Project86B4Z

My 86 iroc speedometer has a 10 mph lag (i.e speedo says 55 actual speed 65!) After the previous owner switched the factory rear to a posi 3.73.

Current drivetrain set up
Assumed factory 700r4 with the factory drive and speedo gear for an lb9 305
tire diminsions 245\16r\50

Which speedo gear should i use and what color would it be

sofakingdom

iTrader: (1)

That's not "10 mph", as in 65 - 55; that's 15.4%, as in, (error at 65) ÷ 65.

Take a look at your existing gears. They'll have a ratio; for example, they might be 15 and 46, which would be .3261. You need a ratio that's 10% lower, which would be about .2935. Then go to places like this http://www.transmissioncenter.net/sp...__va.htm#700r4 gear driven and see what gears you can actually buy and that will go together, that are close to that. For instance 16 ÷ .2935 = 54.5; that won't work. 14 ÷ .2935 = 47.7; if you can find a 48-tooth driven gear you'll be within 1% of dead-on (your tires change in size more than that based on their temp... i.e. getting any closer is an exercise in futility) 13 ÷ .2935 = 44.3, so 13 and 43 would work. And so forth. It's just 5th grade arithmetic (well, maybe 4th grade if you weren't in the "special" "social promotion" section of 5th grade). Not rocket science or anything; after all, it's just a car.