Truck tires
 

I have a choice of replacing my stocks 265-75-R16 tires with 235-85-R16 or 315-75-R16. Heres' the relevant data for BF Goodrich tires

315 68 pounds 601 revolutions per mile 3.42 (effective) axle ratio
265 54 pounds 656 revolutions per mile 3.73 axle ratio
235 47 pounds 654 revolutions per mile 3.72 (effective) axle ratio

The big 315 tires lower the axle ratio, but they will add 56 pounds of rotating mass to the truck; and they increase the frontal area of four tires by about 2 square feet.

The narrow 235 tires lower the axle ratio a fraction (almost no effect); they remove 28 pounds of rotating mass from the truck; and they reduce the frontal area by 1 square foot.

Will the 315's lower axle ratio make up for their added mass and frontal area, or would the 235's tires get better mielage?

I dunno where you live, but aroudn here the thinner tires will help you cut through the snow/slush and corner better where as the wider tire will help you drive over the soft powder with out sinking in as much.

I'm thinking hte 315's will hurt mileage, they are a rather large amount wider than the 235's this will not only hurt frontal area, but disrupt more of the flow under the vehicle. The higher vehicle will have slightly more drag, not to mention i'm guessing the 315's will have more rolling resistance.

What kind of BFG do you plan on running?

I was thinking of the BFG All Terrain T/A KO's, but that tread pattern looks noisy, and could add wind resistance. I'm now considering 235/85 16 Michelin snows or all season tires. Michelin has a great reputation for economy, but BFG has a reputation for traction and off road toughness. <stumped>

Never heard of Douglas tires

I'm about to drive 500 miles in a blizzard tonight, so I went out and got new tries for the F350. I replaced the stock Goodrich Long Trail 265/75-R16 with Michelin LTX M/S 235/75-R16. They're skinnier and lighter, but have nearly the same diameter (Michelins are ~0.1 inch taller)

Although driving in snow tonight is not a fair test, in a few weeks I'll be able to see whether there's a mileage improvement.

I had douglas tires before. There not that bad. Just don't expect a whole lot. I had a set on my sunfire. I did end up with some cupping but the car had 180k on it and the wheel bearings could have needed replacement.

Narrow tires
 

I got 18 mpg on the first tank after installing the 235 tires. The first 200 miles were in 4wd because of snow, so it was hardly a fair test. Still, the truck feels lighter with the smaller tires. I 'll wait for a fair test when warmer weather arrives.