The Tires
First off, we want to make it clear that this was not a tire "shoot-out." The two tires we compared were from different companies and designed for different venues. Both were LT-rated but that's where the similarities end. One was a street-oriented all-season tire and the other an all-terrain on the aggressive side in that class. Apples and oranges-and we don't want you to interpret what we learned here as anything more than what it is: a comparison of tread patterns and their relative effects on fuel economy.
For our "aggressive" tread, we used Dick Cepek FC-II radials, a fairly new iteration of the Fun Country line. Our set had about 7,300 miles on them at the time of the tests, which gave them a bit of an advantage. Our all-season tires were Michelin LTX M/S2s with about 2,000 break-in miles on them.
Test Results
We were floored at the difference the tread patterns made. Our 182-mile loop delivered 14.929 mpg on the Cepek FC-IIs and 17.160 on the Michelin LTXs, a difference of 2.231 mpg, or 15 percent. We had been keeping our everyday mileage regularly with the Cepeks and we did the same with the Michelins, though over a shorter period, and the averaged results were 1.75 MPG apart. And the Cepeks were relatively mild trail tires. What would a set of really aggressive tires cost in mpg?
Bear in mind the divergent purposes of the two tires. The Michelins are street-biased all-season tires. The Cepeks are much more trail-oriented. Yeah, the Michelins delivered 2.231 mpg better on the highway, but take them into the goo and the Cepeks will leave them far behind. A street tire will also usually outlast a more aggressive tire. Yep, it's all about choices and compromise.
The point is not to diss one tire but to make you think about tire choices. The street tire would be the most fuel-efficient, with each step to a gnarlier tread costing just a little more. If you drive your rig every day, the questions become:
(a) How much time do you spend on the trail versus the street?
(b) How hardcore are your trips to the outback?
(c) Do you really need mudders day-to-day, or would A-Ts meet your needs? Or an A-S, like the Michelins?
(d) But, like John Wayne said in The Shootist, "When you needs six, load six." If "six" means mudders, there's the best choice.
If you drive 15,000 miles a year, the difference between 15 mpg and 17 mpg is $306 a year (assuming a stable $2.60/gallon price). At that rate, you could soon pay for a set of day-to-day street tires and then pocket the difference in a coupla years-and save your mudders for when you need 'em most.
Test Results & Parameters
Truck: 2005 Ford F-150, 8,200-lb GVWR, 4.10:1 ratios, 5.4L V-8, automatic
Fuel: Shell Regular, 87 octane, RVP- 9
Date of Tests: May 25, 2010
Trip Length, Miles: Dick Cepek, 180.3; Michelin, 181.9
Gallons Used: Cepek, 12.077; Michelin, 10.600
MPG: Cepek, 14.929; Michelin, 17.160
Average Ambient Temp: 88 (F)