Approximate boost readings at 70 MPH no load

[sgoochds650]

My truck now has almost 1500 miles on it so it should be broke in fairly well by now. I have changed to the larger tires and my mileage numbers are pretty weak. At 70 MPH my truck is reading 5 plus LBS of boost (probably closer to 10LBS in reality but I'll have to verify that again tomorrow) on flat roads with no load and just maintaining that speed. The indash avg mileage reading is reading just under 14 MPG with mixed city and highway driving. 34" tires are definitely hurting my MPG which I knew they would. My good friend with his less than 100 mile 2011 F-250 ext cab SB 2x4 is getting amazingly high fuel mileage figures compared to my truck. So much higher than mine that I don't want to even post them because I don't want to be laughed at and accused of not being truthful. I'm just curious to know what level of boost other trucks are reading at 70 MPH and mostly flat roads just to see how much more my truck is under a load compared to others. Seems like other trucks are making virtually no boost maintaing speed compared to mine. FYI i have 3.55's, 34" tires with 80 psi of air pressure. Any numbers or advise is appreciated. As of now my old dually that is getting just under 20mpg on the highway is still making me proud as ever.

[Balla6.7]

At 70mph I can see about 4lbs of boost.

3.55 Gears

I only get 18mpg @ 70mph. I've heard others say 19.4mpg @ 70mph but thats not the case for me. 65-67mph is were I see bst mpg 19.1-19.4mpg

[SFC1984]

I'm not sure why 34" tires would hurt your MPG when the trucks comes with 34"s from the factory. I think it's probably the 80 psi you're running them at. Back em down to about 65-68 so the TPMS doesn't go off. Ford only calls for 80 psi in the rear tires for a SRW and that's usually for just towing heavy. Our '09 came with 34" tire with 20" wheels from the factory and they were set at 65/80. Those rear tires running 80 psi wore down to slicks within 20k miles.

[LRTMatt]

The 34.1" tall factory 20s hurt fuel economy because of rotating mass more than increased diameter. Put an 18" wheel & tire on the scale and do the same with a 20" combo. It will start to make sense then.

[eleventhtruck]

If your tires have an aggressive lug pattern, I hear the air resistance has an effect.

[sgoochds650]

My truck is making 4 or 5 lbs of boost as long as its flat. When it slopes up it will easily hit 10 maintaining speed. My truck came from the factory with 245/75r17's I didn't realize the factory 20" wheels had 34" tall tires. I wonder what it would take to calibrate my speedo to be accurate with those tires since programmers aren't available yet. I haven't measured my old tires yet but would think they are closer to 31' tall max. Any ideas?

[NotStock]

Are you hand calculating your mileage or going soley off the dash estimater? Add larger tires, increases the load which will decrease your mileage but if you havent re calibrated your speedo your going farther with each revolution. The extra distance is not being taken into account with the MPG calculator.

[sgoochds650]

I realize there will be a difference since the tire diameter has changed and hasn't been corrected. This is one reason I was curious about boost levels just to get an idea of how much harder the truck was working to maintain a given speed. I would guess my speedo is off by at least 3-4 mph at 70 mph. I would love to find out if there is a way to program my truck as if it had the factory 20" wheel option which another person mentioned was 34.1" If so my speedo problem would be solved. I have a friend that works in the service dept at a local Ford dealership so I'll be calling him tomorrow.

[cobrat03]

My truck came with 33's....I put Nitto Trail Grappler 35X12.50x20's on it and lost mileage but not as much as I thought. I run around 5 psi on flat ground but my ODO is off 7.8% so if I add that to the mileage on my trip meter I'm averaging 16 or so MPG on the highway. I know if I went to an all-terrain tire I could probably pick up 1 or so MPG...just my .02!

[abi motorsport]

stop by your dealer, they can reprogram your truck to four different tire sizes, they did mine for free. they can only program to tire sizes that the trucks came factory with tho.