[dfeldt91]

I noticed in my own testing that I was never really utilizing fifth gear because it caused my fuel economy to go down. The only time I used fifth was following behind trucks at highway speed. So I might as well have a four speed transmission now that I have larger tires. I'm not complaining though. I am getting better gas mileage in fourth with the larger tires than in fifth with the smaller tires.

My drive shaft fell off the transmission the other night as I was leaving a rest area so I ended up staying overnight in my car, so with four hours of idling for heat and 265 miles I have used 9.5 gallons. Taking into account the .5 gallons per hour of idle, I got about 35 miles per gallon so I would have to think the extra 6 miles per gallon over EPA were in part due to the larger tires and my new driving techniques. Only 150 miles of the 265 were with the new tires.

Also, keep in mind that lower RPM's does not automatically equal higher fuel economy. Engine load has a lot to do with it:

If I am in third gear going up my hill which is pretty steep then I get about 28mpg with a very slight slow down. If I throw it into fifth then I get about 12mpg even with the lower RPM's because the engine is requiring more fuel to do the same amount. This is more so evident when you have the larger tires on when going up hills.

On long flat roads drafting is your friend, especially with the larger tires. Your car will use much less fuel to turn the extra tire size.