Perhaps someone can help me out with a technical question about my F250 6.0L Truck. I put 35" LT315 75R-16 Toyo Cross-Country tires on my truck when it was one day old. I have about 7000 miles on it so far. The digital LED temperature/fuel-mileage/miles-to-empty guage shows I'm getting only 10-miles per gallon. If that is close to normal or actual, then I feel ripped off to get such horrible gas mileage. I was expecting around 13 to 15 mpg new and 18 to 20 mpg after about 35,000 miles (break-in).
I'm stumped, and so is the shop foreman at the Ford dealership. They can't reprogram my truck's computer because Ford's computer program software only goes up to size 285 tires. I'm just wondering if my tires are a 'big' contributer to my lousy fuel economy? Is 10-MPG normal for a new 6.0 liter diesel truck? I'm sure that the digital fuel economy readout may be somewhat skewed by the larger tires, but I can really tell that I'm sucking the fuel down.
My brother has a Black F250 7.3 liter diesel that looks almost identical to mine. However, he gets 17 mpg with his truck. Is there a solution to my delimna of lousy fuel economy? Is this normal? Everything on my 6.0L is completely stock with the exception of the 35" tires I mentioned and I also added a leaf to each 4 corners which raised it up about 2.5 to 3 inches.
265's, stock, runnin on the speed limmiter for almost a whole trip with a 16foot trailer full of 4 wheelers got about 12mpg. going easy unloaded I get about 15-16. Tires can make a fairly good change in fuel milage but surley the diesel doesn't notice them that bad. try an intake and an exhaust and see if that will get it up
I have a stock California model with the 5 speed transmission and 3.73 gearing. The truck now has 13000 miles on it.
Unloaded on the highway I have seen over 20 mpg driving at 75+ but average around 17.5 mpg on general highway use.
In town commuting 4 miles to work and back I get around 13 mpg because the truck never really gets warmed up or into high gear for any length of time, plus I climb a 400+ foot hill on the way.
I tow a 30' Weekend Warrior trailer with a typical truck and trailer gross weight of 18000 to 20000 lbs depending upon the number of toys and gals of water in the tanks.
On a trip this last weekend from San Diego, CA to Pinetop, AZ and back, I averaged 8.66 mpg. This was towing at speeds of 65 to 75 mph. As the speed increased, the mileage decreased - most likely due to the increased wind drag on the trailer.
I've seen over 10 mpg towing the trailer on some segments, but over the course of most trips, this mileage seems about par for towing this trailer.
I was under the impression everyone was calculating the mileage using your technique. That is how I derive mileage, anyway. On my truck I didn't get that fancy computer thing with the display. I am amazed that some trucks get over 20. I only got about 17 on a highway trip. Maybe the air in Texas is too thick.
Here's the story on your 35 inch tires and fuel mileage.
I just measured the rolling radius on my stock 265s at between 15 1/4 and 15 5/16 inches which translates to 8 feet per revolution or 660 revolutions per mile.
Your 35 inch tires will have a rolling radius of around 17 3/16 inches depending on inflation pressure which translates to 9 feet per revolution or 587 revolutions per mile. For each mile your odometer registers, you have actually traveled 1 1/8 miles - 8 miles on the odometer equals 9 miles traveled. That could account for a 12.5% fuel mileage discrepancy.
However, that is not the whole story. It takes more energy to overcome the rolling resistance of the wide footprint on the road and probably more energy to just turn the increased mass of your tires. Lower inflation pressures will increase the rolling resistance. You have also increased the frontal area for increased wind resistance - larger tires being forced through the air and a raised profile. The air doesn't flow as freely under the vehicle as around and over the body due to turbulence from all those projections underneath.
Wind resistance is no small issue. I had an E350 Van that got in the neighborhood of 16 MPG at 45 MPH and dropped to about 10 MPG at 75 MPH.
Many delivery trucks use skinny high-pressure tires to overcome rolling resistance.
Energy used equals fuel burned!
F250 6.0L 5 speed auto XLT extended cab, Short Box 4X4. 18 - 19 MPG empty mountain highway miles over 10,000 - 12,000 foot passes and 12 - 13 MPG towing a 6000 pound trailer over two 10,000 foot passes.
What has been said so far, is true, also, it may be helpful to change your gears to get you back to 3:73's (if that was your stock ratio).
Mine, on stock 265's gets 20.5 mpg @ 77mph. Towing my 29ft 5'er with my 2 place ATV trailer taggin along,(see my camping gallery), cruise set at 75-77 mph, i get 13 to 14mpg.(there has been times I've got that @ 65mph, oh well) This is a Canadian model & I now have 6300 miles on it since new. Born on 1-24-03.
Forgive me, but what do you expect? If you put on oversized tires, it is naturally going to make the engine and the transmission work harder. My suggestion, get a chip or a programmer to give yourself a little more muscle to make up for those tires. An exhuast system wouldn't hurt either, every little bit helps. Besides, your truck is nowhere near broken in. That and the overhead console is almost always off by a couple points when it comes to the gas mileage. It shows the average miles per gallon not what you're actually getting. Calculate it yourself, you might a difference and then again you might not. Don't always rely on the console. I hope this helps in one way or another.