i just bought a 91 camaro rs with the 3.1 v6 about a week ago. today was "lets see what she's got" day. my girl friend and i were goin down the interstate and i topped out at about 90 according to the guage.

later on while i was driving it home on the interstate, she was following me close because i have no tags yet. she called me up and asked why we were going so fast. i said "what do you mean? the guage reads 87" and she said "thats not right. we're goin 109".... so wtf?!

 

Lemme guess - PO put huge wheels on it, and didn't correct the speedo.

 

Time to adjust some speedo gears in the tranny I'd say.

 

Could be a combination of wheel/tire size, gear ratio, and speed sensor gear in the trans. Have fun!

 

na not really. its just got 205s in front and 215s in back.

 

But what diameter? What's stock size, not sure - about 25"? If the car has say a 27" tall tyre on the rear, the circumference is larger. So for the same speedo reading and wheel rpm, the tyre is actually laying down more distance for the same time. Ie actual road speed is higher than speedo expects.

 

215 60 15
how tall of a tire is that?

 

(215mm / 25.4) x 60% x 2 + 15" = about 25.15". Sounds about right, though like I said, I'm not sure of stock tyre size exactly. Same size as my rears. However, I doubt it would be smaller by the degree necessary to make your speedo read about 20% low.

Maybe can rule this one out. Gets back to different rear gears/total gearing, wrong speedo gears, or dodgy speedo itself.

 

i think i know what the problem is. the first owners had swapped the engine to a v8 and made it into a race car. so more than likely, they calibrated everything to the new motor. when the sold the car, they stuck the v6 back in a probably didn't put the gears back to v6 settings