View Full Version : Speedometer Off?


my bandit
01-21-2010, 01:31 PM
i was crusing down the highway at 75 one day and look over and im passing a civic whos speedo said 80 :confused: so i had my girlfriend follow me in her 08 Mazda 3 stock. while i was crusing at 75 i called her up and had her pace me. she said her speedo said around 80. but whenever i pass those speed checkers that the cops set up it said that my speed was correct. but that was going 35. i have put 18" rims so could that be the problem and it cannot be detected going at low speeds?

RPM WS6
01-21-2010, 01:40 PM
It's not about the wheel diameter, but rather the overall tire diameter.

The stock tires on these cars are 25.66" tall. If your current tires are significanty taller, then this could be the reason.

What size tires are you running?

my bandit
01-21-2010, 02:20 PM
245/40/18 in front
285/40/18 in back

RPM WS6
01-21-2010, 02:42 PM
285/40/18 in back

Ah, that explains it then.

Stock 245/50/16 or 275/40/17 is 25.65" and 25.66" tall respectively.

Your tire (285/40/18) is 26.98" tall. That's a big difference.

So with those specs, the calulator at Discount Tire says that if your speedo is showing 75mph, you're actually going 78.89mph.