First a short story to explain the subject.
Just got back from a weekend at the cottage.
I drove down w/ my 91 and my friend took her 91 Civic. We had 2way radios to talk to each other. She was constantly whining that I was going "too fast". I thought oh well... women driver and she'* driving a Civic, so whatever right.
Well anyways we were over 1/2 way to the cottage and she comes on the radio, "slow down, I'm doing 130kph and I'm not even catching you".
I look at my speedometer... I'm doing 120kph on the mark... maybe even a little under.
I said to her, "your speedometer must be wrong, I'm doing 120". He car has loads of problems so we just naturally thought it was hers.
I let her pull up beside me and we measured our speeds.
When I read 100, she was reading 110.
So anyways next day (with the 2way radios again) I'm driving and my friend Andrew has his dads truck. We try the same thing, only same results. When I was at 100, the Ram was reading 110. My speedometer is off by like 10kph @ around 100kph.
Might explain why a few years ago I got pulled over for doing 117kph and I could have sworn the cop tagged me just as I was dropping below 110kph.
Noticed something on the drive down.
Now onto the real question....
Any ideas what could cause my speedometer to be off?
I'm guessing it'* probably off by 10-12% at any given speed.
I'm using the stock rims, proper size tires. There is no error codes in the system.