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Speedo/Tire size

Feature this

Factory tire size is 225/75R15=28 1/2 inch Diameter
Just put on 31x10.50=30 1/2 inch Diameter

The speedo is right on

If it’s correct now it was significantly in error before.

Thats about all i could come up with but how could that be? isnt it mechanical? Gear driven…

How did you determine that it is now 100% accurate? And did you check it before the change?

I check it using GPS and yes it was off with factory size tires on it. I also check it with the lit up speed signs. The how fast your going signs agree with the GPS

Often the factory size tires results in the speedometer reading higher than actual, which you’ve now fixed.

Well what kind of madness is that. You mean in short its a factory fudge up?

No, just a way to ensure that minor errors don’t result in a ticket.

I think my post went bye bye or I’m asleep or this is a double post. Like I said, sometimes if there are several tire size options available at the factory, someone on the line might have grabbed a yellow gear instead of a blue gear or something. Or maybe someone swapped something out on you at some point or even at the dealer if someone wanted different tires.

My experience has been that speedos are usually slightly (5%) high from the factory. I suppose so that they’d never be found partially liable for a speeding ticket. I calibrate by setting a speed (say 60MPH) then timing between mile markers on an interstate (does it take 5:00 to go from MM 67 to 72?)

IIRC, Honda got sued for over-optimistic odometers that put owners “out of warranty” too soon. (IME, odometers are closer, but still 1-2% optimistic.)

Most manufacturers intentionally produce cars that indicate a higher speed than real. That helps to reduce the chance of having the speedometer reading too low. A car with a low speeding speedometer will result in the possibility of the driver going over the limit with out knowing it and that could result in ticket or an accident.

Remember the old Ramblers with the “Police Certified” printed on the speedo?

Changing tire size should change the actual speed of the vehicle at any given MPH on the speedo.
My '09 Caravan checks out very close to my GPS, with stock tires.

Badbearing, you should know that whether the speedo is “mechanical” or not is irrelevant to its accuracy. Those that aren’t “mechanical”, which includes almost every car still on the road, take their signals from a Vehicle Speed Sensor (VSS) that measures the rotations of the tranny output shaft. A change in tire circumference changes the number of times the shaft rotates for every mile, and thus affects the speedo accuracy.

Thats common sense. The number of rotations to cover a distance. Yes i did and do understand that. Hence Speedo/Tire Size


Thanks for posting the picture of the 4Runner

You seem to have aftermarket rims . . .

Is the truck lifted?