So when i bought my jeep i had 255 stock tires and ive since upgraded to 275s. I know my speedo is off (70mph really is 77 ive been told.) Recently changed to full synthetic and i am struggling to keep my mpg at 16mpg. Friends tell me i drive like a granny and i really try to drive
Fuel-efficiently. Why am i getting a piss poor average mpg? When upgrading to bigger tires could my mpg be reading wrong?

 

I have a 13 jk 5spd auto btw

 

If you didn't recalibrate the computer for the larger tires, then yes, the computer is off. The computer still thinks you have smaller tires on there.

Best way to figure out mpg is manually.
Fill it up.
Drive and keep track of miles.
Fill up again.
Divide the miles you drove by the gallons you put in the jeep.

This will give you the best reading, but remember, your computer milage is off because of the bigger tires so you have to find another way to keep track of how many miles you drove after the fill up.

 

Your odometer, trip odometer, MPG indicator (not that it's very reliable regardless), and speedometer are all off. If you have an auto, I think your shift points are also off. It's really a good idea to get the Jeep calibrated--the AEV ProCal is probably about the cheapest route to it.

 

Thx guys. Good word.

 

Btw, 275 is tire width. Really effecting speedometer is next number after 275. Also I assume tires are much heavier than stocks? That could negatively impact mpg even if you will recalibrate everything.

 

275 is width? Are you sure? I amm so confused at tire measurements then haha. Just fuckin measure in american ass inches lmao.

 

Yes, I am sure. It is in mm (1 millimeter is about 0.04 of one inch)


Tire code - Wikipedia, the free encyclopedia

 

285/70/R17

First number is tyre width as above: 275mm

The second number is the side wall height as a percentage of the tyre width. As above 70% of the tyre width which would be: 192.5mm high.

The third number is the size rim/wheel/mag you mount your tyre on. Again as above R17 is a standard Rubi 17" rim.

R18 would be an 18" rim and so on and so forth.

LT stands for light truck.

There are heaps of other conventions but these are the easiest to explain.

A tyre 285/70/R17 should measure 33" it will probably also star something about being mounted on a certain rim width only as well.

I hope this helps.

 

Wider tires usually have higher rolling resistance. This will decrease mileage.

Wider tires with the same aspect ratio will be taller and therefore greater in circumference. A taller tire would cause the speedometer, odometer and computer calculated mileage to be falsely low if that information is derived solely from rolling tires.

I have a question though. I have noticed that my speedometer remains exactly correct compared to my in dash GPS, even though my tires have worn (and therefore become shorter). Do nav equipped jeeps get their speed and odometer readings from the GPS?

Regarding the Procal, I'm sure it is an excellent unit. You might also consider the Superchips Flashcal. I like it because it does the same basic modifications, but uses a LCD and buttons instead of dip switches. It's also internet upgradable and the cost is similar. Even better, Superchips now produces a Flashpaq tuner for the 2012-13 Wranglers. It includes all of the Flashcal features and adds engine tuning capabilities. I have a question in to them now regarding a possible discounted upgrade path.

Best regards,
Dave