Some believe that gas mileage and tire size seem to be related in terms of how you measure the mileage factor. Your odometer may reflect better mileage with bigger tires. Some experts refute this by pointing out that larger tires will make the odometer read a greater distance traveled-unless the odometer is recalibrated for the larger tires. Your actual gas mileage and tire size issues may have more to do with the fact that your odometer isn't calibrated for the bigger size than an actual savings. Your best bet to increase gas mileage is to use fuel efficient tires with better rolling resistance, and keep those tires properly balanced and inflated.