Hey everyone,

I took a trip yesterday, and kept the speed down to see how good of gas mileage I could get. It was about a 250 mile trip, and I went between 70-75 steady the whole way. I had 2 dirtbikes in the back with gear. ( ~600lbs.) When I filled up, I figured it out and I only got 14 MPG Check out my mods below...I know I have 4.56 gears, but I also have 33" tires, chip, intake and exhaust, and I drove with a VERY LIGHT foot! I can't figure out what gives? My plugs are new, fuel filter is new, intake is clean, transmission fluid is fresh, fresh Mobil 1...etc. I can't figure out what else to try, or am I just going to have to accept the fact that this thing sucks at gas mileage? Thanks in advance!!

Chris

 

I think it's funny that keeping it down for you means 70-75mph....

There are a few factors here. The first is: Did you have the speedometer recalibrated after the tire and gear change???

According to this calculator you should be at 2275RPMs at 70mph. If your speedo has not been recalibrated, you cannot go by the odometer to determine your mileage.

Another factor is the 3.5" lift. This makes your truck even more wind resistant as your speed increases.

I think a big factor here is your gear ratio. In stop and go traffic, the 4.56 gears will move your truck with less initial effort than a 3.08 ratio, but on the highway this higher RPM will cost you in fuel mileage.

I have 3.08 gears and 30" tires. At 70mph, I'm at only about 1700RPMs. If I were to put 22.3" tires on my truck, this would be the equivalent of your 4.56 gears and 33" tires.

At 70mph on the highway, my truck gets 18mpg. In the city, with my 3.08 gear, I can get less than 13mpg. I think it's probably the gears and your lift that are decreasing your fuel mileage on the highway.

 

That is actually very GOOD mileage considering that you have almost every factor possible working against you.
600lbs. of gear
33" tires
4.56 gears
70-75 mph
I can't believe that someone who made these modifications is even thinking about gas mileage. It is almost like you were trying to drive down your MPG. You have built a performance truck and it is performing perfectly. By definition performance vehicles are not fuel efficient. With these mods the only thing that you can do to get the MPG up is to slow down. The 55mph speed limit was chosen, in part, because it is a fairly fuel efficient rate of speed. Either stop worrying about MPG or start removing your aftermarket gear until you get the MPG up to a point where you are satisfied...personally, I'd keep the cool mods and forget about gas money.

 

Not true!
He could still get a fat girlfriend.

 

ooh, that hurts!

Anyways guys, thanks for the replies! First, the speed limit was 75 MPH, so trust me I WAS going SLOW compared to the flow of traffic. (Almost got rear ended many times) Second, my speedo is correct, and I used mile markers to verify. Lastly, I probably wouldn't have even brought this up, but I keep hearing stories about how people with mods just like mine are getting 18-20 MPG! Are they just exaggerating or what? And the other thing that gets me is that my truck's mileage has stayed the same from day 1. Added a lift and tires, same mileage, less power. Added "MPG increasing" chip, intake, and exhaust, same mileage, little more power, then I added the gears because I had read people saying that smaller gears could actually hurt gas mileage because of the power it takes to get the truck moving, and I mostly do city driving, so I gave them a shot, more power, same mileage. Also, 4.56 gears with 33" tires makes the effective ratio 3.73, right? If so, that is a factory option, so I wouldn't think it would hurt MPG THAT much. Lastly, and probably the weirdest to me, I get the same mileage whether I am driving city or highway. I don't know what to think, are other people making up stories or should I look for something else? Thanks again and sorry for the length!

Chris

 

You live in CA. Our 'clean air' fuel has lower energy content than most other places, so the engine has to work harder. Anecdotal evidences suggest that fuel quality really effects mileage on our Ford trucks. Those people getting great mileage do not live in CA.

I check my mileage every tank. When my truck was new, I would get around 15.5 or so, mixed driving. At 11,000 miles, it seemed like my truck broke in and got more power and slightly better mileage, usually 16 with the occiasional hard driving.

After a few mods (chip, Airraid, electric fan), I would usually get about 16.5 - 17. I have got over 20 mpg on three tankfuls in 70,000 miles, all 60-65 mph with a tailwind.

IMO, the mod that helped mileage the most was an electric fan.

When I put on slightly bigger tires (265/70/16) and started driving I-5 at 80-85, my mileage went down to 15. Slowed down to 75 mph (slow for I-5) - mileage up to 16. Still down from 16.5 - 17 I used to get with the stock (shorter!) tires.

Road trip to Reno hauling dirt bike and gear. Took off the Superchip and fill up on Nevada regular gas. Drive home at 75 mph. Got 19 mpg!

Back in CA - no chip, mileage back down to 16 mpg mostly highway.

Moral: Our CA gas is bad, and big trucks with big tires suck down the fuel at high speed, no matter what gear you are running.

 

Thought I'd toss this in for whatever it's worth.

Picked this truck up in Los Angeles with 110,000 miles on it the other day. Beautiful condition, very well maintained, but needed a tuneup.

Ran great on I-5. 85MPH up the grapevine no problem at all, could have gone faster but wanted to keep my buddy in his old 350 chevy in my rearview.

Averaged 16mpg on the way home, ran it at 80-90MPH most of the way home to Oakland.
This truck still had the stock sparkplugs and a dirty air filter.
Feels much quicker and smoother now after the tuneup.
Haven't gotten it back out on the highway for any distance to see what the mileage is.

But sixteen miles per gallon with our crap gas and a truck that needed a tuneup. Motor is a 4.2, the rear end is a 3.08, stock tire size and five speed. No lift or other suspension mods, no bed cover, tailgate up.