Fuel mileage at lower speeds?

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Fuel mileage at lower speeds?

January 22, 2019, 07:27:56 am

As I'm planning our Alaska trip this summer one issue is the fuel efficiency at lower speeds. I'm used to driving 62-65 in 6th gear and getting ~8mpg towing the truck. But looking at other's blogs once we get up into northern Canada and Alaska we'll probably be driving somewhat slower, 45-55. At this speed I'll be in 5th gear. Any idea how that'll affect my fuel mileage?
Not a problem with lower mpg's, just trying to figure out how the lower speeds might affect my fuel tank range.

Re: Fuel mileage at lower speeds?

Reply #1 – January 22, 2019, 07:55:05 am

Depending on how our dollar is doing at the time of your trip will most likely be more of an effect! In the Yukon, as well as Alaska, we stayed around 45 mph. First off, we were not in a hurry and secondly, out of nowhere, the road could turn to gravel or you would see the little red flags on the side of the road indicating a frost heave. We saw 2 Class A diesel's run off the road coming back. The dust was so bad, they missed the curve in the road. Yes, there can be lots of dust at times although they do try and water the roads down when in construction. Now, you have mud, dust and gravel. However, well worth the trip!

Re: Fuel mileage at lower speeds?

Reply #2 – January 22, 2019, 07:58:22 am

I would'nt even worry about the range,you'll be filling up more often anyway and as for the lower RPM, why worry about something you have no control of.Use the mode on the trans and use the economy setting is about all you can do.Once you get into Canada
and mountains I would start using one of the fuel additives,we use Diesel kleen all the time,just my opionion.

Re: Fuel mileage at lower speeds?

Reply #3 – January 22, 2019, 09:17:31 am

I think you will find that lower speed in 5th with comparable engine RPM will yield BETTER MPG.

Re: Fuel mileage at lower speeds?

Reply #4 – January 22, 2019, 09:46:34 am

I agree with what Brett Wolfe said. On one occasion I drove about 200 miles at 40-45 in 5th gear and after refilling the tank found that I got 10 mpg instead of the usual 8.

Re: Fuel mileage at lower speeds?

Reply #5 – January 22, 2019, 10:19:03 am

Aerodynamic drag is the biggest factor in fuel mileage. Fifth at a lower speed will get better mileage than sixth going faster. Heavy toad and topography can really effect mileage also. With a manual transmission and a light engine load, we could always go to fifth at a lower speed and really get the mileage in our bus but our Allison four speed won't shift out of third until almost 50 mph so we can't loaf along on back road without going 50. You six speed owners have it made.

In mixed mountains and flatland, we go from 8 mpg to 10 mpg w/o our 3,000 lb. toad.

Pierce

Re: Fuel mileage at lower speeds?

Reply #6 – January 22, 2019, 10:48:32 am

Mine gets 8 mpg @ 1750 in 5th or 6th. While I have an ISC I would imagine M11 would do the same, lower gear less engine load.

Re: Fuel mileage at lower speeds?

Reply #7 – January 22, 2019, 12:23:25 pm

Our 97 shifts into 6th at part throttle at 55. Really helped it's low rpm torque to change to a resonator. Change the air filter to a Donaldson blue-tec for better filtering and take a spare with you.

Re: Fuel mileage at lower speeds?

Reply #8 – January 22, 2019, 12:46:26 pm

Our '99 won't go into 6th until 61 mph min. even in economy mode. Have seen mileage improvement on the VSMpc after the shift. It is hard to get a good comparison as road changes are involved. When I leave the Q I will try 62 mph towing the Grand Cherokee while level while it still legal to do so. In Ca. I will drop to the legal 55 and see what the VSMpc shows.

Re: Fuel mileage at lower speeds?

Reply #9 – January 22, 2019, 05:40:22 pm

I understand the upshift points are adjustable.

Re: Fuel mileage at lower speeds?

Reply #10 – January 22, 2019, 06:16:00 pm

Quote from: Caflashbob – January 22, 2019, 05:40:22 pm

I understand the upshift points are adjustable.

Yes, but (no, large BUT) it will affect all shift points, not just 5-6. Been there,done that. Lowering may make it possible for you to operate the engine under high load UNDER PEAK TORQUE RPM in some gears. OK if you are a savvy driver-- a no-no if you are a "put it in D and go" driver.

It is one of the perimeters programmable by an Allison dealer.

Re: Fuel mileage at lower speeds?

Reply #11 – January 22, 2019, 06:42:37 pm

What would it hurt to upshift BEFORE peak torque? I appreciate the lower upshift point myself.

