Folks,
Im a Jeep newbie so please cut me some slack. Im looking to learn something here. One of the common things I see is how people are saying how they lose MPG's when they go from stock wheels and tires to new wheels and 35in tires. So Ive done a little research and found out the the 2012 Stock Rubicon comes with a 24lbs wheel and a 48lbs tire. Total weight 72 lbs.

I just upgraded, but have yet to install, new wheels (Mamba M5's) and 35 in tires (Duratrac 325/65/18). The new configuration weighs 98lbs. Wheels are 34 lbs, and tires are 64lbs total weight 98lbs.

This comes out to a 26 pound difference between stock versus new configuration. Multiply that by 5 and the added weight is now 130 lbs.

What doesnt make sense to me is how 130 extra pounds results in a 3-5 MPG loss? Logically, this would seem to indicate that if I gained 130lbs on my personal self, and kept the jeep stock that the outcome should be the same. I doubt however this to be true. I also doubt that the more rotund Jeep owners, would have to install Axel Gussets because of their personal weight. This would suggest the stock Dana 44 axels are really weak to begin with wouldnt it?

So my question is, is it really a matter of size and not added weight, which results in the MPG loss and need to install Gussets?
If tire size is what is causing the loss in fuel consumption and need for gussets, why? It just makes no sense how height alone would impact MPG's and axel strength.

 

not so much the weight, but the leverage the increased tire height. i would even gusset a stock jeep. the c's are weak part of the dana 30 and 44
better explanation on increaces tire size and fuel
http://www.ehow.com/about_5571630_do...lower-mpg.html

 

When you spin all that weight it takes a lot more energy to spin it at the same rate as a smaller tire. Also due to the larger circumference there are more turns of the engine for every turn of the tire resulting in reduced torque, and thus power to effectively spin that tire.

 

As far as the axle strength goes, with larger heavier tires they are getting further and further away, just think about when you try and snap something in half you don't place both hands in the middle you grab the ends of it and it's a lot easier to bend/break

 

Its totally different types of weight!! Sprung vs unsprung weight. sprung weight is anything that rotates ie crank, drive shaft, wheels and tires, to name a few. it takes power to rotate things. Ive read that 1lb of sprung weight is roughly equal to 10lbs of unsprung weight. Thats why you see a lot of super light wheels on sports cars, people spending a ton of money to shave even the smallest amount of weight from any rotational mass.

 

To begin, it's not so much the added weight that drops your MPG, it's the added rotational force and rotational mass. You are installing a larger diameter tire than was it was designed for and the rotational force is moved outward. This causes your axle to apply more work to turn the larger tire. While weight does play a role, it is not the only reason MPG is affected. Take a piece of rope about 10' long and tie a 5 lb weight to it. Hold the rope two feet away from the weight and start spinning it. Doesn't take much effort. Now move out to 5' away. Takes a little more to spin it at the same speed. Now hold it at the end of the rope. Takes a lot more work to spin the exact amount of weight at the same speed. Take the added rotational force and use the stock gears and transmission ratios, which transfer the power or work to the wheels, power is lost and you have more energy to over come to produce the same amount of work. That is why you re gear the axles when installing larger tires. Re gearing will give you a better ratio of power needed to over come the added weight and rotational mass of larger tires.

As for gussets, Yes the axle c's are weak to begin with. Mine started to bend with stock tires. I have a Rubicon so I have a D44. The D44 is essentially the same as a D30 just different housing. As for the reason needed them, it's more about physics than weight. Again goes back to the rotational force and with the c's, you also have to take in moments about the axle and forces being applied to it.

Sorry about the way i answered, but being and Engineer by trade, it's pretty easy for me to see the reasoning behind it.

 

So its more about loss of Aerodynamics due to the increased height than weight. This makes sense. It doesnt really address the need for gussets however unless the stock axels are not that strong as you have indicated.

 

Weight mainly comes into affect when regarding the stock rear tire carrier. Many people have had their tailgates "warp" with heavier tire/wheel combos.

As to tire size and mpg, there's a good explanation up there.

 

No apologies necessary. This makes sense even more than Earodynamics does because I can see how the increase in rotational force would impact both MPG's and the need for Gussets. Aerodymamics just explains the loss of MPG's. So its settled then, I need to buy Gussets and probably sleeves. Its a reality I cant talk my way out of. :-)

 

well here is a comparasion from the stock c's and a pr44.

the stock c's are weak. if you dont think you need them thats fine. but if they do bend, its gonna cost more in the long run to fix than the 60 bucks for quality gussets.