Tires & MPG 103 - PowerStrokeNation : Ford Powerstroke Diesel Forum
 
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post #1 of 1 (permalink) Old 08-04-2007, 08:56 PM Thread Starter
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Tires & MPG 103

Tires matter if you are interested in MPG.

At low speeds (under 35 MPH) rolling resistance and dynamic effects dominate the road load equation. Above 40 MPH aero drag becomes boss. We all do a lot of driving <35-40 MPH.

In general, rolling resistance is a function more of width than diameter. The simple fact is that skinny tires have less rolling resistance.

Ribbed, road pattern tires are no used by the big boys by accident. Rib-pattern tiresare more economical and they have found that out through both research and long experience. If you never drive off-road or in the mud/snow you might want to think about rib-pattern tires. Conversely lugged-pattern tires have high rolling resistance. Lug-pattern tires also high higher aero drag, too. If you can hear your tires "roar" down the road that is the sound of diesel fuel being turned into noise.

Everyone who goes to larger-diameter tires reports (if they are honest with themselves) lower MPG. They mess up your resistance in three ways. First, they are wider (that aspect-ration thing) so they have more rolling resistance and more aero resistance. Second they raise the truck up, making the double boundary layer under the truck bigger and imposing more aero drag at any speed. Third, they are bigger "flywheels." When you accelerate or decelerate your truck, you not only must change the energy state of the vehicle itself but the rotational energy of the wheels. We all know about the flywheel behind the engine but each wheel for a pickup is nearly as effective as the heavy diesel flywheel. The parameter for measuring a flywheels ability to absorb and hold energy is rotational moment of inertia. Moment of inertia goes up with the square of the radius, and bigger tires are inevitably heavier, so this parameter is very sensitive to increasing wheel/tire diameter. Every time you pull away from a light, you must not onl acelerate the truck but those huge flywheels as well. It takes energy (fuel burn) to accomplish this, so the smaller the tires, the less energy (fuel burn) needed. This also explains why truck with big tires eat brakes. More energy must be converted to heat & noise to slow the flywheels down - more brake wear.

If you can gear the truck right, smaller tires are better. This is definitely related to gearing and I will post on that later, but all super-high MPG vehicles run on dinky little clown-car tires.

Wheels matter somewhat. Obviously, wheels have to accommodate tires. weight matters - less is better. Lower wheel weight reduces rotational moment of inertia. Ever see a Bonneville racer without Mooneyes? they try their best to minimize aero drag and long racing experience is that the outside of the wheel is a significant source of drag.

Look at the running gear of Bonneville racers - skinny, nearly bald tires, aired up rock-hard, with Mooneyes. They live with fairly large diameter tires because of limitations in gearing availability but their flying mile is after a long run-up so the dynamic effcts of big wheels is minimal for Bonneville racers.

What do I use? I have 235-85x16 E tires in back and 225-75x16s up front. I have not yet done the Mooneye thing yet but it is on my priority list. I am looking for ultra-low (numerically) gearing and if I find it I will drop to 215 tires. I have to drive in snow so I do not use rib pattern tires. I should get a set of "good weather" rib tires and keep my lut tires for winter only.

But then...if I were made of money I would not have a weight problem.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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