Are my rims and tires causing bad mpg's
 

I have a 99 f150 flareside with the 4.2l manual 4x2
18" lightning rims with Mich Pilot 285/60 R18 tires

255/70 R16 is the stock size

How much is this effecting the gas mileage of my pickup?...I love the look of the rims, and the width of the tires surely adds traction...but is it costing me more then they are worth...

I have a pretty good feeling that they are robbing me of mpg's..

Thanks

http://www.daproparts.com/pics/truck.jpg

Bigger tires make your engine work harder to turn the tire, and they way more, so yes, i think they hurt your mpg's but, how bad is it .5 mpg, or like3 or 4 mpg's???? theyre not much bigger so it should not be that bad

I have 2 guys at my work with the same motors, saying they get 18-20 mpg's, I have a figure at 12-13 mpg's on my truck....i have been all over the motor doing routine stuff, its in great running shape...the mpg's are bugging me...thanks for the quick reply

yeah thats a difference that i would be concerned about.....

If you havn't recalibrated the speedometer/odometer, your mpg calculations are off because you are traveling more miles than your odometer is showing.

I am checking on that now, I read something about it last night and I am hunting around my area for someone to fix that...does it consist of gear changes in the diff, and I think i was told a pinion in the tranny? If its more then that then please let me know, It really bugs me not to know..thanks for the replies...

Just do the math. Determine the original circumference on the stock tires then determine the circumference on the new tires. Take new Circ divided by original circ. Then multiple resulting number by however miles your odometer is reading for a trip.

To make it even simpler, since circumference is just pi x diameter you can do the same math by just dividing the new outer diameter by the original. All of the ODs can be found on Tirerack.

(New Diam/Original Diam) X trip miles on Odometer= correct miles.

Lets say your new tire is 10% larger circumference. That means for every mile your odometer is stating you are really going 1.1 miles. This is an exaggeration but your odometer is tricking you into thinking your mileage is worse than it is.

excellent point.......feel stupid now thanks!!:)

Your soeedo is controled by the computer. It has to be reprogrammed.

is that a dealership visit?

Yes, or buy one of the aftermarket computer tuners like a Superchips or Hypertech.

While his tires are larger than stock and the speed/odo are off, it isn't by the 40% that his fuel mileage are off. It's only 5%.

There are way to many variables that determine fuel mileage, mostly related to driving style. What kind of mileage were you getting on the stock setup?

I purchased this truck with the rims and tires on it...I am trying to find and extra rim so I can have a spare...