Speedometer Error

[golonaus]

But there is other issue too
Wheel size
Not many. Of us running stocks, right?
If You run stock tho, speedo is calibrated to the height of brand new tire
When miles go on tire is getting smaller so it's got to spin faster to be at the same speed
I don't know how much does it affect the speedo, but in some .0... It should

Happy speeding!!!

[CoBeerToad]

I was just looking this up because I passed a radar sign that showed I was doing 36 mph and I had the car holding at 40 mph. Tires are new-ish and original sizes. I'm going to double check on the way home today with a GPS app. The 4 mph difference just seems to be way to much for that low of a speed.

But about the odometer... What other method could they use to measure miles besides using the speedometer which I imagine is driven from the transmission?

[Joel-Box-ster]

According to Magellan GPS my speedometer reads 2MPH over the actual speed which is slower in real world. Changing into bigger diameter tires can also affect your speed reading.