random thought on big tires

[cleatus12r]

If the engine has to work too hard to maintain velocity, then your fuel economy goes right in the toilet.

I had a 1984 Bronco II with the 2.8L V6 and manual 5-speed transmission. Because of trouble with the engine control system, I put in a normal carburetor and a duraspark distributor and ignition module. The thing was a gutless turd no matter what, but it would consistently get 2 MPG better in 4th gear than 5th gear....and all of the miles were highway miles at 70-75 MPH. No in-town driving.

It's all about engine efficiency range and load.

[Jackpine]

Right! It's a bit of a "balancing act". And, ticopowell, it's gear "ratio", not "ration" which is something you might eat.

IF the rolling resistance of the big feet and the aerodynamic drag on the vehicle don't change, then, once you get the truck to cruise speeds, because the engine is operating at lower RPM, you COULD get better gas mileage. BUT: The bigger feet weigh more, they offer more frontal area drag, lifting the vehicle may add to the interference drag (turbulence between the underside of the truck and the ground) and, they will almost certainly have more "rolling resistance", due to a somewhat larger footprint. So, even though you ARE operating at a lower RPM, you may have a slightly more open throttle, putting more gas into the cylinders just to keep it there. (That's the "engine load" effect that cleatus12r was talking about.)

And, due to the slightly increased weight and the significant hit on mechanical advantage, the load on the engine is increased appreciably (maybe 8-14%, depending on the size of the tires) to get the vehicle from a standstill to cruise speed. This load has to be overcome somehow and it comes from more fuel needed to get to cruise speed.

When automakers design vehicles, they try to marry a wheel and tire size with a gear ratio to achieve the best combination of power and fuel efficiency. But, it's always a compromise. With Government restrictions on fleet mileage, you can bet the pendulum swings pretty strongly in favor of efficiency. This compromise can be calculated with pretty good accuracy, using linear programming principles. Any time you change either the tire size or the gear ratio, you might find you don't really like the result.

So, when you put big feet on your truck, in my opinion, it's for one of three reasons: You like the look, or, you want to increase the ground clearance slightly, or, you want to improve off-road, "muddy" traction slightly.

- Jack