When I bought my '07 NBS Silverado I got about 17.5-18.00mpg combined. It had factory 17" wheels when I bought it. I changed to factory 20" wheels, and my gas mileage has decreased to about 13.0-13.5mpg. Does this sound right? I know they weigh more, but that seems like a big decrease. Do I need to have it programmed, or buy a programmer due to the wheel change?

 

It is possible that your odometer is now "off" and that you are actually driving more miles (and getting more per gallon) than indicated.

Go down the local highway and check your odometer (and speedometer) between those measured mile markers. You should be able to go sixty miles per hour and cover one mile in EXACTLY one minute - if everything is accurate.

Good luck!

 

What a coincidence, i just received a phone call from a friend that is offering me a FREE (I Love Free) set of 20" avalanche wheels for my 08 nbs silverado and I was just coming on line to look into this exact question you are asking.

Here is the result of what I looked up.

You WILL need a programmer if you go from the factory sized 17" to the factory size 20" tires. What I mean is I currently have 245/70R17 factory setup and I want to go to the factory size 275/55R20.

The diameter is off and if I was going 60MPH in the 17s, I will be going 62.64 MPH with the new 20s when the speedometer is reading 60MPH. Of course, this is a percentage (approximately 4.61% off) and will create a larger change the faster you go.

So to answer your question directly ctopher, YES, your fuel mileage probably did go down a little because you NOW have a greater rolling resistance (larger width of the tire). However, your computer is not accurate because it thinks you are going a shorter distance than you are ACTUALLY going. If you get a programmer, it will fix the speedometer and subsequently your mpg computer will come back in line. But you mileage WILL probably be lower because of the before mentioned rolling resistance.

Hope this helps.

Eric