QuestionQUESTION: I recently purchased a 2010 Ford F150 4WD Supercab. The OE tires were Goodyear Wrangler SA/R P275/65/18 all season radials. I replaced them with an all-terrain LT tire one size up...Goodyear LT275/70/18 (E Rated). I will seldom if ever carry a full load of people or cargo. I wanted a tire a little larger mainly for looks and E rated tires are all that's available in that size. The placard only list the OE tire size and psi (35) and no optional sizes. What psi do I need to run in the E rated Silent Armors to equal the recommended 35 psi with the OE SA/R's? Also, the Silent Armors are 18 lbs heavier per tire, plus the weight of the extra psi than the OE's. Is that a problem? The truck is a 4.6L 3v with a 3.73 rear end, I average less than 1000 miles per month, and I live on the Gulf Coast where summer daytime temps are in the 90's...if that matters. Thank you in advance for your time.
ANSWER: Jeff,
In order to carry the same load a P275/65R18 does at 35 psi, an LT275/70R18 needs to use 48 psi.
Why, you may ask, would a physically larger tire require MORE inflation pressure?
Because LT tires are built to be able to carry much higher loads than Passenger type tires, even considering the physical dimensions - and in order to do that they not only use higher inflation pressures, but the materials used are stronger, but stiffer. LT tires don't like flexing as much as Passenger car tires - and in order to get the rubber compounds to survive that, they call for less deflection for a given load - and that means more pressure.
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QUESTION: Will the additional weight of the LT tires cause mechanical problems? Also, are the factory machine cast aluminum wheels designed to handle that sort of psi? Thanks.
AnswerJeff,
The weight of LT tires is small compared to the forces generated by road input.
The pressure inside the tire forcing the beads apart is also small compared to the forces from the road surface that impact the tire and are transmitted to the wheel.
