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View Full Version : Changing tire size to increase fuel economy & speed.


CharlieBingo
01-04-2006, 01:52 PM
A friend has put tires on his pickup truck 1 inch taller and 1 inch more narrow to inrease fuel mileage and speed . He claims it does both and it's simple physics. I've done this on a mower and it sped it up quite a bit(2 inches taller but same width). Any opinions?

Richard Martin
01-04-2006, 02:25 PM
It's far from simple physics. Bear with me on this. First off putting a different diameter tire on anything changes the effective drive ratio, usually in the wrong direction with taller tires. My neighbor has a '70 Dodoge Challenger with a 3.90 gear ratio. It came equiped from the factory with 25.5 inch tall tires. He put a 28 inch tire on the rear over the summer and then complained that the car no longer had the same "gitty up" that it used to have. I told him to drop back to a smaller tire and he picked up everything he had lost. By changing his rear tires to a taller tire he changed his effective gear ratio to a 3.55. You still with me? How does this effect anything to do with mileage you may ask? Torque is the force that moves weight. Horsepower determines how fast torque can move that weight. The effective gear ratio is a torque multiplier. In effect the lower the gear ratio (lower means higher numerical in gears) the more torque you will have.

So basically if you put a taller tire on a vehicle you will be lowering the amount of available torque to move the vehicle. You will have to press on the gas pedal harder to get the same results. There is a sweet spot where the benefit of increased torque becomes a hinderance to increased fuel mileage due to the engine rpm being higher but at normal highway speeds you shouldn't see it.

I replaced the 3.08 gears in my truck this summer with 3.55s. When pulling my load (about 4,000 pounds) I saw an increase of better then 2 mpg.

Eclipse
01-04-2006, 07:30 PM
...but on the highway you may benefit from a higher effective gear ratio. So on the highway a taller tire may help with MPG's. A thinner tire will help no matter if it is taller or not.