I have a 96 F250 with 4:10 and 4EOD transmission. After replacing the plugs, plug wires and distributor cap I decided that I would make an MPG test on my next trip to relatives in Southern Ohio. This is a 120 mile round trip in totally flat country. Traffic was very light, no towns or stops and probably the best place to conduct an MPG test. My goal was to see just how much econonmy I could get from this old 7.5 (460) without becoming a hazard on the road. Since this is just about all straight highway I figured faster vehicles would be able to pass without concern. I placed the cruise control on 48 mph and only on occasion increased my speed if being tailgated or felt I was being a nuisance. After traffic passed I would slow back to 48.

At first I nearly gave up on this test because it seemed I was moving so slow I would never arrive! This feeling subsided however after a few miles. I guess hauling a** is just a bad habit. When filling the tank I was careful to get a good full load, clicking the nozzle several times until the neck of the tank was spilling over.

This is unbelievable...........I drove 122 miles and on my return to the same station near my home I pumped exactly 8 gallons of gas to fill the neck of the tank again. This is 15.25 MPG!!!!!! SO....speed has tremendous effect on gas mileage. When this truck was new I saw 14 mpg a few times while driving around 50 mph for long periods of time but never 15. There was no wind, nothing magic about my tune up, just ran with a light foot.

Now I'm not suggesting everyone run as slow as I did in this test. However, the government is forcing the issue on higher mileage vehicles from the auto makers and the answer may be at the end of your foot. The states have increased speed limits now to 75 on interstates and many folks now run 85. Don't complain about gas prices if this is your "habit"

 

yep....i usually try to run a little slower than i would otherwise as well. with my new tach, i can really see how speed affects rmps. just going from 70-65 makes a huge difference. i usually go about 68. speed limit is 70, most go 75+. truck feels 10x better at 68 than at 75

 

I have a 96 Silverado that gets 17/17.5 on the highway when I bought it. I drove a full tank around town without going over 2,000 rpm. Took off really slow, but I got 16 or 16.5 in town.

 

What you experienced is no surprise to me. I figured it out years ago. You had two major things happening:

1. 48 mph is not too far past shifting into OD--top gear, low rpm.
2. The friction from air moving over your vehicle goes up with the square of your velocity. Meaning, if you double your speed, you quadruple the wind resistance. The effect of this on vehicles is particularly noticeable (from engineers doing wind tunnel tests) above about 40 mph. Below that, acceleration and lower gears have a much greater detrimental effect on fuel economy.

Personally, I feel if you speed, you have no right to complain about fuel prices. If everyone in this country started following the speed limits tomorrow, we would see gas prices drop dramatically.

Jason

 

I have read that the "Highway mpg" listed on new vehicles has been calculated at 48 mph at optimum conditions like I had.

 

Yeah.. some people just don't get it. There's a guy up on another page with a late model 5.4 supercrew he uses to tow a Lightning on a trailer. That's a 5500lb truck towing another 5-6000lbs, and he wonders why he only get 8mpg at 80mph on the Interstate!!

 

You guys are right.
I have had mostly diesel powered trucks for many years due to the increased fuel economy.
The F-250 I bought a couple of months ago is gas. $2500.00 was just too good of a price for a mint 95 with a perfect R/K Service body on it. The bed alone was probably 3-4K.
Anyway I had never been convinced that rpms made a bunch of difference in a GAS powered job. It makes a hell of a difference in a diesel.
If I drive:
55 i get 13-15mpg
65 I get 11-12mpg
75+ I get 8-10mpg

I used to think my time was extremely valuable. I put a pencil to it and found If I slowed to 55mph I could work less service calls per day and make more money. It really woke me up. I get to work less and make more
Remember I live/work in a rural farming type community and most of my calls are 25-50 miles apart. With 2.89 gas and $35.00 for a trip charge it just doesnt work out.
I have tried adding mileage fees but my old time customer feel like I am robbing them.

Thanks to all for another great thread.
Y'all are the best!

 

Speed makes all the diference especially in a truck with its aerodynamics etc