I went from 225/50-17 stock tires to 215/55-16 winter tires yesterday.
Before, my speedometer was 2 mph off in the city and 3 on the highway. So if I set my cruise for 70 mph I was actually doing 67.

Now, I'm 3 mph off in the city, 4 at low highway speeds, and at 70 I'm off 5 mph! So now when I set cruise for 70 I'm only going 65.

Just FYI, in case anyone cares.

Tony

 

This might be a stupid question, but how did you know your speed was less than the cruise control setting? I'm thinking of downgrading to 16's, but I want to make sure I end up with the same rolling radius so the speedometer isn't off. Did you notice a height difference bewteen the tires?

 

I've got a friend that's a police officer and I use a radar to check the accuracy of the speedometer. I just plug it in to the cigarette lighter socket and drive down the road with it in my car.

I have no idea what the difference in height is of the two tires, but evidently there was a difference.

Tony