I put Mamba 20X9 wheels on my truck this year and noticed a decrease in my calculated MPG's. I was getting 18.4-18.6 with the stock 18's and then I was getting low to mid 17's while running the 20's. Now with winter coming I've switched back to the 18's and I'm back up to 18.6 on the last tank. The overall diameter of the tires I run is very close so I'm guessing it might be that the 20's are heavier and wider and there may be more rolling resistance. Any thoughts?
Anyone running factory 20's who can tell me what their average MPG's are?

 

Yup, most likely the added weight and maybe width

 

It is not just the added weight, but that it is rotational mass. That makes a huge difference compared to carrying dead weight in the truck. The added rolling resistance of the wider tires is likely to make a difference. Every little thing adds up.

 

I would consider two things when looking at why there seems to be a difference.

First, did you change tire size such that the OD of the tire is now larger then the original tires. Weight, rolling resistance, rotational mass, etc are all certainly factors. But if you've increased the OD without adjusting the PCM for tire size, your mileage isn't accurate to begin with.

Next, you may be seeing tank to tank mpg variations. Just based on some of the temp swings we've had here in NH in the past month, that could definitely be a factor.

Just my 2 cents. Where in NH are you at?

 

I doubt that it's tank to tank variations - if you read the OP again, the mileage went right back up when the stock wheels went back on.

Another factor involved with rolling resistance is tread compound and aggressiveness. The OEM tires are hard compound, optimized for wear and low rolling resistance. I have lost about 1 mpg simply by putting soft compound ice and snow tires with a more aggressive tread pattern on my stock wheels, same size tire.

 

Tire width makes a big difference. When I pull fertilizer buggies with my F250 they weigh in at about 10k lbs. When I haul hay on my gooseneck flatbed, it weighs even more but pulls easier and gets better fuel economy. The fertilizer buggies have 12.5L x15 (or 16.5 x 15) tires. The gooseneck has 6.75 x 15

As a matter of fact, they have so much rolling resistance that my truck will light up the tires in first gear (due to the resistance of the tires giving the engine time to build boost). The only other way the truck will do that is to power brake it up to about 1800 rpm.

 

STX,
What is the tire pressure in the Mamba wheels?

If you have room to increase pressure try to pump them up to or close to the max rating noted on the tire sidewall it should help get you back some of that lost mph.

 

I'm in Jaffrey and drive to Hatfield MA for work. 58 miles one way. As far as the OD, I didn't measure it but the tires are only a little taller, maybe 1/4 inch, than the stock size so that shouldn't account for almost 1 MPG difference. I don't use the trucks computer for MPG's. It's been reading 20.3 when my calculations put it at 18.6. Both my Subaru Legacys (2005 2.5i and 2008 GT) read high also so I don't put much stock in factory ratings. I think they pump them up a little to make people feel better

 

I was running them at 45 psi and there is a noticable difference in the ride. Much rougher at higher psi which is to be expected. I run the factory 18's at 35 psi and get better MPG's and a smoother ride.