I have a set of BFG AT's 265/75/16 on my 98 4X4. It came with 255/70/16 which I run in the summer months. My question is if I were to replace the smaller tires with a 265 tire should I have Ford reprogram my computer to compensate for the bigger tires? In the winter months using the 265 BFG's the truck is always dropping out of overdrive and is harder on gas along with a messed up speedometer. Would changing the computer program bring back the feel of the stock tires?
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Your 98 doesn't use a computer controlled speedo. Thats only on the 99+ models. The 97 and 98s still used a VSS gear. Now remember by going to larger tires you raised your effective gear ratio. If you originally had a 3.55 ratio you may have a 3.45 ratio which means its going to take more power to turn the tires. Going with a different VSS gear will help, but it will still most likely downshift more on hills. Mine with 265s did compared to the 255s. If you notice most of the trucks with the 265s from the factory have 3.73s for a few reasons, this being one of them.

 

If that little gear controls the speedometer then is there anything that I can do to the computer to help out the shifting? What does Ford do to their trucks with different size tires that have the same gears?

 

Ford has different gears for different tires/ratios. To make the speedo read right on the 97/98 trucks you need to replace the gear. Only the 99+ has anything to do with the computer.