MPG at 40-45 MPH

Topic: MPG at 40-45 MPH (Read 1762 times) previous topic - next topic

MPG at 40-45 MPH

July 23, 2012, 01:18:52 pm

Recently did a 3908 mile trip from Austin, TX to Las Vegas, a 2 week tour to the Grand Canyon, Bryce Canyon, 2 Utah state parks, Zion and back to Las Vegas. After that we went to Cedar Breaks, via Panguitch to Escalante, Capitol Reef, Blanding, Hovenweep, Cortez, Durango, Pagosa Springs, Chama, Santa Fe, Clines Corners, Santa Rosa, Ft. Sumner, Clovis, Lubbock, Abilene, Austin. 8.3 mpg average for the whole trip with no deduction for generator run time as I forgot to log the time when I filled up. From Austin to Las Vegas, mostly on the interstate with the generator and 2 AC running at about 62 mph most of the time -7.43 mpg. For the 2 weeks visiting the Grand Canyon, etc. 8.54 mpg. From Las Vegas to Austin - 9.13 mpg. I used economy mode all the time for the entire trip. The one thing that was really different on the return was that a lot of it was driven at about 40 to 45 mph in 5th gear as a large amount of it was on scenic roads without much traffic. About half of the return we didn't have to run the generator. Still there was a climb to 10,000 ft at Cedar Breaks, and another climb after Escalante and a lot of just up and down. Towing a 3000 lb toad too, but no aqua hot to use diesel. At one time I thought I got better than 11 mpg between Las Vegas and Santa Fe, but decided that I didn't get the tank really full in Santa Fe, still it was probably close to 10 mpg on that portion. After Cortez we were in 6th gear most of the time.

I always thought from comments on this forum that the best mpg was in 6th gear at about 62 mph. Apparently one can get even better mpg in 5th at about 40 - 45 mph. Can anyone with a fancy program on their computer with all the engine parameters, mpg, etc. verify this? I'm talking about the bus style coach, I know the Grand Villas have better mileage.

Re: MPG at 40-45 MPH

Reply #1 – July 23, 2012, 02:28:40 pm

Quote

I always thought from comments on this forum that the best mpg was in 6th gear at about 62 mph. Apparently one can get even better mpg in 5th at about 40 - 45 mph. Can anyone with a fancy program on their computer with all the engine parameters, mpg, etc. verify this? I'm talking about the bus style coach, I know the Grand Villas have better mileage.

Two years ago I got 12 mpg going slow East To West on MI-41 in Michigan's Upper Peninsula.

I often travel at 62 mph, too.

But better mpg is at slower speeds. Wind resistance is the key.

All measured by SilverLeaf's VMS 240 CL

best, paul

Re: MPG at 40-45 MPH

Reply #2 – July 23, 2012, 03:23:49 pm

Best mileage is top gear at rpm that produces maximum torque for your engine. On My ISM that is 1500 rpm, which happens to be geared to produce 70 mph in my 40 foot U-320

Re: MPG at 40-45 MPH

Reply #3 – July 23, 2012, 04:22:08 pm

Tim.... do you have data to back that up? I'll bet mileage would be better at max torque in 5th gear. For that matter the slower the better at max torque RPM. I think wind resistance goes up with the square of speed. See http://en.wikipedia.org/wiki/Drag_(physics)

George

Re: MPG at 40-45 MPH

Reply #4 – July 23, 2012, 04:43:26 pm

Yup, sure you are correct, meant best mileage at Max torque. Don't intend to travel at 45 to minimize wind resistance. Drive as fast as your budget allows, when the money runs out park it and enjoy it where you stop!

Re: MPG at 40-45 MPH

Reply #5 – July 23, 2012, 04:54:21 pm

According to this link, there is an optimal speed for each vehicle and the slowest is not the best. The question is what is best for a FT.

Mpg For Speed - Fuel Efficiency Vs. Speed

Any ideas? We normally cruise at 55mph. That may not give optimal mileage, but Pat is "optimized" so to speak!

Re: MPG at 40-45 MPH

Reply #6 – July 23, 2012, 05:00:03 pm

For those that like reading, take a snoop at what Cummins sez.
Enjoy
Dave M

Re: MPG at 40-45 MPH

Reply #7 – July 23, 2012, 05:34:41 pm

Yes, I suspect that peak torque RPM in 5th gear would give better MPG, but unless you are about to run out of fuel, probably not worth it (frustration factor> MPG improvement).

