trust me your speedometer is why you think your getting ****ty gas mileage. its more off than you think. just go get it fixed and you will like the numbers your actually getting

 

That's marginal. His original tire size was 30". Now it's 31.46" high. That's only a 5% increase in tire diameter and circumference, so he should only be seeing a 5% variance in his fuel economy (approx. 13 miles per 1/2 tank).

Here's where the "rub" is in fuel economy. Nyuck, Nyuck:

1. His truck is now almost 1" taller (less wind resistance/poor aerodynamics).
2. His tire is now 1" wider (more friction)
3. His tires are now 20-30 lbs heavier a piece (>120 lbs total).
4. His tires being bigger have lowered his overall gear ratio by 5%, meaning shifts are lower

Put all that together and he really is losing around 5 miles per gallon, or about 60 miles per 1/2 tank. Adjust for speedometer, that's about 66 miles.

Not sure where the other 40 are going, but I would assume it could have something to do with gas. Or the tank is a different size (30 gallon instead of 26 gallon). If you're using 15 gallons per 1/2 tank that's 75 gallons, or 83 gallons unadjusted.

 

Just an update. Had the speedo fixed a hile back. Probably used 4-5 tanks since. Theres basically been no change. Gas mileage still sucks compared to stock. Guess the larger tires/wheels had a much bigger effect then I thought/was lead to believe it would.

 

It's been several years since my truck was stock... BUT. Out the door when I got the truck I had the OEM 235/70R17 Hankooks. The truck got 15.9 mpg stock, out the door.

I have since added plenty of mods, not the least of which is...
35x12.50/17 Hercules Trail Digger MTs mounted on 17x9 Pro Comp 1089s.
Brush guard with off road lights (more wind resistance)
102" whip CB Antenna (more resistance).

The speedo has not been corrected. I am getting 320miles on the trip odometer between needing to fill up. That figures out to 12.8 MPG.

Now according to http://www.4lo.com/4LoCalc.htm and knowing my OEM tires were 30" dia, and the new ones are 35" dia, My mileage reading is 16.6% off.

So 320 miles *.166=53.12 additional actual miles covered. So lets round down and say I covered 370 miles out of the 25 gallon filling. This calculates to 14.8mpg. That is my current mileage calculation, but mind you, I have not cleaned my K&N in at least 30K miles, and have been through hurricanes and all kinds of dirt, filth, and gack in that time... I bet if I serviced the filter, and threw a couple of cans of Seafoam at the truck my mileage would go up...

 

My '07 Supercrew 5.4 gets 425-450 on a tank (30 gal. tank).

 

Believe my tank is 25 gallon. I'm at 178 miles and already past a half tank. Before the bigger tires/wheels I would usually be around 280 miles(give or take ) at the point im at now.

 

I think I have the 27 gallon tank, when I fill up it takes about 26 gallons and I usually have 430 miles on the tank.

 

You nailed it right on.

 

i have 30 gal tank and usually fill up around E and i've got about 350-370 mi mix of highway/city running 305/55/20

 

I replaced my fairly wore BFG's with the same size Goodyear Wrangler's and my mileage (especially highway) has gone done considerably... I only got about 14mpg driving 70mph on the interstate on a three hour trip, should be 17-18 easy. The ride is much rougher and louder too, anybody else have these issues with the Wrangler's?

 

Badyears suck. Of course people have these issues, among other things like failing to balance, wobble, poor handling, crappy wear, etc.