I've had my 20"s on for about two weeks now and I love the look. They are Incubus Shylock 20 x 8.5 up front and 20 x 9.5 out back, running 255/35/20 and 275/35/20 respectively. One thing that I have noticed, though, is that my fuel mileage has decreased 5 MPG! WTF?! Is this normal for you other 20" guys/gals out there or is there something wrong with my car?

Disclaimer: I'm well aware that we don't buy Mustangs for the mpg's but come on, we're feeling it at the pumps like everyone else.

 

takes more energy/tq to get bigger tires rolling.

 

pics please

 

+1

You could do a gear swap to help overcome this. Also a CAI and Tune if not already done.

Also are those tires the exact same diameter as the stockers? If not then that is also part of the problem. You need to recalibrate the speedo to the Revs of the tires your running.

 

That might be a little bit because I live in Oregon and we just got nailed tonight with a snowstorm so the car's covered in mud and whatnot. Don't want the first pics I ever post to look like *** They look like the Foose Nitrous II's. I loved the design and the fact that nobody had them.

 

Did the CAI and tune prior to the rims. I picked up 2 mpg with the CAI/Tune, so I was that much more disappointed when I lost so much with the rims.

Recalibrated the speedo for the revs/mile, but I was a little wary of how well that worked since you can only change it with the XCal2 in increments of 5. Had to round it off.

A gear swap seems like it would be a wash, wouldn't it? Since you already lose mpg's with a gear swap?

Do you think that some suspension mods would change anything? Currently I'm running the stock suspension, but I'm putting together a suspension kit to do next.

 

No because your gaining the mechanical advantage back. Most do not see a MPG drop when doing a gear swap anyway and many see and increase.

 

It's a muscle car, not some prissy-boy porsche! Let's see those grunge pics!

 

You could pick up some mpg with Under drive pullies from steeda. Those and the CAI almost brought me back even when I put on the 20's.

 

so its normal to lose that much mpg when going with 20's????

 

I didn't lose any with mine.
Maybe the cool factor leads to lead foot....

 

I think its your speedo calibration. Using a Xcal II on my wifes Explorer when we changed to different wheels, I could never get the speedo calibrated accurately. It always seems off. Chances are the 20's are probably in the 28" height range compared to the 26" stock range. In theory, this would make your rear gears taller, thus lowering cruising rpm speed. So, that would save gas. Ask your tuner if he can recalibrate in the tune and email it to you.

 

see if you can borrow someones gsp unit to see how far off your speedo is off

 

I bought my 07 with 20" on it but I go the same mpg as my 01 with 17". My mpg did go down a little with the SC install but that because of my foot getting in to the boost 99% of the time ...

 

Incubus Shylock

 

i put 20's on my car and didnt really notice a change in gas mileage. if there is any i didnt really notice it. i am getting about 23 mpg on 20's and with 4.30 gears.

 

I don't pay attention to my gas mileage so I have no clue

 

I have an 07 with 295/30/20 on the rear with a manual trans and 3.55's out back. These tires are a little shorter then the stock ones and I am still getting about 21 mpg. The motor is all stock for now and I do have a lead foot. Also I checked my speedo against my gps in the car and a hand held one and at 60 I am actually doing about 58.