Changing the muffler to a resonator and changing the fan drive incorrect controller and a Donaldson blue-tech airfilter woke the 55 mpg torque up majorly.

With stock components the engine pulled slowly from 55 and downshifted a lot under medium throttle.

The 62 might allow smoother less downshifting power with stock stuff.

The other way to fix this was what I did. Does not downshift now in 6th under fairly aggressive throttle.

Way different drive. If you drive a m11 with these mods set for 55 into 6th you would notice it right away.

Brings a grin that's hard to wipe off versus before. Fun to chug up grades in the mountains in higher gears.

Finally. Useable torque

Re: Fuel mileage at lower speeds?

Reply #12 – January 22, 2019, 07:06:47 pm

Quote from: Caflashbob – January 22, 2019, 06:42:37 pm

What would it hurt to upshift BEFORE peak torque? I appreciate the lower upshift point myself.

If the driver understands what is happening ( BIG IF) no issue. But it would allow a full throttle application at below peak torque RPM. OK for a few seconds, but on a long up-grade....................

Full throttle below peak torque RPM is not good for the engine-- any diesel engine.

Re: Fuel mileage at lower speeds?

Reply #13 – January 22, 2019, 10:03:20 pm

One needs to read up on the operating parameters in their engine manual. As Brett said operating any Diesel engine at to low an RPM "lugging" is not good. Ideally operate where peak toque and RPM curves cross, makes happy engine.

Re: Fuel mileage at lower speeds?

Reply #14 – January 23, 2019, 12:06:21 am

In similar displacement/torque engine families like the 8.3 the horsepower/torque relationship is close. Coach runs as fast up grade X upshifted lugging as it does when downshifted and revved. Square chart. Straight line at a 45% angle on a chart versus a 11 liter bored to L10 then stroked to m11 engine.

The bigger engines displacement lowers the max torque rpm enough to have the coach pull grade X lugging versus downshifting the less torquey bore/stroke engines.

I thought the m11 was a dog when I got it. Boring to drive. Revvy horsepower versus torque.

Not electronically fixable with marine engine top end made for fixed rpm. No overrev. Less low end but strong mid rpm.

Too exotic. Too trick. Not needed that bad other than just a toy.

So the other way to unload all power from the motors rotation?

Exhaust. Air filter. Cam position sensor. Ambient air temp upgrade. Adjust the valves.

Add solar to turn off alternator down the road. Change the hydraulic fans controller to the correct temp rating to reduce up to 60 hp draw in mild conditions.

Replace the restrictive muffler with a really well made welded up system with a blanket insulator and a 5050xl resonator.

Change to new energy efficient Michelin tires at 97/87.

Blue-tech Donaldson. Rebuild alternator. Synthetic oil in front hubs.

All possible hp to the trans, now what?

Engine now can pull light grades without downshifting. Torque up things.

Does the ISM drive the same? I bet it does.

Re: Fuel mileage at lower speeds?

Reply #15 – January 23, 2019, 11:56:27 am

Opening a can of worms but what about synthetic in the engine.

Re: Fuel mileage at lower speeds?

Reply #16 – January 23, 2019, 02:53:08 pm

Quote from: John44 – January 23, 2019, 11:56:27 am

Opening a can of worms but what about synthetic in the engine.

No stuffing the worms back in the can now.

Excellent if you can budget for it. It could be a lot cheaper to run as it covers up operator errors like heat soak for the turbo, less deposits if engine slow idled for extended periods, etc. These are procedures 95 percent of RV owners make as they just can't seem to learn new ways.

Fuel mileage gain is going to be very small, not enough to make up for the added expense. But like I said...

Sorry about broken toes.

Pierce

Re: Fuel mileage at lower speeds?

Reply #17 – January 23, 2019, 09:41:22 pm

I doubt any current diesel oil is not a large percentage synthetic?

Re: Fuel mileage at lower speeds?

Reply #18 – January 23, 2019, 09:43:54 pm

Heat soak may just happen running full throttle up a grade, and your a fuel filter which is also running full bore decides to clog. Than that expensive oil just got cheap. Stuff can happen to a seasoned operator and a well maintained rig.

Re: Fuel mileage at lower speeds?

Reply #19 – January 24, 2019, 09:54:18 am

Quote from: Old phart phred – January 23, 2019, 09:43:54 pm

Heat soak may just happen running full throttle up a grade, and your a fuel filter which is also running full bore decides to clog. Than that expensive oil just got cheap. Stuff can happen to a seasoned operator and a well maintained rig.

Well said. I've had a couple of car diesels clog on the way up grades and if they are going to clog/lose power, it's always on the uphill that is first noticed.

Pierce

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