Speedometer calibration?

[RikRunr]

I have a 97 SR5 with stock 16" wheels. I'm going to need to replace my tires soon (definitely before the next inspection in August). I'm considering moving up to 17" wheels (probably go with stock Tundra/Sequoia). My questions are: What is required to recalibrate the speedometer? Do you have to swap some gears, or is it much simpler than that? I realize 16 to 17 doesn't make that much of a difference. Just looking for some opinions on whether I should even bother.

 

[VA07]

The size of the wheel wont matter, and wont change anything in terms of accuracy of the speedometer. Tire size is what will change the accuracy. Are you planning of changing tire size with the upgraded wheel size?

 

[RikRunr]

Maybe I'm missing something in your question. If I find a set of 17" wheels (say from a Tundra, Sequoia, or FJ Cruiser), I'm going to have to get bigger tires. Even if I had the 17" wheels now, I couldn't use my current tires because they're 265/70R16s.

 

[20034RunnerV84x2]

Maybe I'm missing something in your question. If I find a set of 17" wheels (say from a Tundra, Sequoia, or FJ Cruiser), I'm going to have to get bigger tires. Even if I had the 17" wheels now, I couldn't use my current tires because they're 265/70R16s.

If I'm not mistaking, he's talking about the width of the tires.

 

[RikRunr]

If I'm not mistaking, he's talking about the width of the tires.

If so, then he is absolutely right. However, I was referring to the diameter. With everything else being equal, a 265/70R16 is smaller in diameter than a 265/70R17 (only by about 1/2 inch). This does have an effect on the accuracy of the speedometer. After switching to to 17 inch wheel/tire combo, your speedometer registers 60 MPH, but your actual speed is approximately 62 MPH. If it's a simple calibration that costs less than, say $50, I might do it. Otherwise, I can live with adding a MPH or two to my registered speed.

 

[VA07]

If I'm not mistaking, he's talking about the width of the tires.

Width wont's effect your speedo at all. Only diameter will effect it.

Maybe I'm missing something in your question. If I find a set of 17" wheels (say from a Tundra, Sequoia, or FJ Cruiser), I'm going to have to get bigger tires. Even if I had the 17" wheels now, I couldn't use my current tires because they're 265/70R16s.

I realize you are going to have to get new tires, but you can buy something that will be the same diameter (or nearly the same) and retain the same accuracy of your speedo.

If so, then he is absolutely right. However, I was referring to the diameter. With everything else being equal, a 265/70R16 is smaller in diameter than a 265/70R17 (only by about 1/2 inch). This does have an effect on the accuracy of the speedometer. After switching to to 17 inch wheel/tire combo, your speedometer registers 60 MPH, but your actual speed is approximately 62 MPH. If it's a simple calibration that costs less than, say $50, I might do it. Otherwise, I can live with adding a MPH or two to my registered speed.

Here is a chart.
I will use the example of the 265/70R16 since that is what you mentioned above. If you look at the Overall Diam. column and look down the that tire size, you will see an overall diameter of 30.6".

Now you can look through the tires with a 17" rim and look for an overall diameter of 30.6" or something very close. From what I see you could go with a 245/70R17 for a skinnier tire that is the same height or you could go with a 265/65R17 for a tire that is the same height and width but will have a slightly smaller sidewall to account for the larger rim diameter.

This only works for this specific tire. If you look at another brand or model of tire, the sizes will be slightly different. You can even look at the different tires that are listed as the same size but have a different load range will have a different diameter within the same model.

My point being in all of this: Unless you are changing your tire size dramatically, there is little point to trying to calibrate the speedometer.

 

[RikRunr]

Thanks VA,
I actually hadn't considered going with a 17" tire that is the same height as the 16". Very good point. I think if I end up with a half inch taller tire, I agree a calibration is probably not warranted.

 

[IanB]

It's actually the circumference of the tire that affects the speedo calibration. You can change the wheel size and maintain the same tire circumference, here's a link to a tool I use to help figure out what size tire is appropriate, and how big a difference it will make to the speedo:

Tire size calculator

FWIW, when I got new tires I went from 265/70/16 to 265/75/16, and my speedo was actually dead on accurate with the new, bigger tires than it was with the factory size tires.