I posted this to the Expy board, but I believe it applies to the F-150 crowd also.

I put 275/70/16's Bridgestone Dueler HT tires (30.1 inch diameter) in place of my stock 255/70/16's Goodyear Wrangler RTS tires(28.875" diameter) on my 98 Expy EB 2wd 5.4. The Goodyears looked to small and were wearing so badly even with rotations and making a ton of road noise. The new 275's really fill up the wheel wells nicely and look like the correct size that should have come from the factory. Plus the Bridgestone Dueler HT is a very quiet tire and it handles much better than the Goodyear. It has been the OE tire on Toyota Landcruisers.

The speedo however was off by about 3 mph or 4.79%. The 97-98's need a new speedo gear when changing tire size, while the 99-up have a reprogrammable PCM computer which can be done at the dealer to accomodate tire size changes. I went to this great website to calculate the new speedo gear I'd need: http://www.finer.com/Internet/pages/speedo.htm

I first entered my stock info - 8 drive teeth, 3.73 ratio and 28.875" diameter and it calculated 21 teeth-which I had stock, so at least I knew this calculator is accurate. I then entered my new tire size of 30.1" and it calculated I need a 20 tooth gear. For $5 I got a 20 tooth gear from Ford and installed it in 5 minutes. Now the speedo is off by only .7% according to my testing. That equates to an error of only about 7 miles for every 1000 miles of driving- not bad at all.

Hope that helps someone out there.

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1998 Expy EB
5.4, 2WD, 3.73LS
Magnaflow cat-back
WMS Velocity Tube
Superchip
275/70/16 Bridgestone HT's