The odometer is factory set for 29-30" tall tires. You're running 35" tall tires.....18% taller. If you're calculating you get 10-11 mpg without taking the taller tires into account, then your mpg is actually around 13.

More importantly, you need to watch your speed on the road. If your speedo says 60, you are probably doing 71 mph.

Tire/wheel combined weight is a big factor on economy as MitchF150 stated above. I weighed my factory wheel/tire and it was ~60lbs. My new wheels/tires weighed in at 82lbs and I dropped about 1.5 mpg due to the weight alone.

 

According to a software program I have, it should be the opposite. The factory tires on my Navigator are 30.5" and with a 35" tire the speedometer would only show 61 mph, where as with factory tires it would be 70mph. This would, however, increase your mileage, because the odometer would be registering less miles than actual.

 

Ooops...I got it bassackwards. Thanks Navi Man; I stand corrected!

 

Thank you Again for the info, My friend has 410 gears. So I was wondering if there is a guide as to what gears I should get for the size tires I have. My truck has the stock gears, so I am not sure which ones those are.

 

Remember your odometer is most likely off with the tires unless it has a reprogrammed computer/chip or gear swap. I had a 2001 V6 4x4 (sorry, have a 99 SD V10 now) and I went from the stock 2.55 70 16 to a 265 75 16. My odometer was off by .06. So every fillup, I multiplied my odo reading by .06 and added it to the total. I dont think you will match the Chevy for mileage. They get excellent fuel mileage. My buddy has a stock 2002 4X4 RC 4.8 5 speed and gets 400-415 miles to 22 gallons mostly highway. They are good on gas. Still a Chevy though.

 

I used to have a ’83 full size Chevy Blazer 4x4 automatic with ’35 inch BF’s tires with a built 400 small block motor and a 7” lift with 411 gears and I got about 8 miles to a gallon around town. You will need to adjust your speed/odometer reading from your large tires. The newer motors should get a few more mpg so 10-11 sounds about right. I also have a stock ’94 Chevy 4x4 Extra Cab with a 350 automatic and only get about 12.5 mpg.

 

I'm going to correct myself again. (I'm sooo confused)

With taller tires, your indicated speed will be less than actual speed. Here's a web page to calculate your speed with different size tires:
http://www.web-cars.com/math/tiresize.html

 

Area 52,

Sorry for confusing you. I think we are saying the same thing, but just differently. I misunderstood your first post.

Here is the web page I use. It's really neat, because it will give you the dimensions of pretty much any size tire and the effects they will have.

http://www.dakota-truck.net/TIRECALC/tirecalc.html


"God gave us a brain, he just forgot to give us the instruction manual"

 

With 3.55LSD 4.6L...

with stock P235/70R16's I got 15-15.5mpg doing 75mph on the highway with AC running, engine at 2150rpm

With new LT265/75R16's, computer reprogrammed for newer tires, 75mph on the highway, AC running, I now get 17-17.5mpg, engine at 2000rpm

 

Why is it when I read this thread I feel like I'm in a barber shop with Yogi Berra and the AFLAC duck has just walked in?

 

Effectively what you have done is that you now have an equivalent of a 3.25 gear ratio, but you have also added wider tires which will create a little more rolling resistance. It seems pretty incredible that you could gain 2 mpg by switching tires though, but great if it did.

 

I have a 2000 5.4/auto supercab 3.55/LS and I get 15-16 mpg. 100% stock and driven only to Church on Sunday.
Putting on huge oversize wheels will kill gas mileage and it'll throw the speedo off quite a bit too. A 4.2L Regular cab F150 would be HARD PRESSED to get 20 MPG anytime.

 

"...and they give you money, which is just as good as cash..."

 

That is either President Bush or Yogi Berra I can't recall which one.

 

Remember that a gear change does nothing to correct the speedometer. Only put your truck back in its powerband. You must get the computer programmed in some manner.