I have 215/45/17s on my xB.

I read that the miles on odometer are off by 3.7% in this setup...Im assuming that means Im going 3.7% further then what the miles read out are?

if this is the case, for figuring out my mileage per tank, I record the displayed trip miles since last fill up and multiply that by 3.7% then add that to the trip miles then devide by how many gallons I just filled my tank up with to full.

I hope the above is correct, if so...Im gettin about 31.5mpg on a mix driving of city and highway per tank.