this sound right to anyone? What are the stock colors? Maybe I should just replace both at the same time...

BKarol

Willcox has this chart:

http://willcoxcorvette.com/instructi...gear_chart.jpg

7T1vette

A PO could have changed the rear gear from a much steeper ratio...and not changed the speedo gears. Or, the transmission could have been swapped out. Most tranny shops would have changed the speedo gears so that you don't have that problem; but it could have been swapped out by a PO. Also, the speedo could have reading error.

Make sure that the speedo gears in the tranny are correct for the rear gear you have in the car. If not, change them and see if that corrects your problem. If those gears are correct and you still have reading error, you will need to have your speedometer repaired...OR, install a ratio correction unit in the speedo cable circuit so that the speedometer reads correctly.

mr_stoop

Alright, after having the tail shaft housing off the transmission, I have realized that what I though was a brown drive gear, was actually purple. So I changed it out for a gear that has one less tooth. The speedometer is much closer to the actual speed now, only 10km/h out at 100. I have ordered a driven gear with one more tooth and that should solve my problem completely.

Thanks for all your help!

mr_stoop

Oh, one more thing, TCI is a great resource for calculating gear ratios for speedometer gears, they have a calculator and more found at:

http://www.tciauto.com/tc/speedometer-gear-calculator/