Speedometer after tire change

I think the outside diameter of the tires, that has contact with the road, is nearly the same for the 15" and 17" wheels. The owners manual says to use the tire calibration button for the navigation system after getting new tires, but I have never had a clear understanding of what it does.

 

Not if you keep the same diameter of tire. Go on tire rack and check things out! Hal

 

Some navigation systems use the odometer data and accelerometers to continue tracking your route when you lose GPS signal. Such as in tunnels, parking garages,etc.

 

.25MPH at what speed?

 

Your speedometer is off by 1-2mph (fast) from the factory so if you size your tires correctly I wouldn't worry about it.

 

A nice calculator for evaluating this question can be found at:
Tire Size Calculator

225 45/17, diameter diff: -0.03% , speed will read faster
205 50/17, diameter diff: +0.36% , speed will read slower

I went with 16" rims and a 205/55 tire...I like the look.
There is a picture posted here.

205 55/16, diameter diff: -0.41% , speed will read faster

I find that compared to my GPS, both the stock tires and these show about 1-2MPH faster on the speedometer vs. the GPS at highway speeds. That's about a 2% safety margin built into the speedometer.

 

The numbers from the calculators are based on perfect tire dimensions to calculate a tire's circumference, which are not real-world numbers. The same sized tire can vary quite a bit from brand to brand. For example, your particular brand 195/65 -15 tire might really be a 197/66 - 15. Fortunately, you can find the actual circumference for each tire on the manufacturer's website, typically expressed as revolutions per mile.

So if you want to be accurate, you would use the calculator as a guide and then lookup the revolutions per mile for each particular tire you are considering. If you want to maintain the same speedometer accuracy, then first lookup the revolutions per mile for your stock 15" tire and then find the 17" tire that's the closest.