fuel mileage and tire size

[Frank Lee]

I've gone larger on the F150 and it did NOT work, but then again, mine has the tallest axle gears available. With the oversized tires it downshifted every time someone sneezed, and it gets driven on the flatlands. Since yours is also a '94, it likely has the E4OD transmission. If you have the tall gears you will not like all the downshifting.

[euromodder]

Originally Posted by Execut1ve

so, in general less flexible sidewalls -> higher rolling resistance? and more flexible sidewalls -> lower rolling resistance?

No, the other way around.
It takes energy to constantly deform the tyre.

Less flex in the sidewall -> less rolling resistance

[CapriRacer]

Originally Posted by Execut1ve

so, in general less flexible sidewalls -> higher rolling resistance? and more flexible sidewalls -> lower rolling resistance?

Sidewalls play very little role in RR, but tread compound - both amount and properties - plays a HUGE role.

[aggie sig]

my jeep wrangler had 32" BF Goodrich Mut Terrains on it, and any little hill made it downshift once if not twice with 3.73 gears.

If I could get on a flat or downward sloping surface with a no wind or tailwind it did fine, but other than that highway speeds were a huge PITA

[Execut1ve]

I have the 4R70W tranny and 3.55 gears

[Execut1ve]

and the thing already downshifts at highway speed on anything steeper than slight hills

[CapriRacer]

OK, Based on what you've told us, you just may be at the limit of the transmission's gearing. What you want to do is keep it from shifting as much as possible. So going up in overall diameter is not a good option.

Your truck came with 15" P metric tires. Those have a tire sizing structure that looks like this:

P235/75R15.

You've got Flotation sized tires on there now. Those have a sizing structure that looks this this:

29X8.50R15LT.

The other common sizing system is LT metric - and they look like this:

LT235/75R15.

You do NOT want LT metrics or Flotation sized tires on your truck. Those tires were designed for more rugged usage. They will not be built out of materials that are good for RR.

You'll want to use as big a 15" tire in a P metric you can find. Unfortunately, your truck came stock with 6" wide rims and that limits you to P235/75R15. There are only a few LRR tires listed on Tire Rack in that size. The problem here is "LRR compared to what". I do see some tires that might be OE on something - Goodyear Wrangler SR-A for example - and those are likely LRR even though Tire Rack doesn't list them as such.

[Frank Lee]

IF it's already downshifting a lot then going larger than stock is a mistake.

[roflwaffle]

Originally Posted by Execut1ve

so, in general less flexible sidewalls -> higher rolling resistance? and more flexible sidewalls -> lower rolling resistance?

I suggest taking that data with a grain of salt since it's only for a specific brand/type/year of tire and excludes most 16" inchers and above. Generally speaking the safe route is to pick the tire with the lowest tested Crr as opposed to taking a gamble.

[d0sitmatr]

Originally Posted by mcrews

unfortunately this post has no relavance to the question because: we don't know what the origanial size was. Maybe your slightly smallers now are actually only about 4% LARGER than the factory size? who knows?? but you can't make a blanket statment that smaller is better when in fact it is not.

im not going to get into a pissing contest with you on someone else's thread, but your wrong.

it doesnt matter how much larger or smaller of a diameter the tires is over stock, my post had nothing to do with either of those in comparison to amount of extra energy spent vs rolling resistance.
I thought it was pretty much common sense, and everyone else seemed to get it, so my guess is think more about it before you post.