Fuel Mileage

[reid9439]

My wife and I recently returned from a "Bucket-list" trip of 27 states, going coast to coast (8,200 miles) over a period of 2 1/2 months. The trip was flawless, with no breakdowns or problems.

Since the 2005 Presidio (Mandalay) was new to me, I wanted to track the fuel and mileage stats. The on board computer reads the avg. mileage to be 7.8 mpg. However, after tracking the trip, the actual mpg was 7.1. My Unloaded Vehicle Weight (UVW) is 27,000 lbs. and we pulled a Jeep Liberty for the entire trip.

Here is my question. My coach has a 330 hp Cummins. What would the correlation be of MPG to horsepower if this same coach had a larger engine such as a 360 or 400 hp engine? I know that it would not be feasible to upgrade engines to save mileage and I have no intention to do so. I am only interested in the correlation of horsepower to MPG. If my coach had a 4oohp engine, could I expect better MPG?

[jroan]

I have a Tiffin Phaeton 07 40QDH 33,000 lbs with a Cat 350 HP and I am lucky if I get 7.5 MPG. I drive at 65 MPH interstates pulling a GMC 4 wheel drive 4door pickup. I just figure 7.5 at idle or running at 65 MPH.

Jerry Roan

[wolfe10]

These kind of MPG threads always concern me as they often elicit answers of "what my coach gets" which is a VERY different question than what the OP asked.

Meaningful comparisons between different coaches/different drivers are VERY difficult to make, as there a large number of variables other than just the one asked about (see below).

Let me rephrase your specific question so we will get fewer "apples and oranges" comparisons: Will a modern turbo, inter-cooled 400 HP diesel IN THE SAME COACH, WITH THE SAME TOAD, DRIVEN THE SAME ROADS, DRIVEN THE SAME SPEEDS, AND OF THE SAME EPA GENERATION, etc, etc, get better MPG than one with a 330 HP engine?

Stated another way, since all other variables are assumed to be the same (therefore the same HP would be required to move the coach over each mile of road), can a larger diesel engine produce XX HP more efficiently than a smaller diesel engine.

The answer is MAYBE-- it is certainly possible. Unlike gasoline engines, diesel engines are most efficient at lower RPM and lighter throttle settings. So a larger engine, turning slower and at lower throttle settings can be more efficient than a smaller engine turning faster and at higher throttle settings.

But, remember, the larger engine will weigh more, so coach weight is up. They will also likely require a beefed up frame (heavier) to handle the extra power. The cooling system will have to be larger (heavier) to handle the higher heat load, etc.

Also, few would back out of the throttle of a larger engine while climbing a grade, and a larger engine WILL use more fuel at WOT (Wide Open Throttle) than a smaller one. Larger engines are quite a lot more expensive-- enough more than smaller engines that you could never pay for the difference if fuel consumption even if you agreed to keep your foot out of the throttle.

Just a few of the variables, each of which has a material affect on MPG that make comparisons between coaches less relevant when addressing your question:

1. Weight of coach (single slide 36' or quad slide 40')

2. Weight of toad (Smart Car or 4 wheel drive crew cab)

3. Terrain (flat lander or living in the Rocky Mountains)

4. Speed driven (cruisin at 55 MPH or keeping up with the car traffic at 75MPH)

5. EPA "Generation" (older, pre-emission requirement diesel of newer diesel with emissions equipment)

Brett

[Roodriver]

Great answer Brett. There is a relationship between hp and torque which essentially tells us how much work the engine is capable of on paper, but there are other factors at play which kind of mess up the pencil and paper calculations. We have all seen the ads for oils which reduce friction and as a plus increase mpg. So adding the slipperest oil to the crankcase should increase mpg. Some claim that inflating tires with nitrogen to the proper level will increase mpg, but some say that the compressed air we normally use is already 78% nitrogen.

Going back to Brett's answer, we got terrible mpg going through the Western mountains on US2, and increased our mpg by 5 mpg going on US27 from Georgia to Indiana. I can't compare my gas coach to a dp but I'd bet the end results would be similar.

The one universal thing I truly believe in to increase mpg of both the coach and the driver is to slow down and smell the roses. My bride seems to be happier as well which I'm sure increases my mpg.

Ward

[Guest BillAdams]

Here's the thing. No 2 people with the exact same equipment will get the same mileage. Your driving habits will be different than any other driver. Some drivers are very conservative. Some drivers (like me) drive the speed limit. That means if the speed limit is 75 I drive 75. I can tell you as an absolute certainty that if you drive 75, you will NOT get the MPG that another driver traveling at 55 (or even 65) will get.

