I was recently given an 87 Pontiac Bonneville; pretty nice car, but one problem. The speedometer is way off; by about 10 mph. How do I fix this so it reads the correct speed? Any help would be appreciated!

 

If it is an unmodified vehicle... I'd first be suspect of the wrong tire size. Alot of times people will go with the next size or so tire down from stock because it's cheaper and the standard person doesn't realize that a smaller (or bigger tire in height) does affect the speedo reading. That's just a guess though...

*edit.. to add... A smaller tire will make the speedo reading progressively faster with increased speed and a larger tire will be opposite. It will not just be a solid 'X'mph off across the board.

As tested through (by me so don't take it as gospel) the police survey things sitting on the side of the road.. 255/50/16 makes the speedo slower by 5mph @ 60mph over 245/50/16s. The effect is nil @ 30mph though.

 

I checked the tire size with the stock size on the sticker in the door.. they match up. Anymore ideas?

 

Not sure about this one. 87 bonneville . it is probably the speed sensor, just can't think of where it si on that car. I will look later, but that is probably the problem.


Happy Birth day to YOUUUU....Happy Birth Day to Youuu....happy b day Mr 82firebird...Happppyyyy Birthdayy toooo Youuuuuu!!!! (singing and applouding)

 

Thanks man! I'll have to look around for the speed sensor. Hopefully thats all it is.