Could someone please explain why you would lose MPG's when installing larger diameter tires on a vehicle.

The way I see it a larger wheel would increase distance travelled for each revolution of the axle shaft, resulting in more distance travel per rev.

I could understand that the speedo would read lower than actual, which would result in the appearance of less distance travelled and lower MPG's per fuel consumed, but, the actual's would be greater, wouldn't they?

 

Weight is one reason.

 

It also takes more power to initially get the larger diameter wheel assembly moving.

 

So the added 20#'s or so would make that big of a difference, I better lose some weight then, I could be saving alot on fuel if I weighed say 30#'s less and is there really 5#'s per tire difference? I don't know that is why I am asking.

 

Wouldn't this be only true in say mud, but on flat pavement wouldn't the longer lever arm from the centerline of the axle to the OD of the tire actually reduce the effort required to move the truck?

 

The extra weight of the tire accounts for a lot more energy loss than if you were to throw that same extra weight in the bed. It's not adding the weight to the vehicle that sucks up the gas, it's getting that weight rotating.

In addition to the added weight, the larger diameter tires changes the dynamics of the torque. Think of the tires as the front sprocket of a mountain bike. The smaller the front sprocket you use, the easier it is to get going. Try taking off using the big sprocket and you have to exert a lot of energy.

Plus, larger diameter tires are usually wider as well, which means more rolling resistance. All these factors eat gas.

 

if you are doing highway most of the time you will get better or the same mileage, but stop and go city traffic will screw you.

 

It all depends on the tire. My Terra Grapplers are about 33's and my Trail Grapplers are 35's, but the trails have a LOT more rubber in them. Just guessing by throwing them around, I'd say the Trails way a good 30 lbs or so more, each.

However, with as much weight as those Trails are over the Terras, the Terra's only get about .15 mpg better on average. So weight is not as much of a factor as size. Terras are (285/75R16) 32.8" x 11.2" vs the Trails at 35" x 12.5" and almost the exact same gas mileage.

Compare that to the stock tires (255/70R16) 30" X 10" which were getting at least 2 mpg better.

I averaged 14 mpg with stock tires and stock programming. With the oversized tires stated above, I was getting 11.5 mpg. With a custom tune, I am now getting 12 mpg.

 

Not if you have wide tires. The added rolling resistance is always eating away at the gas mileage.

 

The question I have is, when looking at MPG, you are getting 11.5 with 35" tires are you correcting the miles travelled indicated? as a 35" tire has a 15% larger circumference than a 30" tire and the miles travelled would be 15% greater than indicated by the odometer.