Once the FlashPaq is synched up with the truck, you can adjust tire size, axle gear ratios, and in some cases even transfer case low-range information. This allows you to fix mismatched speedometer settings. We found that it is often best to undersize the tire input. For example, if you're running a 37-inch tire, input 36 inches, as most tires are smaller than their sidewall markings. The programmer can also optimize engine performance for towing and mileage and allow the use of 87-octane fuel in vehicles that recommend 91-octane.