Tires and Fuel Economy

Bigger Tires, New Pressure
Once you install oversized tires, the OE-recommended pressures on the tire placard, or in the manual, are no longer valid because the internal volume of the tire has changed. Here's how you can translate those OE tire pressures to a new tire and give yourself a starting point. Increased volume usually requires less pressure to carry the same weight in the same class of tire because it isn't air pressure holding the tire up, it's air volume.

First, you need to find the Tire and Rim Association load inflation table (LIT) for the size and type of tire you are switching from, and to. If you Google "Load Inflation Tables," you will find many sources of such info. The tire manufacturers will also supply this information directly, and tire shops often have it.

Step 1: Look at the tire placard on your vehicle (or owner's manual) and write down the pressures listed for the stock tires. In our case, they were 50 psi front, 60 psi rear for LT245/70-17 tires, which will be our example. Those are the pressures listed for the truck to carry a rated capacity load, but yours lists both loaded and unloaded specs; either will translate.

Step 2: Look up the old tire on the load inflation table and note the weight each tire is rated to carry at the psi rating you want to translate, in our case 50 and 60 psi.

Step 3: The LIT is broken up into 5psi increments, and your recommended pressure may fall between. Yes, it matters. If that happens, you will want to determine how many pounds of weight that 1 psi will carry. Let's say your pressure is 47 psi and you are going to do the same tire swap we are. The weight rating at 45 psi is 2,010 pounds, and at 50 psi it's 2,205 pounds. Use the following formula:

Subtract 45 from 47 and you have 2 psi. Multiply 39 x 2 = 78. Add 78 pounds (2 psi worth of weight) to the 45psi load rating and you get 2,088 (2,010 + 78 = 2,088). That's the load the tire can take at 47 psi.

Step 4: Look up the new tire on the LIT. Find the pressure needed to hold up the same weight at the rated placard pressure. Our new LT285/70-17 tires can carry 2,105 pounds at 35 psi, so using the formula in Step 3, we know that to support 2,205 pounds, we need 38 psi. In back, our 60 psi load rating dropped to 44 psi with the larger tires.

The Michelin LTX M/S2 is the latest iteration of the LTX M/S line introduced in late 2009. It's an all-season light truck tire that comes in both P-Metric and LT versions. Sizes cover the majority of light trucks and SUVs. LTs come in Load Ranges D and E and, commonly, with a 13.5/32 tread depth. The tread compound is high in silica, which makes it outstanding for use in rain. The tread design also lends itself to good snow and ice performance. The LT versions are part of Michelin's Green-X program, which is their commitment to improve fuel economy by updating their tire designs to be class leaders in the fuel economy department.

Size tested: 285/70R-17
Load range: D
Sidewall: 2-ply polyester
Tread: 2-ply polyester, 3-ply steel
Tread depth (in): 13.5/32
Tread width (in): 8.7
Section width (in): 11.5
Overall diameter (in): 32.8
Max load capacity (lb @ psi): 3,195 @ 65
Revolutions per mile: 635
Speed Rating: R (106 mph)

Dick Cepek's F-C II is an all-terrain light truck tire that debuted in 2008. It was designed to compete in that new aggressive A-T category, which some call the "missing link" between A-Ts and M-Ts. With 18/32 inch of tread depth, it's a little less gnarly than some deeper-tread mudders, but our trail tests showed them to be highly capable. They come in 36 size choices and in load ranges from C to E.

Size tested: 285/70R-17
Load range: D
Sidewall: 2-ply polyester
Tread: 3-ply polyester, 2 ply steel
Tread depth (in): 18/32
Tread width (in): 9.1
Section width (in): 11.4
Overall diameter (in): 32.8
Max load capacity (lb @ psi): 3,195 @ 65
Revolutions per mile: 617 (est.)
Speed Rating: Q (100 mph)

Choosing a Tire for Mileage

• Never forget: Optimized tire pressure will glean the best mpg out of any tire. Testing done in the '80s showed that for tires with standard inflation pressures ranging from 24 to 36 psi, each single drop in psi resulted in a 1.4-percent increase in rolling resistance.
• Radials deliver better mpg than bias-plys on the order of about 30 percent.
• Lower-profile tires have less rolling resistance than higher-profile tires.
• Less tread depth equals less rolling resistance.
• Rolling resistance improves with tire age and wear. Not only does the carcass get more pliable, thus rolling more easily, the tread depth decreases, the compound gets harder, and the tread blocks stiffen up. These all serve to reduce rolling resistance.
• A rough road surface can increase rolling resistance from 5 to 20 percent.
• Among the often-quoted figures is that a 10-percent increase in rolling resistance results in a 1- to 2-percent loss of fuel economy, and vice versa. Thing is, there can be a 60- to 100-percent difference in rolling resistance between the OE tire and the mudder you installed, and maybe more.