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Does MPG drop as MPH rises??

3K views 21 replies 11 participants last post by  06SIXLITER 
#1 ·
On my truck I get about 18mpg @ an average speed of 60 mph I get 15.5 @ 75mph

both of these were done at 10 mile stretches on the highway..

Is this normal? I dont think so but who am I????
 
#3 ·
According to a news article I read, Your MPG INCREASES as you increase MPH UNTIL you reach 55mph. Beyond 55mph it slowly declines until you reach 75 mph and then it falls off like a cliff. After that, you spend more fuel overcoming air resistance then anything else. Keep in mind this information was based off of regular passenger vehicles, so our bricks may be different!

How many miles are on your truck, and how did you calculate mpg in only 10 miles? I assume that is because you have a lariat and you read off the overhead console. These have been reported has inaccurate and the only real way to identify mpg is by filling up with a specific technique, driving until nearly empty and then filling up the same way. Then you divide miles driven by # of gallons used.
 
#4 ·
I believe the reason mpg increases from 0-55mph is because when driving 5mph you spend more time with the engine on, then you do driving the same distance at 50 mph. Make sense?
 
#5 ·
To answer your question, YES....MPG does dropp as MPH increases. It takes more fuel to allow the motor to turn the higher RPM required to drive at a faster speed.......BUT

This is a very complex equation. Your example is pretty cut and dry, but others are not. If you are in 3rd gear at 30mph turning 1400 rpm and you are in 4th gear turining 2200, you are actaully gonna get better MPG at the slower speed
 
#7 ·
As speed increases, the energy required to overcome wind resistance increases exponentially. The best illustration is riding a bike. It's fairly easy to pedal at 15 mph. Twenty gets a bit harder and 25 is much harder than 20 and the difference between 25 and 30 is huge. The same principles apply to vehicles.
 
#9 ·
Like Marty said, it's pretty well tested and documented. although I don't think 55 mph is the magic number on todays cars. 15 years ago that was the number. I think my truck actually gets better milage at like 62-63 than at 55....the RPMs are just right at 63 and its not luggin on hills thinking about downshifting and it also isn't wound up like a top. Anything over 63 seems to drop milage in my rig.
 
#10 ·
When my Cummins was bone stock I got about 18.5 mpg doing 80 all the way home from South Dakota after picking it up. I slowed to 60 when I hit the Indy state line as the limit decreased. After resetting and driving about 100 more miles it told me about 24.4. That's a little more proof....
 
#12 ·
The Cummins has 2 less cylinders. Each cylinder has a percentage of losses so with less cylinders and virtually the same displacement you negate those losses of the extra cylinders. Rotating mass and other variables play into it as well. The most efficient diesel engine for nearly 30 years was the John Deere 2 cylinder diesel. The Cummins benefits from the same things that made the Deere work so well. Fewer cylinders, lower rpm and a longer stroke that equals more torque. Doing the same work at less rpms = less fuel burned.
 
#13 ·
Ford powerstroke milage issues

I just bought a 2000 PS diesel with automatic trans. Hauling a 3000 lb stock trailer on interstates I have made 3 tests of milage, each about 150 or so miles. The computed results are between 11.2 and 12.8 mpg. The truck is stock and I drove most of it at 65 mph and dident pust it at all as I was trying to get good milage. No wind issues. I am disapointed in the results and wonder if my expectations are off or is something wrong. BTW the truck has a new air filter (Ford), came from a dealer who serviced the truck before I picked it up.
 
#16 · (Edited)
Displacement is displacement. The advatange to a V-8 over a straight 6 is that you have an extra power stroke per revolution of the crankshaft. At the same time the disadvatange is that you are also dragging two more cylinders during non power stroke operations (i.e intake and exhaust). This is essentially the case on all non turbine internal combustion engines.. The more the cylinders the more power strokes per revolution of the crank but also more "dead" cylinder time... You'll find that the very large diesels used in trains and ships are usually anywhere from 8 to 12 cylinders (there are even 14 cylinder engines) because of the number of power strokes for such slow turning engines. In the smaller applications it does in fact make more "economy" sense to have fewer cylinders... The RPMs go up, the fuel consumption goes up, it's that simple... At some point efficiency drops off due to drag and load, further increasing fuel consumption....

Ken....
 
#18 ·
I don't know a whole lot about the Dmax, but the one my friend had wouldn't do much better than 15 on average. I personally drove that truck and I can usually squeeze the mpg out anything, but it didn't respond any better to me!
 
#20 ·
The last trip I took was 200 miles round trip, filled up before I left and when I got back. Probably 60 or 70 of those miles was slow 55 mph driving with crusie on most of the time and the rest was highway with the cruise set right at 65 mph. I after I did all the math I came up with just a hair over 20 mpg while the overhead consol said 18.5 I think it was.

I have also had times were the over head consel would be saying high 16 to low 17 mpg but once I did the math I came to somthing like 15 mpg. So dont rely on your overhead consel to give you acurate readings cause its hardly ever acurate.
 
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