The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Ugh, apparently my speedo reads off.. by a lot

**MikeB** · 11-02-2002, 11:10 AM

If the cop said you were doing 72 and your speedo said 58, that means your speedo cable needs to be spinning 24% faster. That's a lot! Usually larger tires will make a 10-15% error max. I had a similar poblem (10% slow), so pulled out my old set of speedo gears. I then determined the current gear ratio as well as the ratio I needed. I found a trans supply shop with a variety of nylon ring and pinion gears and ended up buying a ring gear with one more tooth than the one I had. I also replaced the pinion gear since it looked worn. The ring gear is secured to the tailshaft with a metal clip. Don't forget to replace the clip, or it will come back to haunt you.

11-02-2002, 11:10 AM	#11
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,666	If the cop said you were doing 72 and your speedo said 58, that means your speedo cable needs to be spinning 24% faster. That's a lot! Usually larger tires will make a 10-15% error max. I had a similar poblem (10% slow), so pulled out my old set of speedo gears. I then determined the current gear ratio as well as the ratio I needed. I found a trans supply shop with a variety of nylon ring and pinion gears and ended up buying a ring gear with one more tooth than the one I had. I also replaced the pinion gear since it looked worn. The ring gear is secured to the tailshaft with a metal clip. Don't forget to replace the clip, or it will come back to haunt you. __________________ Mike 1969 C10 LWB -- owned for 36 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- converted from 250-six to roller cam 350, Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! Last edited by MikeB; 11-02-2002 at 11:13 AM.