Some observations on MPH vs. MPG

[rv_safetyman]

I guess we all know that increased speed can significantly reduce MPG.

Before I get directly to the subject, let me set the stage.  My Eagle has a Series 60.  The ECM can give very accurate speed and fuel used information.  I capture that information on my SilverLeaf VMSpc system (I love the system so much that I have become a dealer).  I have checked the data that is presented several ways.  The speed is exactly the same as my GPS and the odometer agrees with my mechanical unit as well as mileage checks along the highway.  The fuel used seems to be very accurate.  I use the data as my fuel gauge.  When is says that I have used 100 gallons, I will put in 100 gallons plus or minus one or two gallons (sometimes more, sometimes less).  So, I think the data I am about to present is accurate.

My driving habits over about 42K miles was to set the cruise control on 68 mph.  Obviously that varied in hilly country, but on the open road, that was my speed.  Over the past 43,681 miles I have averaged 7.8 MPG (includes some idling) and some major mountain driving.

On this trip I set my cruise on 63 mph.  The first leg of the trip I got right at 10 MPG with a bit of a tail wind.  I then hit some head wind the mileage became more realistic.  So far on this trip (1246 miles) I have averaged 8.5 MPG.  The 8.5 is about 9% better than my overall average.  Granted, I have not hit terrible weather, or big mountains, but I am convinced that I have gained what would probably amount to a least a 5% gain on a long term basis.  That helps a bit with the fuel cost.

I does not bother me to drive slower.  On the interstate, I am not causing a problem.  When I drive two lanes, I try to drive a speed that will not cause a problem with the folks behind me, but 63 works pretty well there as well.

One interesting side benefit is that I do not have to pass a lot of traffic.  At 68, there are a lot of trucks ("Swift" is one ) and some cars going slower.  At 63, that is not an issue for the most part.  Makes driving less of a chore. 

It sure seems to me that there is a lot less traffic on the interstates, and that trucks are driving a bit slower.

Just some random thoughts.

Jim

[Charles in SC]

I agree, my bus has an 8-71 and gets the best mpg at about 55. Hmmm that brings back memories of what Uncle Sam made us do in the 70s. Somewhere in my mind I remember a rule of thumb that said if you double the speed it takes 4 times the hp. That means it would take 4 times as much to go 70 than 35. In reality I think the best mpg in my bus comes a the fastest speed I can run in high gear without making the engine work hard. That seems to be about 55. I have no data on this, just my feeling.
This is worth what you paid for it.

[boogiethecat]

I've always found it interesting and a bit strange that my Crown gets 10 mpg, regardless of the rear end ratio (I've had three different ratios now), how fast I drive (Max has been anywhere from 63 to 80 depending on that ratio) and how loaded or unloaded the bus was (ie I got 10mpg bringing it home riding the governor at 2500 rpm, totally empty bus at 63mph, and now with the perfect rear ratio driving 68-70 at 1800rpm with another 10,000 pounds of stuff added to it, a raised roof and two A/C's stickin out there.... guess what... 10mpg.

The only time I've gotten less is when I tow a 6000 pound trailer or VW vanagon... then it goes down to 8-9 depending on the grades I encounter.  But speed, rear ratio, and load to a certain point don't seem to make any difference at all... 10 mpg....

G

[Tom Y]

Thanks for the info Jim.  Tom Y

[zimtok]

I don't have a working speedometer so I base my fuel usage on my regular route per week.

We do on a regular basis 4 gigs a week and I top off the tank every week at the same place on the same day. Normally I drive the bus with a heavy foot and in the city rarely get into 4th gear. I would use about 45-50 gallons of fuel a week.

Lately I have been a little lighter on the pedal and try to get to 4th gear more often. I have noticed about a 5-10 gallon drop in fuel consumption.


I will soon be able to do an actual miles per gallon as I just bought a GPS for my bus.



.

[RJ]

Somewhere in my 4106 file folder I have a statement from GM Engineering that the coach was designed to get 10 mpg @ 60 mph running 1650 rpm using tires that turn 495 revs per mile.  (DD 8V71 with 4-spd manual, 4.125:1 rear axle and 0.808 bevel gear ratio.)

Wonder what the difference would be today with an electronic four-stroke. . . maybe 12 - 13?

One can contemplate, anyway.

FWIW & HTH. . .

[beatsworkin]

Fuel mileage depends on a lot of different factors

For example:  The number of times the pistons go up and down per mile   example try low gear and 1800 RPM compared to high gear at 1800 RPM for a mile.

Other factors such as weight.rear axle ratio, aerodynamics, wind resistance.     

I have learned over the years that for each five miles per hour over 55 mph you decrease your fuel mileage by 10 percent.  Therefore at 65 mph your fuel  cost will be increased 20 percent from driving at 55.


Red

[HB of CJ]

Looks like the common theme is that slower is $better$ regarding fuel mileage.  However, I would have to agree with boogiethecat; with my old Crown 40-foot 10-wheeler, it didn't seem to make all that much difference.  At 55mph, (9th or 10th) she got well over 10 mpg, at 70mph, about 8 mpg.  I never really ran at 80mph long enough to measure anything except that we going way too fast. Yikes!

Guess it is just a matter of aero-dynamics.  Crowns are just a little bit slicker than an Eagle.  However, one pays that price by the lack of headroom (75" unraised) and very little under-the-floor storage area.  Oh well.  That important fact may (may?) be offset by the other advantages the Crown Super Coach gives the owner, like quality, ease of maintenace, cheap parts, durability, etc.  Thanks.