I've got a s331 and i typically get about 12mpg around town and 14 on the highway. But I've noticed that it does better over 70 mph than under. Ive wondered in the parasitic drag of the supercharger is making my fuel mileage worse at low rpms

 

Have you corrected, either electronically or mathematically, for your taller tires? If not, the odometer is grossly underreporting your mileage travelled, and therefore your fuel mileage. For reference, just stepping up to a 70 series tire (from the stock 65 series) underreports my fuel mileage by about 1 mpg when calculating based off the odometer. You're actually probably around 15-16 mpg, which seems right for a 5.0 turning 35s and pushing a 6" lifted brick down the highway.

Highest I've ever gotten with my 5.4 B6 on an all highway trip (must've been downhill with a tailwind the entire 200 miles) was an odometer-corrected 19.5 (fuel station's pump probably had shut off early on the next fill-up, too). Usually, unladen, I'm around 17, trying to keep it at a max of 70. With my empty 700 lb trailer, I'm under 15, and with the entire load this past weekend, I'm probably 10-11 (guessing, 170 miles on a half-tank; I'll know when I fill up tomorrow. I've started running 89, as it seems to buy a few more mpg and possibly more power, and the 5.4 needs as much as it can get! My last truck, a 5.7 Ram Crew cab with a 3.92 rear, did over 21 mph on a trip NY-OBX and back, so this truck was a huge drop for me... what a difference a transmission with double the gears makes!

 

How do you correct this, "Have you corrected, either electronically or mathematically, for your taller tires?." Is there table/chart or electronic calculator that will calculate these numbers? Thanks

 

Divide the ACTUAL stock tire size by the ACTUAL new tire size (rim size doesn't matter, it's the overall height of the tire that makes the difference; you'd have to research to find those true specs from the manufacturer), and then divide your mpg by that number. If you do "New/stock", then multiply that number by your mpg. The odometer under-reports in inverse proportion to tire circumference, so the numerator in "miles/gallons" is smaller than it actually is. Back in the day, we'd change out speedo gears, but today, I'm sure the speedometer/computer could be reprogrammed to account for larger tires. Don't base miles driven off a GPS, as that only reports horizontal distance, but actual mileage will differ based on changes in elevation. With a lift and more rotational mass, you'll see a drop in mpg, but not as severe as what you're seeing.

 

We came back from Michigan doing the same speed as you (@OpsDad #1209) did. We have pretty much the same truck too. Here are our numbers : 467.5 miles, 22.48 gallons, Hand calculation = 20.796 mpg computer read 21.3 mpg We have 6975 miles on the odometer. Change to Mobil One at 5500 miles. Wondering if we bought "winter gas" in Michigan....hmmm

 

I should mention that I use 93 octane gas. Winter gas is a possibility. Also, you can, without need of ForSCAN, change the AFE bias to make your fuel economy meter (AKA "Lie-O-Meter") closer to hand calc. (hold OK button while starting then scroll thru everything until it comes up AFE Bias. Figure out the percentage difference, multiply by 10, and subtract that many 1/1000s). I adjusted about 4 times before I got it to where it's always within .1 or .2 mpg of hand calc.

 

Did the math for you. Change your AFE bias to 976 and it matches the hand calc you did.

 

"Winter gas" is mostly myth; fuel usually has 10% ethanol everywhere, year round. Vehicles get lower gas mileage in the winter for 2 reasons, in order: 1) air is colder and therefore denser, therefore more fuel must be metered to maintained proper air-fuel mixture, and 2) air is colder and therefore denser, which offers more aerodynamic resistance.

 

Also to add - warm up time. If you idle to warm up (like using remote start) that will make a big difference in your MPG. Even if you don't let it sit to warm up the engine will use more gas driving with a cold engine - plus the fact that the transmission and rear end are cold along with your tires for the first 20 or so miles. All this makes a noted difference.

 

Never use remote start or doing any "warm up" unless the windows a frosted over. In fact the "Start / Stop" feature makes me a more patient driver when it comes to pulling out into traffic. This trip was the first use of the heated seats and the steering wheel....nice features