I like to get where I am going as soon as possible. That may not be your desire. The faster you go the less your MPG will be. I seems that somewhere around 55 MPG is the most efficient speed, but if I fall asleep and crash into the bridge overpass due to boredom of driving at that speed, then the somewhat increased fuel mileage will be offset by my shortened life span! YMMV.

[sgureasko]

As your moderator has said and Bill, also, there are many variables in fuel mileage. In your case, there are many "tuner" kits out there for your engine, that will give you more mileage and power. It all depends on your wallet, and how much do you use it.

[MikeCanter]

You might all be interested in reading this attached article on MPG. This CAT MPG article explains what effects the MPG. I changed my driving habits after reading it.

CAT MPG Help.pdf

[tbutler]

Some time ago we had a discussion of some ways to improve fuel mileage and fuel economy. One of my points in starting this discussion was to highlight some driving techniques which can be used to maximize your fuel dollar. The main thrust of this idea is that with a few changes you can make improvements in your fuel mileage and these techniques are completely free. Well, not truly free as a few of them will cost you time. But the fact is that your driving habits can be changed and making a few key changes can result in better fuel mileage. What we were discussing in the days of high fuel prices nearing $4.00 still apply at today's prices!

Food for thought!

[desertdeals69]

Getting the best possible fuel milage has always been a goal of mine. Slowing down will usually get you better milage. Back in the 70's when the speed limit was 55 I discovered after getting 5-6 mpg I was now getting 7-8! This was with gas class A's which I had to slow down from 75+ mph.

With regard to the first post, I have found that an engine change delivered my desired results. I started out with a 5.9 mechanical Cummins 190 hp. It was getting 7-8 mpg. I changed the injector pump and injectors for increased power. It ran much better however ran hot and I didn't have enough gears, only a 4 speed no overdrive. Slight increase in milage. I decided to change engines. I selected a ISB electronic Cummins coupled to a Allison MD3060 six speed. I changed out the steel springs with air suspension and changed the front end to a Tuthill independent front suspension, with a Roadmaster swaybar. Changed the rear axle with a Rockwell 1520 and also went from 19.5 wheels to 22.5 Alcoas in all six positions.

In March we left Arizona to go to Perry FMCA and got 11.003 mpg and when we left Perry and went to Pigeon Forge Tn we got 11.034mpg. Our toad is a Chevy Silverado 1/2 ton

[Guest BillAdams]

Wow, that is an amazing mileage increase. I suspect that the $1,000,000 you paid out to make these changes will be repaid at the rate of 3 additional miles per gallon in about 1,000 years!!!! Thanks for helping save the enviroment, no matter the cost.

(No one ever gets me so let me just say right up front that this is all tongue-in-cheek.)

[desertdeals69]

I was in the rv repair business for a good many years, retiring 12 years ago. My hobby is modifying and upgrading my motorhome. My total investment is about 1/5 of what most have spent on their rigs.

Anyone interested in seeing our coach, we will be a Madison in August.

[Guest BillAdams]

Cool. While I over estimated the cost the fact that you could do it for only $200,000 doesn't really change the equation. Again, thank you for helping to save our environment. I doubt anyone else (us common folk anyway) will be able to afford to duplicate your actions but it's very neat to know that it "could" be done. Just speculating here, but I suspect that I could buy a newer coach with the more fuel efficient engine/transmission and be miles ahead (yes, pun intended) of trying to make the massive changes that you have accomplished. Congratulations!

[desertdeals69]

I have invested about $75k including the purchase of the coach. What I had listed was about half of what I have done.

[Koliver]

Reid, Bill, 69er:

Here are some real world figures. Not on the road, but on the water, where there are fewer variables.

I changed my boat engines from Volvo TMD40 @ 145hp to Volvo TAMD41 @ 200hp, so a 38% hp increase and a move up in engine technology, without other changes, what Reid is asking about.

Without changing speed or propellers, my mileage went up. I couldn't resist using some of this extra hp, so I added 1 knot to my cruising speed, a 14% improvement that isn't counted in the figures below, but if substituted, will improve the numbers even more. Then I changed my propellers, engine rpm went down, mileage went up. I did this in June 2000. it took about 5 years to pay back the cost of the engine swap, so for the last 6 yrs, I have been a $winner on the fuel bills. Maybe too much, cause now I am spending it in a Motorhome too.

