random thought on big tires

[ticopowell]

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:

[ChuckD]

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

[ticopowell]

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

[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.

[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/conversation-pit/3301-finding-sweet-spot-mileage.html

[Jackpine]

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

[ChuckD]

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/conversation-pit/3301-finding-sweet-spot-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]

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:

[Jackpine]

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

[88Racing]

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 thought the same motor was used?:shrug:

[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 :hehe:

[Longshot270]

I thought the same motor was used?:shrug:

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 (http://physics.about.com/od/glossary/g/mass.htm)

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. :cheesy smile:

[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]

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.

[cleatus12r]

Yeah it did. You wouldn't believe the amount of attention that thing gave me......I even got "flashed" twice with the wife in there with me.

Those were the days.

[AZFXFOUR]

When i went to 35s on my f150 I DID notice slightly better mileage on the highway because the engine was not working as hard to MAINTAIN the speed. But I also suffered worse mileage around town or stop and go because the engine had to over come the mass and increased rolling resistance of the larger tires. So you pretty much break even. Now if you ran a taller very skinny tire you might get a little better result.

[matt7.350]

Ok so what im getting from this is 49" tires = :D