Brett

Re: MPG at 40-45 MPH

Reply #8 – July 23, 2012, 05:42:13 pm

Excellent Cummins reference! I need to read it every year. Thanks!

Re: MPG at 40-45 MPH

Reply #9 – July 23, 2012, 05:47:47 pm

And here is another excellent one, by Caterpillar Corp: http://www.catrvclub.org/PDF_Docs/Understanding_Perf.pdf

Both documents apply to all heavy vehicles, irrespective of engine manufacturer.

Brett

Re: MPG at 40-45 MPH

Reply #10 – July 23, 2012, 06:01:03 pm

Maybe if the fuel mileage issue it too big, staying at home and watching "Wheel of Fortune" is always an out.

Dave M

Re: MPG at 40-45 MPH

Reply #11 – July 23, 2012, 06:14:12 pm

I remember a period from my past life when I was studying power requirements and streamlining. I found a lot of data at Drive55.org - Your Brain on Mathematics

There is some very good reading there. I set up a little spreadsheet into which I could put in a little data and see how the increased air drag affected the horsepower requirements. The key thing to remember is that the horsepower required to overcome air resistance is cubed as a function of speed. That means is takes 8 times as much power to double your speed. (2*2*2 =

For example, consider the power requirements to run 50 mph versus 75 mph. Let's say we have a frontal area of 80 sq. ft on our coach, with a coefficient of drag of 1. (basically a flat plate) It will take about 69 horsepower to push our coach through the air at 50 mph. Now, the total requirements are much higher due to rolling resistance, etc., as the Cummins PDF explains. Right now I'm talking just air resistance.

If we decide to run at 75 mph instead of 50 mph, we are now running at 1.5 times the original 50 mph. So, 1.5^3 = 3.375 times as much power to run 75 mph. So at 75 mph it now requires 232 horsepower to overcome the air drag.

Basically what this means to me is to drive about 55 to 60 mph and enjoy the scenery.

Glen

Re: MPG at 40-45 MPH

Reply #12 – July 23, 2012, 06:40:46 pm

A couple of questions concerning the Cummins Doc:

1. Graph on Pg. 27 states that the max time in the highest gear is required for max mpg. The ISM seems to go into 6th at 1600 rpm which is over 60 mph. Has wind resistance killed 6th gear savings ( which I guess was the orginal point of the topic)?

2. Aeodynamics: Do GV's get significant mpg improvement? How about a big clear pointy cowl on the front of a flat nose?

Re: MPG at 40-45 MPH

Reply #13 – July 23, 2012, 07:14:40 pm

Quote from: LBoyd – July 23, 2012, 06:40:46 pm

1. Graph on Pg. 27 states that the max time in the highest gear is required for max mpg. The ISM seems to go into 6th at 1600 rpm which is over 60 mph. Has wind resistance killed 6th gear savings ( which I guess was the orginal point of the topic)?

I've wondered this myself. Our 450's sweet spot for 6th gear is around 1350 rpm/62-63 mph. I have noticed, however (especially now that we have the VMSpc to hawk), that our mpg seems to be better when speed limits keep us between 55 and 60 mph, which means 5th gear.

Would be a fun "Mythbusters" segment. Or a lab project for a college engineering fluid dynamics class. Anyone got a kid/grandkid looking for a master's thesis project?

Michelle

Re: MPG at 40-45 MPH

Reply #14 – July 24, 2012, 07:43:35 am

at that speed- and that gear- would you be lugging your engine? I believe they like some rpms- to keep everything lubed ad cool

Re: MPG at 40-45 MPH

Reply #15 – July 24, 2012, 09:53:39 am

The Cummins guide recommends letting engine rpm's approach peak torque before downshifting. With ISM peak torque at 1200 rpm and Allison in econ mode and 6th gear, can you then back off the throttle and cruise at 50-55 in 6th gear? I've noticed when climbing grades, doesn't downshift until speed drops down to about 50. Would this maximize economy?

Re: MPG at 40-45 MPH

Reply #16 – July 24, 2012, 09:54:32 am

While this is contrary to everything I've ever heard, I recently discovered my Powerstroke pickup gets significantly better mpg running 65-70 than 55-60. I've always kept the rpm under 2K cruise but no more. F350 4x4 with 4:10 rear.

I wish I had a Floscan on the coach.

