Weight or Diameter? Biggest MPG killer

[foo.c]

Weight: a heavier tire will take a bit more energy to get it rolling, but once rolling it’ll have more inertia, and therefore a greater tendency to stay rolling. 10-15 lbs. per tire is an insignificant fraction of the weight of the truck, so it won’t noticeably effect fuel consumption. More unsprung weight will effect handling performance, but we’re talking a Jeep, not some hyper-performance sedan.

Diameter: a larger tire will effectively lower your final drive ratio (think 4.10 to 3.73,) and while it will tend to reduce performance, it’ll also tend to improve economy. More distance traveled per tire revolution.

Kevin

It's not as simple as adding 40-60 lbs of unsprung weight. Adding weight in the driveline adds resistance that takes extra engine torque (fuel) to move and in this case braking force to stop. It's a much bigger penalty than say having a heavier axle housing. (Trying to keep it Jeep focused.)

You also can't rely on the number of revs per mile as an absolute indicator for fuel consumption. It takes X amount of force to keep your truck moving regardless of gearing. If you move it (numercially) lower, you lower the engine rpm but you increase the load on engine. If you move it up, you decrease the load and increase the rpm. A loaded engine at low rpm can burn more fuel than an unloaded one at high rpm. There's probably some equations to figure out what's optimal in a given situation.

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[Jeepin' John]

This isn't my field, but if i recall correctly, the mass (roughly weight) is a linear difference, but your radius is an exponential difference. So i would initially think size

 

[foo.c]

This isn't my field, but if i recall correctly, the mass (roughly weight) is a linear difference, but your radius is an exponential difference. So i would initially think size

You jogged a memory ...

I = mr^2

https://en.wikipedia.org/wiki/Moment_of_inertia

 

[Blade1668]

No one considered rolling resistance - the friction of a wider tire on the pavement vs. the standard width, the more aggressive tread being harder to roll, etc.
I see where weight is considered - and it's said that a heavier tire will be harder to get rolling but not so bad to keep rolling...... well, unless you always drive with cruise on and it never changes speed or never goes up an incline (the engine must lift the truck up that hill - tires and all) yes, the weight does matter. The mass, thus inertia, only doesn't matter if you are flying a perfectly straight line and perfectly steady speed.
Otherwise yes, the mass matters for multiple reasons.
I moved from H/T to A/T tires of the same diameter, a bit wider, and saw a loss in MPG - and the dealer that sold me the tires "warned me" although I told him I was aware - I wanted better tires for our nasty weather. I knew it would drop some - and it did - SAME diameter, same weight!

A interesting side note I installed a set of OEM HT on OEM rims same as the what came on mine replacing AT and I've noticed a slight drop of 1-2 mpg not what I expected.

 

[Chocolyle]

I have been stuck at about 16MPG since I bought the truck with the stock M/T tires and manual transmission. Planning on going with 37's once money is saved up. Last thing that crossed my mind when buying the Gladiator was MPG. Was doing some research though and I have found a few options to get better MPG:

 

[AmishMike]

I have noticed a huge difference in mpg at highway speeds.
65 mpg ~ 19-22 mpg
70 mph ~ 18-19 mpg
75 mph ~ 16-18 mpg
80 mph ~ 14-15 mpg
85 mph ~ 11-13 mpg
I find this to be mostly from aerodynamics. Drafting greatly increases these numbers, while my RTT only drops them 0.5-1 mpg.
That being said, (without recalibration) if you go to a bigger tire your over the road speed will increase compared to your speedometer reading and quickly jump you into the next speed bracket and decrease your mpg.
If you are properly calibrated, taller tires will give you a higher gear set and decrease revs/mile. That normally would increase your mpg. BUT as we have seem, due to a lean calibration the Jeep does not make enough power at highway speeds to make good use of the overdrive that it already has.
Arguments can also be made that a same size but lighter weight tire has more sidewall flex/greater rolling resistance and actually uses more energy than it's stiffer heavier counterpart. When I go for an offroad tire, I want it to be thicker than a highway radial and take more abuse resulting in a heavier tire.
Then you get into tread design and compound...........

