It's hard to say accurately, but going to larger tires could decrease your mileage. The factory tried to maximize mileage for government regs and they have the tires matched to the RPM to do it. OTOH, if you have low gears, larger tires could help the mileage.
At any rate, you'll need to have the speedo recalibrated for larger ones. You used to have to replace the speedo gear on the transmission but now it's done electronically.
