finding right cruising speed for economical gas mileage

[shark14]

We just purchased a used 2008 Winnebago View with 5400 miles. Does anyone know a good speed to travel at to get the most out of the mileage? It is a V-6 diesel motor.

[Old-Crows]

Shark, I assume by "V-6 diesel motor" you are referring to the Mercedes Benz Sprinter 3.0 liter V-6 turbocharged engine (OM-642) and the five speed automatic transmission. Everything, including the variable output turbocharger is electronically controlled.

First... this engine does not have a 'torque peak'.... it generates the same torque from about 1700 rpm to 3700 rpm. There is no 'peak'.... so forget references to running at, above, or below peak torque.

Second.... the Sprinter drive train tends to keep the transmission up shifting sooner and keeps it in a higher gear... longer, when slowing .... than conventional automatics. I don't do a lot of shifting and just let the engine/drive train management pick the right gearing. On occasion you can get on the back side of the power curve...too slow, gear to high, rpm too low.. like going up a hill and the trans wants to stay in a higher gear than I sense is appropriate or slowing and making a turn in town... you step on it and it wants to bog down and then downshifts. When it's 'boggy,' I shift it down to keep the rpm up in the middle of the torque band. That is not often.

The concept there... without using add on engine monitors and getting more wonky and complicated or increasing the cockpit workload is to keep the engine rpm within the 2200 to 3500 rpm band. Or, simply... keep the tach needle between 11:00 and 12:00 o'clock on the tach. Anywhere in the 'band' is fine and that depends on grades, speeds and headwinds, etc., etc.. It's a balancing act....

As far as the old saw about not running in OD (5th)... that is non-sense that started 30 years ago and just doesn't seem to go away. There were reasons 'back in the day' for not running OD... among which was that the american transmissions were weak designs and just couldn't handle it. Unless your operator manual specifically recommends not running OD under some conditions keep it D. With the Sprinter... it will run all day in 5th at highway speeds. Running OD reduces... most of the time the fuel burn. There are occasions when you might want to go to 4th... you are battling a 30 mph head wind. Put it in D and forget it... unless the rpm drops.

Our VP and old "B" with the Sprinter chassis seems to run most efficiently around 2400-3200 rpm. In the VP 2400 is about 63 to 65 mph .

The three things that increase fuel burn, IMHO, are: weight, high speeds, headwinds. Reduction in weight carried will improve your fuel burn. Moderate speeds will improve your fuel burn. Headwinds increase your fuel burn but you can manage that by adjusting your speed downward (still keeping a safe speed)to avoid 'fighting' the headwind. Got a tail wind...? Yeah... push it up a bit.

The problem here is the shape of the rear of the RV. It's a box. Not good for aerodynamics. The faster you go the higher the drag due to the low pressure 'bubble' that forms behind the square box back of the vehicle. Not a heck of a lot to do here other than to keep the speeds reasonable.

ALSO.... since it is a new to you Benz with the beautiful OM-642 direct injection turbo diesel.... you need to follow the MB recommendation of using ONLY MB specification 229.51 oil. NEVER>>>EVER>>>EVER a non 229.51 spec oil in your engine. Not a single drop!!!! 229.51 is required to keep the diesel particulate filter and egr systems from clogging up. Mobil Formula M is what MB stores use. Valvoline has one. Lubri-Moly 229.51 is available from NAPA. And there are on line stores who carry it.

If you are a DYier .... use only Hengist or Mann filters (NAPA).

[Snowman9000]

I accelerate slowly, and am getting good at coasting up to stops and turns. That helps the town mileage a lot.

I only have a reset-able running average MPG readout, but it is accurate. When I've filled up and then driven in town for a while before getting to a highway, the MPG will be 12+. This is not city stoplight-type driving, but town highways with occasional stops or turns, going say 40 MPH. When we are out onto the highway, that town average starts dropping. I can't quote MPGs at various highway speeds, but I do get 10 or 10+ at 65 in calm level conditions. One of these days I will get results for 55 and 60. Based on limited observations, I would expect to see the 11's under 60. We only have 4700 miles on it so far.

