random thought on big tires

[ChuckD]

Originally Posted by Longshot270

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

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.

[Longshot270]

Originally Posted by ChuckD

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.

[Jackpine]

Originally Posted by Longshot270

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.

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

[88Racing]

Originally Posted by Longshot270

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.

I thought the same motor was used?

[88Racing]

It shouldn't be the extra weight of the tires being larger.
Mass is the better term for the added size.
Mass deals with more than the term weight does.
http://physics.about.com/od/glossary/g/mass.htm

[ticopowell]

Depends on the motor, the gas V8 is the same for the 150, 250, and 350, but the 150 doesn't get the v10 or the diesel, so it doesnt always have the same motor.
and yeah ration's are kinda interesting here, i much prefer a good ratio

[Longshot270]

Originally Posted by 88Racing

I thought the same motor was used?

It shouldn't be the extra weight of the tires being larger.
Mass is the better term for the added size.
Mass deals with more than the term weight does.
Mass - definition of mass

I was comparing a ranger sized truck to a F250. The largest motor for a ranger is the 4.2 V6, a motor that isn't even an option for the F250. There is probably a ton difference between the two vehicles.

Mass and weight can get complicated, just keep in mind that weight is conditional and mass is not.

I know Jack recently talked about tire size on another thread where he compared the mechanical disadvantage of the larger tires but I dont remember what it was called. I'm sure he's got the formula somewhere.

[cleatus12r]

Just for the sake of discussion......

I ran 49" tall tires on my old 95 F350 PSD. The truck had 4.10:1 gearing in the differential. I could NOT drive the truck 75 MPH in overdrive (5th gear) because the engine simply could not keep momentum. I went from 15 MPG highway with the stock tires (265/75-16) to roughly 5 MPG highway with the 49" tires even though gearing remained the same.

Mathematically speaking, I went from a tire that made 637 rev/mile to a tire that made 413 rev/mile. This is a nearly 35% difference. As far as MPG goes, the difference is nearly the same (33%). The truck would barely get out of it's own way, turned 1200 RPM at 75 MPH in 5th gear, but the mileage decreased significantly.

[majek5]

Originally Posted by cleatus12r

Just for the sake of discussion......

I ran 49" tall tires on my old 95 F350 PSD. The truck had 4.10:1 gearing in the differential. I could NOT drive the truck 75 MPH in overdrive (5th gear) because the engine simply could not keep momentum. I went from 15 MPG highway with the stock tires (265/75-16) to roughly 5 MPG highway with the 49" tires even though gearing remained the same.

Mathematically speaking, I went from a tire that made 637 rev/mile to a tire that made 413 rev/mile. This is a nearly 35% difference. As far as MPG goes, the difference is nearly the same (33%). The truck would barely get out of it's own way, turned 1200 RPM at 75 MPH in 5th gear, but the mileage decreased significantly.

But it looked good gettin the 5 mpg.