Bill, actual numbers: Cost of engines, excluding injector service, which I would have had to do anyway, net $2500.00, labor that I couldn't do myself, $under500, $propeller change $350.00 (repitching the existing pair), so with all the things I forgot to mention, say $5k.

Mileage improvement from the engine swap, 10%, from reducing the rpm by changing props, 5%, net 15%. At today's fuel price ($1.239 per litre Chilliwack BC Pilot/Shell =$4.93 /US G) 15% = $.73/gal.

Reid, 69er: Just as I don't know of any other boaters who have access to a smoking deal on newer engines (mine had 1500 hrs on them, so equivalent to 60k miles) and the inclination to do all that work themselves, I don't think you will find any way to do this in a Motorhome without opening your wallet a lot more than I did. 69er may be that exception. Brett is also correct in that a more powerful engine will require a lot of other things to be looked at, as you can't just add hp in a vacuum. In a boat, the hull is usually built for the max hp configuration offered, so other modifications are not usually required.

A lot cheaper to pay attention to the pointers in the CAT article.

[Guest BillAdams]

2 replacement engines for $2500? OK, but my one replacement engine (8V92) cost $18,000. Congratulations on however you pulled that off.

[Guest Wayne77590]

Aw shucks! Texas Gulf Coast to Alabama, 10.6 MPG. Alabama hills to Gaffney, dropped to 9.6. Accidentally hit the reset button after Gaffney. but the hills of Pennsylvania, New York, Connecticut, cut me down to 9.1. Shux! That was in the 340 HP DP, with an Allison 2500 6 speed. I hope the return trip is all down hill.

[Koliver]

Bill:

Here is how the deal worked. A guy with a smaller boat had the 200s, but wanted go go faster still, so moved up. Volvo took his money, lots and lots of it, then had these two used engines sitting around. I got them for 10k, for the pair. I advertised the engines in my boat, still in use, $5k each or $7500 for the pair. I got the $7500, so all I had to do then was the swap. The newer engines were the same block as the old ones, so trannys and mounts etc all fit without modification. Like I said, not too likely a smokin deal like that will come along for someone looking for a bigger engine.

[Spike45]

My wife and I recently returned from a "Bucket-list" trip of 27 states, going coast to coast (8,200 miles) over a period of 2 1/2 months. The trip was flawless, with no breakdowns or problems.

Since the 2005 Presidio (Mandalay) was new to me, I wanted to track the fuel and mileage stats. The on board computer reads the avg. mileage to be 7.8 mpg. However, after tracking the trip, the actual mpg was 7.1. My Unloaded Vehicle Weight (UVW) is 27,000 lbs. and we pulled a Jeep Liberty for the entire trip.

Here is my question. My coach has a 330 hp Cummins. What would the correlation be of MPG to horsepower if this same coach had a larger engine such as a 360 or 400 hp engine? I know that it would not be feasible to upgrade engines to save mileage and I have no intention to do so. I am only interested in the correlation of horsepower to MPG. If my coach had a 4oohp engine, could I expect better MPG?

Diesel engine HP ratings do not necessarily correlate to expected fuel mileage. The controlling factors on fuel mileage are drivetrain gear ratios, road speed, head wind, grade, GVW of coach and toad, and how hard you press on the throttle pedal. Actually, your pressing on the throttle pedal is in direct response to the other factors and how they are affecting the speed you wish to travel. Higher HP engines are as capable of fuel mileage as smaller engines. Chassis manufacturers get drivetrain recommendations on gearing selections form the engine OEM. Engines get their best fuel mileage when operated at the lowest point on the BSFC curve, Brake Specific Fuel Consumption. That usually is above the peak torque RPM of the engine. Higher HP engines can handle increased loads easier and still remain closer to the BSFC RPM with less effort. It is possible for a 600 HP Cummins ISX to achieve 7.5 mpg+ depending on hills, speed, head wind speed and GVW of the 18 wheel truck configuration. Engines do not produce maximum HP continuously. The above factors in your operation combine to present a total load to the engine. The engine ECM (governor) reacts by adjusting fuel delivery to achieve a response to how hard you press on the throttle pedal. Smaller engines will struggle more than larger and may actually achieve poorer fuel consumption figures against a larger engine that is being stressed less.

Gary Spires

Field Service Engineer

Cummins Filtration