[Gjac]

Snowman9000 wrote:
I've noticed that my average MPG is better in town than at any highway speed, unless there is a tail wind. Even though that means not being in high gear. Otherwise, on the highway, the slower the better. Just my observations.

That is hard to understand unless your average highway speed is 65 mph or higher. My experience is the opposite. When I went to Alaska I averaged 8.5 mpg on the highway driving at 55-62 mph. In Alaska I averaged 8.3 mpg because the roads were dirt could only travel at 40-45 mph which means I was in 3 rd gear most of the time. Weight plays a role also. I was heavily loaded on that trip always traveled with full tank of water and a lot of stuff. Ave for most trips was 8.7 mpg.

[Snowman9000]

I've noticed that my average MPG is better in town than at any highway speed, unless there is a tail wind. Even though that means not being in high gear. Otherwise, on the highway, the slower the better. Just my observations.

[Rollnhome]

KD4UPL wrote:
Around here the minimum speed on interstates is 45 but the limit is 70. Are you saying everybody is happy if you poke along at 50 in a 70? I'm pretty sure they're not.

OP, slower is the answer to a point. You need to be at the engine's peak torque RPM while your transmission is in it's top gear. That should theoretically be your best MPG.

This is what I've been taught. That pushes me along at about 63-65 MPH.

[jefe_4x4]

A lot of interesting experiences here. When trying to push a garage door into the wind, try to make the wind slower. I've been in the RV/4x4 biz for a lot of decades and remember the results of a study about what was the best mileage per gallon of the then current (I'm guessing 1953) automobiles. After much testing, the best mpg was attained @ 43 mph. This is the cutoff on where wind resistance made less difference. I don't know why that stuck in my mind but I have my own experience with a smallish Lance truck camper mounted on a diesel pickup. My experience was similar to the diesel types above in terms of speed v. mpg. Simply, the slower I went the better the fuel mileage was per hour. We recently did a trip over the Mojave Road and were motating maybe 15-20 mph for days on end. I drove three solid days on about 1/4 tank of fuel. About 22 hours on 9 gallons of #2. So, it depends. In Afton Cyn:

The slower you go, the better mpg you get, to a point. Once you have lost the headwind, it makes no sense to go slower yet.
A gas vs. diesel engine have differing curves of that ultimate 'sweet spot' where you get the most speed for the least fuel expenditure. Most of that traces back to where the engine of note makes it's peak torque. On my Dodge Cummins, HO, 6 speed manual, the sweet spot has more to do with the rpms than the headwind. My personal sweet spot for the engine and how much frontal area i have and how much gross weight i have (10,200 pounds) is when the engine is running at 1850 to 1950 rpms. Even though the peak torque is at 1600 rpm. The earlier 12 valve Cummins 6BT made it's best at 1600 rpms. Bump stick and valve timing, plus the differences in injector pumps and compression ratios make the differences. With a gasser, the rpms are usually up higher with modern engines to get to the sweet spot. Still one of my favorite gassers is the tried and true GM454 and its offspring. You need to learn to play it like a violin, but some pretty impressive mpg's can be had, under circumstances you can control, if you can 'feel' where the engine/trans need to be humming. It boils down to a Zen thing. You need to breath with the engine. Feel what it feels. Let'er go down hill. Ease up when climbing. If you know what you are doing you can do better than your factory cruise control. I had a friend who used a manual throttle cable when driving long distance. Kind of dangerous in traffic, but he proved he could go with the flow and get astounding mileage on his Cummins powered pickup with a camper in the bed.
The sad part is you can't teach this 'feel' business. You just need time in the saddle and keep track of your mpg's.
regards, as always, jefe

[Dakzuki]

shark14 wrote:
We just purchased a used 2008 Winnebago View with 5400 miles. Does anyone know a good speed to travel at to get the most out of the mileage? It is a V-6 diesel motor.

