I'm sure this question has been asked and answered somewhere on here, but I can't find it. If I go from my stock 195 55R16's to 205 45R17's what will the impact be on the accuracy of the speedometer and odometer? Thanks!

 

Google "tire size calculator." For example, here at Discount Tire. You'll be going 64.52 when your speedometer reads 65. Assuming it was exactly calibrated to begin with. Which it wasn't.

 

That's very helpful. I'm buying them from tirerack.com and I didn't know this calculator was there. Does it impact odometer accuracy since there are about 6 more revs per mile on the new tires?

 

Yes. If your odometer thinks you went 1000 miles, you actually went 1000 x 825.04 / 831.2 = 1004.62 miles. You'll go an extra 69 miles before your 15k mile service indicator says it's time. Which is nothing. Tire pressure, tread wear, and the inherent error in your speedometer and odometer are a lot greater than a mere 4.6 miles per 1000 miles.

It's too bad you can't recalibrate a car speedometer and odometer easily with a $115 dongle like Speedo Healer, like you can for bikes.

 

Yes. Any change in tire size will impact the odometer as well as the speedometer. You will need to have the car tuned for the new tire size. Mini should be able to do this, some tuners can do this also. I have a DiabloSport tuner for my Challenger (won't work on my wife's MCS) that allows me to change the tire size in the computer.

 

Hmmm, I did the math you suggested and I got 992.60 rather than 1004.62. That would mean you haven't gone as far as the odometer indicates. Did I do something wrong? I guess it's a moot point though because the difference is small either way.

 

If your go to larger diameter tires then you will be going further than the odometer indicates and your speed will be faster than indicated. Don't confuse rim size with overall tire size.

Almost every web site that sells tires has an on line tire size calculator.
Here is one I use:
http://www.1010tires.com/TireSizeCal...?action=submit

 

The answer actually depends on what tires you are switching from and to. 195/55-16 is a nominal (not actual) size. For example, MINI's 2010 OEM 16" tires had 24.4 or 24.6 inch diameters. OEM 205/45-17 tires had 24.3 or 24.5 inch diameters. So the very minor impact on the speedo and odo could be an increase or decrease. In any case a tire's diameter wears down by as much as half an inch over its lifetime, so there's no need to tune anything for a few tenths change in initial size.