Hi guys, I tested yesterday two of my cars, 1974 Century 455 stage 1 gran sport and 1970 Riviera 455, both original rear ends and rims, tires very close to original size, on Gran Sport speedometer shown speed higher than real speed in Riviera is opposite speedometer shown lover speed than reality. It is around 6 to 7 miles difference on both cars when driving in around 50 miles\h and difference will go higher with higher speed. Because both transmissions (both TH 400) are overhauled before I guess the speedometer gear on transmission side is changed with different size. anyone knows what is the true teeth\color speedometer gear for both models, or maybe problem is anything else?
Calculated gears will only be approximate, due to variables such as rolling dia of tires. I usually calibrate so the odometer is right, then assure the speedo is also OK. To get it exactly right, measure the actual miles vs the indicated miles, the ratio of those is the correction that must be to the current trans speedo drive and driven gear ratio. Count the teeth on those gears, then find a new pair with the needed ratio. Bruce Roe
I've always gotten very close with the calculator, https://www.tciauto.com/speedometer-gear-calculator You need tire size, # of teeth on the DRIVE gear, and the rear gear ratio. With that, you can calculate the # of teeth for the DRIVEN gear.