 

[ShadowsPapa]

I have noticed a huge difference in mpg at highway speeds.
65 mpg ~ 19-22 mpg
70 mph ~ 18-19 mpg
75 mph ~ 16-18 mpg
80 mph ~ 14-15 mpg
85 mph ~ 11-13 mpg
I find this to be mostly from aerodynamics. Drafting greatly increases these numbers, while my RTT only drops them 0.5-1 mpg.
That being said, (without recalibration) if you go to a bigger tire your over the road speed will increase compared to your speedometer reading and quickly jump you into the next speed bracket and decrease your mpg.
If you are properly calibrated, taller tires will give you a higher gear set and decrease revs/mile. That normally would increase your mpg. BUT as we have seem, due to a lean calibration the Jeep does not make enough power at highway speeds to make good use of the overdrive that it already has.
Arguments can also be made that a same size but lighter weight tire has more sidewall flex/greater rolling resistance and actually uses more energy than it's stiffer heavier counterpart. When I go for an offroad tire, I want it to be thicker than a highway radial and take more abuse resulting in a heavier tire.
Then you get into tread design and compound...........

It's been realized since the time of Newton that the "wind resistance" doesn't increase in a straight line compared to speed. Once you hit roughly 60, depending on shape, and other factors, wind resistance ramp gets much steeper. For every mph over ~60 mph, wind resistance jumps even more.

No, decreasing "revs" vs. mph doesn't necessarily increase mpg - it ain't that simple. Move any engine out of it's more efficient power band and mpg drops, not increases. An engine may actually give better MPG if the rpm is increases slightly.
It's not the Jeep "lean calibration" - it's true of any engine. I could really relate when I had my diesel tractors. I found running them at higher RPM in a lower gear actually improved the number of acres I could work as far as gallons used. I'd found the same for gas engines. If I pop my SX4 into 5th gear and drop the RPM too low, my MPG suffers compared to leaving it in 4th gear where it's making more power.

I suppose I could dig into my own college texts but hope not to as far as mpg vs. rpm, etc.
Another comparison for mpg vs. rpm vs. engine size - a given car with a V8 may give BETTER MPG for the same speeds driven because of the fuel the smaller engine must consume to make the same hp vs. a larger engine working in it's happy power band more efficiently.
It's not all about "lean tune" for Jeep - it's about where it makes power, where it's most efficient - pumping efficiency (and these can go way over 100% efficiency - dynamic compression ratio can really jump based on VVT and VVL these things have programmed in them)
If an engine is lugging or close to lugging - it's not going to make as much power vs the fuel being consumed. Higher RPM, similar fuel consumption yet more power produced means raising RPM a bit could improve mpg.

ANY thing going through the air - bicycle, train, human, car, truck, the brick shape of a JT, will see wind resistance jump quickly at speeds of ~60 and above. Even my little SX4 or my long sleek Javelin with a pointed nose or the Camaros my wife owned over the years.
But look at the drag coefficient of a cube - basically a Wrangler or JT -

drag equation that represents the drag force F_{D} experienced by a body due to the movement through the air (or any other fluid). It is composed of the density of the fluid \rho (which we can’t change), the reference area A (the frontal area in the case of cars or motorbikes), the drag coefficient C_{D} (defined by the body’s shape), and the flow velocity u (relative to the object)

 

[RCKLNDR]

40s on beadlocks, about 135lbs per wheel.

Dang, I may as well go 40s since my 35s+wheel is already 122lbs… lol

 

[JD101]

I'm running 38s , 5.13s , fully corrected via tazer . Basically at sea level with trash winter fuel and 20deg temps .

Last two tanks : 50/50 city 45mph /hwy 70mph 13mpg ave .

If I behave on the hwy at 70 I've seen 14.8+15.

 

[mbush91]

40s on beadlocks, about 135lbs per wheel.

Any other mods to get that mileage? I'm still getting 16mpg on my stock Willys

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