Re: MPG at 40-45 MPH

Reply #17 – July 24, 2012, 10:06:22 am

Yesterday we drove from Asheville, NC to Raleigh, NC, a distance of about 250 miles. One looses a fair amount of altitude (about 2100 feet to 300 feet) which may explain our mileage of 9.5 mpg on the Silverleaf. Our average over about 20,000 miles is 8.5mpg. Normally the cruise was on and the mode switch on. The route has a lot of ups and downs and I noticed on the Silverleaf, which shows the engine load (from 0-100%), that the transmission waits until it has reached 100% before it shifts down. If it can maintain close to the set cruise speed, it will stay at 100% and not shift down.

Re: MPG at 40-45 MPH

Reply #18 – July 24, 2012, 10:28:26 am

Quote

The Cummins guide recommends letting engine rpm's approach peak torque before downshifting. With ISM peak torque at 1200 rpm and Allison in econ mode and 6th gear, can you then back off the throttle and cruise at 50-55 in 6th gear? I've noticed when climbing grades, doesn't downshift until speed drops down to about 50. Would this maximize economy?

Nope. You cannot cruise in 6th gear with my 6 speed tran. You'll drop yo 5th.

best, paul

Re: MPG at 40-45 MPH

Reply #19 – July 24, 2012, 06:26:34 pm

Quote from: erniee – July 24, 2012, 07:43:35 am

at that speed- and that gear- would you be lugging your engine? I believe they like some rpms- to keep everything lubed ad cool

Erniee,

You are correct, for a Detroit Two Stroke engine-- lugging will kill them.

But, the opposite is true of all modern 4 stroke engines by DD, Caterpillar and Cummins. For all these, most economic way to climb is at or just above peak torque RPM. They can be run all day at peak torque RPM at WOT.

This was a VERY hard adjustment for most truckers used to the DD two strokes!

Brett

Re: MPG at 40-45 MPH

Reply #20 – July 24, 2012, 07:37:41 pm

Drive it like you stole it is the old DD adage. I was always used to a 2300 or 2400 rpm fuel cutoff so the 2100 in the U300 seems like it's barely turning over. We were taught to shift down anytime the rpm dropped below 1900 if we had much of a load on the engine. Naturally, cruising down the boulevard with no load, we could ghost along not far above idle in fourth (manual trans).

The big exception to the rule was my old 4107 Buffalo bus with a 8V-71. It only had 225 hp configured for the bus but pulled well above idle all the way to a 1900 max rpm. The engine was so flexible that GM did not even put a tachometer in any of them. Just a 4 speed non-syncro Spicer trans (no CL). The afternoon I bought it, the bus company had just gotten an 86 mph speeding ticket with a full load of passengers outside of Boston. Got great mileage with 9.3 mpg through the Rockies with toad but was slow on the hills. Over 12 mpg at 55 on the flat.

Pierce

Re: MPG at 40-45 MPH

Reply #21 – July 24, 2012, 08:15:17 pm

Pierce, My Knee still hurts from driving an Old BUFFALO on a city route with a convention in town. That clutch was a bear. Any one that can shift that tranny can shift anything.
Gary B

Re: MPG at 40-45 MPH

Reply #22 – July 24, 2012, 08:59:17 pm

Gary,

We had Spicers in our Crowns but big tachs and with the engine right behind the driver, it was easy to listen to it so shifting was OK. Just had to time it exactly. The Buffalo with no tach and the engine in the back, was just a guessing game shifting. Non-power steering was tough if you couldn't roll it a couple of feet.

Sure was a super well built rig with excellent handling too. Sure miss the compartment doors.

Pierce

Re: MPG at 40-45 MPH

Reply #23 – July 25, 2012, 06:45:26 am

I do laugh a lot reading about the old DD and the Buffalo etc, why when I did the MCI, I removed the Spicer 4 speed and the push clutch that went with it, changed to the Road Ranger RTO910 with the pull clutch, worked the linkage for a easier clutch pedal, MCI had a air cylinder assist on clutch, on & on etc & etc.
Cheers
Dave M

Re: MPG at 40-45 MPH

Reply #24 – July 25, 2012, 10:00:27 am

Can someone provide a little clarity for me and my 2005 Cummins 400 ISL? I guess I'm looking for my engine's sweet-spot. In 6th gear for the best mpg, my tach should read...? How about for 5th gear?

(400 HP - Peak torque 1200 LB-FT - Governed speed 2200 RPM; Allison 3000)

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