Saw a truck around town today with a ridiculous lift kit and chunky off-road tires that were clearly much larger than factory standard, and it got me thinking; if you install this kind of modification in a car, do you need to adjust the speedometer to compensate? What about the odometer?
My logic is the only absolute measurement the car has is how fast the wheels and drive shaft are turning, so presumably there is some sort of multiplier - 1 revolution = X meters - that is then used to show speed and track distance travelled, but that factor would need to change if the circumference of the tires did
Your speedometer likely wasn’t accurate before the tire size change
I use my phone as a nav device pretty much everywhere I go with different rental cars and the speedometer is never accurate to the GPS measured speed or the radar based speed limit warning signs I see sometimes
The GPS always matches the radar signs, so I tend to trust it and use it more than the factory speedometer
AfaIk, the tachometer is allowed to be overestimating your velocity by 10 %. On my car, it’s pretty accurate when using 15" the factory default summer tires, but it’s off by 10 % when using the factory default 14" winter tires.
Check your tire pressure.
Being low or too high is enough to put it off.