Bill, when going to a larger tire size you'd need to change the plastic gear in the speedo drive assy bolted to your tranny. This is simple and I've done it in a P'up before without draining the tranny oil. Just elevate the rear and unbolt/slip out gear-install new/reinstall drive assy.

Jerry Lemond can probably get a good used gear with the correct # of teeth for your application so you might get hold of him. I don't know if they're still selling these new.

I was planning on going from LT235's on my 3.4 Trooper to larger tires. I have a nice set of 31X10.5-15's which should provide about "1-gear-taller" gearing. I'd expect that'll cut down on hiway rev's a bit and the 3.4 should certainly be able to pull it! Turning way too fast at 60 right now and with gas prices going thru the roof, any bit helps.

You can figure out the percentage change with the larger tires you plan on installing. That'll let you compensate for odometer readings until you change out the speedo drive gear. Otherwise your mpg calc's will be skewed and you won't be able to tell if the larger size actually helped or hurt.

Here's a real handy tire dia calculating tool:

http://www.1010tires.com/tiresizecalculator.asp

HTH & let us know how it works out............ed