BlueBruin

Does anybody know with some degree of certainty what effect tire weight and wheel weight have on MPG's and ability of keep speed on the highway (power while towning especially?)

Would say Toyo MT's or Nitto Mud Grapplers at 90lbs plus a steel wheel have a noticable difference compared to say BFG's at 70lbs and aluminum wheels?

Am I worrying about nothing? I just don't want to shell out 10 grand for lift, wheels, tires, and gears only to find out I can't go over 55 mph on the highway with my trailer in the mountains here in Colorado.

Any physics majors out there?

bamabrugge

Don't have any "scientific" knowledge on the subject, but I just but on 285/75r/16 Duratracs on my JK and I am pleased with the power.....I thought it would be way more sluggish. Plus I run steel wheels. No lift, 3500 miles on the Jeep, 3.21 gears and no BIG loss of power IMHO with 33's.

hammontroy

I can't speak to the weight side of it, but I have first hand experience with the effects of increased diameter. If you go up in size without changing your gears, you will definitely feel it. On my last Jeep, I ran tires that were too big for my gearing. Oh boy! The best thing I can say for it is that it was easy not to speed. I basically had cruise control consisting of putting the pedal on the floor the whole time. There was no way to speed on the highway unless I was going downhill.

On my current Jeep, I only have the 3.21s and with my bumpers and rack, I have all but lost 6th gear on the freeway. Then again, I am in Idaho and our posted speedlimit is 75. I can keep up in 5th but in 6th, my speed drops below 65.

I should re-gear but I probably wont.

AZJeeper

I don't know how much heavier wheels will impact MPGs, but here are some semi-random thoughts.

• Heavier wheels/tires means more unsprung weight. There's no hard "rule" about equivalent sprung weight, but some web sites have a multiplier (as much as 10) that tries to make an equivalency argument. That is 5 lbs on the tires (times 4) impacts the vehicle as much as 200 lbs of cargo. Not sure about the "science", but there is definitely a negative impact.
• More weight means more energy (torque) to get them moving. More energy = less MPG, especially in stop-and-go traffic.
• Conversely, once moving they are harder to slow down. That means more wear on the brakes.
• On the positive side, high-speed bumps have less effect on the vehicle as the tire's inertia is harder to overcome vertically too.

That's why I went with the lightest compromise I could find with 35" tires - BFG KM2s and AEV Pintlers.

style_front

8 lbs = 1hp



This is a sort of general rule of thumb I remember being applied when I was in the performance field. Every 8 lbs of rotating mass you could remove from your driveline, wheels, more importantly tires, driveshafts, lightened fly wheels, etc........ could untie 1hp.

Red

Let me try to build on what style_front said...

As I remember learning it, 1 lb of unsprung weight corresponds to about 10 lbs of sprung weight. Reducing sprung weight by 10 lbs or unsprung by 1 lb, "frees up" about 1 hp. Where sprung rotating mass is concerned, like flywheels, you are looking at about an 8:1 ratio.

So going with that, if you increase the weight of your tires by about 15 lbs, then it will feel as though you lost 15 hp. That being said, it really isn't quite as easy as saying that you need 15 extra hp to make up for your 15 lbs heavier tires. There are a lot of things that also come into play, i.e. gear ratio, wind resistance, etc.

Will you feel the difference from lost or added weight, yes, especially on bigger tires, like 35s and 37s. But, you have to remember that what you are feeling in your butt dyno is not the perceived change in hp, but the effectiveness of of the engine's torque pushing you forward, which is why this perceived gain or loss of hp can be overcome by changing gears.

I guess what you can say with certainty is that bigger tires will reduce your gas mileage and ability to maintain speed. The ability to maintain speed can be compensated with gears, but your milage is not going to get better, but it is not all due to tire weight. You can also say that reducing vehicle weight will result in a faster vehicle, but exactly how much faster the car will go without the A/C pump, good luck trying to find a real formula for that. You have to put that car on the track, but most people aren't running 1/4 miles in their Jeeps, so I guess that really isn't that important. You can also say that unsprung weight and rotating mass suck up more power than sprung weight.

I hope I got that somewhat right and didn't make it more confusing. If I got something wrong, someone please correct me, it has been a few years since I found myself shaving bits and pieces off of little Hondas and drilling holes into body panels in an effort to reduce 1/4 mile times, and, well, the mind is the first to go.

JPop

These 10 to 1 numbers are real overstated in terms of unsprung weight. While unsprung weight is really import in a lot of motor sports with lots of shock and spring flexing, this isn't the same as when your JK is rolling down the highway. Anything that moves on a vehicle that isn't directly related to going forward robs power, although not in the kind of numbers being thrown around. If you were a rally racer running on dirt roads, the 10 to 1 numbers could come into play. However in normal conditions on paved roads those numbers are less than 2 to 1.

Ironhead

I just put on a set of toyo open country mt's 285 75 16's on an aluminum rim and there is a difference but not as much as I thought there would be. I still have full use of all my gears and I have the 3.73's, even fuel economy and highway driving is fine

style_front

Okay, lets clarify some definitions here because some lines have been blurred. Were calling rotational mass unsprung weight when they are actually two different things although at times the same part. For instance wheels and tires are considered unsprung weight and are also a part of the rotating mass.

Rotational Mass: Everything that spins from your crank shaft to your cam shafts, axles, wheels, tires, gears, transmission, flywheel, axle shafts, brake discs, SIMPLY PUT ANY THING THAT ROTATES OFF OF THE ENGINES POWER. Just as important as considering the weight of all these things is to consider the moment of inertia.....


Moment of Inertia: The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. For example, consider two discs, A and B, made of the same material and of equal mass. Disc A is larger in diameter but thinner than B. It requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed farther from its axis of rotation: mass that is farther out from that axis must, for a given angular velocity, move more quickly than mass closer in. In this case, disc A has a larger moment of inertia than disc B.

And lastly

Unsprung Weight: In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels and other components directly connected to them, rather than supported by the suspension. (The mass of the body and other components supported by the suspension is the sprung mass.) Unsprung weight includes the mass of components such as the wheel axles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.


More in depth definitions of each can be found at Wiki if one would like to do more research.

JPop

That's all well and good, what's needed here is some math.