random thought on big tires
 

So from what I have read, and what I understand about putting on oversized tires on a truck, it essentially changes the gear ration, and makes it so less power gets to the ground. I also understand that if you have a lower gear ration (3.55 being lower than 3.73), you get better gas mileage in the truck because the engine works less at higher speeds. so since lower gears add gas mileage, and big tires make the effective gear ratio lower, if we ignore other factors like aerodynamics and heavier and harder to turn tires, you should get better gas mileage? is this right? or do the weight and lack of power to the ground make enough of a difference to negate any gear ratio improvement? :shrug:

Negate. Also you got to look at the height of the vehicle. Lower vehicles get better milage.

yeah i forgot to add vehicle height in, and that is what I guessed, but it was a good thought right? lol

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.

It is a good theory, engine turns less = better mileage, but physics works against you. What is one thing that many of the most fuel efficient vehicles have in common? Ex. trains and economy cars. They have small wheel sizes but make up for it through gearing. It is much easier to spin a gear than a tall tire.

For the ratio, that is also not an absolute rule. The peak gearing for mileage depends on the vehicle's weight. A 3.55 gear ratio may get great mileage on a ford ranger but it will get worse mileage on a F250 of the tire sizes are the same. Like you said, the lower ratio delivers less power to the ground. Take out too much power and you have to make the engine work harder to move the truck attached to it.


Also you might want to look at the pictures I have included in this thread
http://forum.gopowerhungry.com/conve...t-mileage.html

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

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

I have 3:55s in my F350 and can get 20 mpg. My OD does not shift till 47 mph, if I keep the rpms between 1750-1950 I get my best.

True, but the F350 also has a significantly larger/more powerful motor. I was trying to compare just weight and forgot to tie engine size into the equation. :embarrassed:

I don't think you "forgot" exactly, but it's like comparing "apples and oranges". A more powerful engine is going to be able to operate at a lower "load" at a specific gear ratio to generate the same speed. I'll call this the "Lance Armstrong effect".

Let's put Lance and me on identical bicycles, and let's lock the gearing at the "highest" point, so we are each turning our cranks at the slowest RPM to maintain a set speed. So, Lance motors along, has enough energy left over to wave at the crowd, carry on a discussion with somebody on his cell phone and maybe even carry someone on the handlebars. Me, the sweat's pouring down. I'm about to have cardiac arrest and I look stupid out there trying to compete with Lance.

The difference is in the native "power" of our two "engines". His is suited for a lower gear ratio than mine. He is seeing a lower "load demand".

The engine design, vehicle weight, transmission and intended use are also factors in choosing a TS and GR. As I said earlier, it's an optimization problem that can be solved through linear programming techniques. I imagine the engineers at Ford do this very well.

- Jack

I thought the same motor was used?:shrug: