Carguy55

Good day, First time here so hope I'm doing this correctly. Just got this vette and the speedo reads about 20KM high at 100 kph. The odometer reads more as well. Having done some research I expect I will have to change the speedo gear with one with more teeth. I am a bit stump as to why though.. Here are my findings, the tires are stock (255 x 60 r15), the rear end should be 3.07 according to the reseach I did (confirmed this by jacking the car up and rotating the rear tire on full revolution = a bit more than 3 driveshaft revolution. again according to my research with the automatic transmission the speedo gear should have 19 teeth which it has. I eliminated the gage by swapping another one and it also read about 20 kph high. Anyboby have any insight?

MelWff

the 1980 Corvette speedometer gear chart shows only the 3.07 rear and only the 19 tooth gear. You are confirming the speed inaccuracy using a GPS?
there is no chance your count of revolutions could be closer to 3.36?

0Willcox Corvette

typically, on tooth is equal to 5 mph... So make the adjustment accordingly. I don't know how this translates to KPH.. but I'm sure it'll give you a starting point.

Carguy55

Yes checked with GPS. I have just found the problem. The driving gear should be 8 tooth (orange) but it is blue which I believe is 10 tooth and would explain my discrepancies. Sooo now I think I can put a 23 tooth driven gear based on calculators on line, should be really close and relatively easy to do. Or pull the tail shaft and install the proper driving gear (which I have on hand on spare tranny), A lot more work from my research but can be done without removing the tranny.

7T1vette

Each worm gear (installed on the trans output shaft) has a range of driven gears that will fit it. And there are a few different worm gears. The [driven] speedo cable gear MUST be compatible with the drive gear or it won't work at all (or not for long).

There are speedo gear 'calculators' on the web which will allow you to find the correct gear(s) for your transmission.

Carguy55

Thanks for the info. I ran across one of those charts yesterday listing worm gear with matching speedo driven gears but for the life of me can't find it again lol. I will be parking the car for winter in a few more weeks so I will decide then whether to try the 23 tooth speedo gear (according to online calculator) or replacing the worm gear. Kinda leaning in replacing the worm gear...

Carguy55

Note: I checked it 3 times its definitely a 3.07 using car jacked up method and counting driveshaft revolution per one turn of rear wheel.

joewill

if you can't get the correct toothed driven gear then you can call these guys and they can build you the correct spec adapter..

http://www.ebay.com/itm/Speedometer-...-/132311016520

CanadaGrant

I had the same problem with my wife's 78. At 100kph it was reading 111. It had a 19 tooth driven gear so I replaced it with a 21 tooth and it is accurate now. With the drive gear you have it should work with a 23 tooth driven gear vs. the 19 tooth you have now. Going from 19 to 23 teeth on the driven should reduce your indication by very close to 20% and way easier to do than replacing the drive gear.

Carguy55

I already have the proper worm gear (8 tooth orange). and removed it from my spare tranny, was quite easy. I know it will be more work to do on the car. But figure when I remove the driveshaft it should not be too bad. decided to go that route rather than to wait for a 23 tooth driven gear. Doing it tomorrow, hopefully everything goes well and my speedo will be much closer to actual speed.

Carguy55

Just an update. I replace the drive gear in the tranny tail shaft from 10 tooth (blue) to 8 tooth (orange). Checked at 80 kph and 100 kph against gps and it is bang on. Took me about 3 hours. Dropped the exhaust from headers collector back, lowered the cross member, and disconnected the driveshaft at the transmission. Everything went easy except for putting the exhaust back on. I had to disconnect the tranny mount as there was a welded on bracket that ran from exhaust pipes to it. Was a PITA to line back up. Other than that all good.