just put 18" wheels on my 88 convert and now the speedometer appaer to be way off. do i need to get another speedometer cable or something? what can be done done to get it back into accuraccy? i put on 18" x 7.5 wide with 235/40/18 tires....

 

Did you kno having larger wheels makes your Speedo reading inaccurate. It tends to be off because of different calibration due to wheel size. Changing out the cable/harness will not change the accuracy *in this case.

 

According to my chart, your tire size is only about 2% difference off from stock...
http://fc3spro.com/TECH/MODS/RT/tires.htm

This shouldn't be that big of a deal...

How do you know your speedo is off?
Are we talking about 10mph+???


-Ted

 

I'm with Ted, it those tires should make much of any difference. You need to find a way of testing it. Either drive next to a friend whos speedo is working, or find one of those "your speed is" things. I've also heard you can get a cop to test it with his radar gun, but I don't want to try it.
Also, my speedo is off even with the stock tire size if that makes any difference.

 

My speedo is about 4-5 MPH off. So say my speedo says im doing 45, im doing about 40. This is with stock 14" wheels

The way i know this is from every single digital speed sign the cops put out, always tells me the same thing.

I think there is a way to calibrate it but im not to sure how... maybe im thinking of the tachometer. I have read a writeup online some where.