I know this was answered before, but I can't manage to find it on the site so here goes

I changed my tranny from a pg to a TCI th350. In addition, the tires went from whatever the heck was on the car originally (not stock) to 15" rims with 275s on 'em. I know the speedo is off. How do I figure out how off my speedo is? Is it a function of just the tranny, or the tranny and the tires? How do I correct this (I know there are Speedo gears somewhere in the tranny (?) but where do I get new ones, or is this not my problem)? TIA--Amanda

I paid the transmission shop to do this for me. They drive the car on a measured course (like the measured miles on the highway)and then they change the gear to one that they figure is close. Then they drive it again and and adjust accordingly.

Hope this helps,

Scott

Find a section of freeway or whatever, that has measured markers on side of road. At 60 m.p.h with stopwatch should take a 60 sec.Then decide fast or slow. 55sec.=65mph,65sec.=55mph. The speedo gear is behind housing cable threads to on trans. Take bolt out and wiggle housing out. Count teeth,add teeth to slow speedo down,subtract to speed it up(trying to remember). Local GM dealer should have gears,maybe chart to get close. gooduck!

Check out this link.
http://www.chevelles.com/shop/speedo.html

CA, basically everything above is true. But first, find a stretch of road that is exactly a measured mile. The speed you travel is unimportant at this time. Look at the odometer in your speedometer when you start. Whatever the tenths is, it should be exactly that when you arive at the end of one mile or 2 miles or 10 miles. If at the end of the measured mile, for example, your tenths is .9 or 1.1 miles then change the speedo gear at the tranny until your odometer shows one mile exactly in a measured mile of road. once your odometer is correctly measuring mileage, then and only then do you want to check the speedometer for speed. If it is not correctly measuring speed over a distance of a mile then it can only be correctly calibrated by a speedometer technician. You want your odometer to read mileage acurately before you calibrate for speed because otherwise if you change speedo gears at the tranny to correctly indicate your speed you will never know exact distance traveled or never be able to acurately check gas mileage. I know this at first seems backwards, but think about it (very probably, after you get your odometer recording correctly, the speedo will be right on the money).

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[This message has been edited by DZAUTO (edited 02-16-99).]