On your View slower is better....period. I got stuck in slow (but steady) moving traffic one day. My fuel economy was epic...something like 17-18 mpg. Trouble is we were doing a steady 35 mph or so....not a good freeway cruising speed.

I keep it to about 60-65 and get about 15 MPG while not towing. 60 MPH is also the speed limit for vehicles over 10k GVW in my state.

[ReadyToGo]

I drive my C at 55 mph no matter what the top limit is. I am not the one causing the problem, it's the ones that insist on going at the top speed no matter what the conditions are. The last I saw which was today, the lower limit is 40 mph.
I remember the advertising that stated that speed kills. It's true today as much as yesterday.
55 gives me the best gas mileage so I drive at that speed.

[OldRadios]

I try to let mine cruse where it likes. Seems I can back off the gas pedal on the flats at about 63 MPH. If there is a lot of traffic I go with the flow. Unfortunately that's usually above 65.

[NWnative]

When I bought my Class C, fuel mileage was not one of the factors I used in making the purchase. Travel at the speed you are most comfortable with and enjoy the trip.

[tatest]

For most motorhomes, no-wind conditions, the "best mpg" speed is going to be too slow for safe highway travel in North America. It will be in the 30-40 mph range for most class Cs. EPA throws around numbers like 55 mph, but that is for vehicles with much lower aerodynamic drag, and parasitic loads (primarily A/C) that are much closer to what it takes to actually move the vehicle.

In a tailwind, best mpg speed might be even slower, so that the tailwind does more of the work for a longer period of time. For the same reason, it is usually better to speed up into a headwind. These ideas seem counter intuitive, but are well established in aircraft cruise practice for the past 80 years.

The engine being more efficient at "peak torque RPM" applies to full-throttle conditions. At less than full throttle, peak efficiency for the lower torque output level is at a reduced RPM. For most IC engines it is the minimum RPM at which the engine can produce the power needed.

The engine manufacturer has likely run the suite of specific fuel consumption tests over the full range of power outputs at selected RPMs, and has mapped them into the engine and transmission control programs. This is why so many modern automatic transmissions now have five, six or more gears (or CVT), to let the engine find that optimal MPG condition at expected city and highway cruise speeds, over a range of loads, vehicle and parasitic.

So practically, for highway driving in this country with a RV, best MPG will be at the slowest speed you are willing to travel, given your time constraints, patience, and tolerance for holding up traffic.

[Jim_Shoe]

KD4UPL wrote:
Around here the minimum speed on interstates is 45 but the limit is 70. Are you saying everybody is happy if you poke along at 50 in a 70? I'm pretty sure they're not.

OP, slower is the answer to a point. You need to be at the engine's peak torque RPM while your transmission is in it's top gear. That should theoretically be your best MPG.

Frankly, I don't care if they're happy or not. There's a passing lane, so they can go around. On a two lane road, I keep up with traffic.

[ron_dittmer]

Every motor home will have it's own best fuel economy via MPH sweet spot pending aerodynamics, weight etc, but it won't vary by much. That is why people make blanket statements like 55 MPH. It's going to be right around 55 MPH.

With a Scanguage-II the TPS (Throttle Position Sensor) display number indicates how far down you are pressing the gas peddle. Between that number and real-time fuel economy, you can get a good idea of the optimal speed for best fuel economy.

[Bobbo]

gbopp wrote:
Welcome to the forum. Slower is usually more economical.
In our Class A gas Southwind I usually travel about 58-62 mph. That gives me the best mileage and isn't unreasonably slow.

I don't know about a diesel but, I would think the same is true.

Mine is a gasser, but I have found this to be accurate. Faster uses more fuel. As a matter of fact, 16% faster (72mph) gives 25% reduction in mileage (8mpg down to 6mpg). Slower (55mph) causes it to downshift more often on small grades, causing it to use more fuel. I usually set it on 62.