Fuel economy: gas vs. diesel

No lack of discussion re. gas vs. diesel-- with strong opinions on both sides, but without getting into all that I'm just wondering about the respective fuel costs involved.

These days at least, diesel is more expensive than gas but is it true that diesel is more efficient? IOW, theoretically if there were two identical Class A RVs, both weighing the same and one with a gas motor and the other diesel, would the diesel's better fuel economy come close to offsetting the higher price per gallon?

Diesel is typically more efficient, ie. better mpg over gas but whether the difference is worth it depends.  While the mpg is better, diesel tends to be more expensive per gallon, the initial purchase is considerably more and maintenance costs are somewhat higher.  For me it comes down what is the right tool for the job, if you are towing a large TT, 5th wheel or carrying a heavy load particularly in the mountains while a gasser can in many cases do the job, a diesel is the better choice.  If one is not towing a large trailer or carrying a heavy load, I would argue a gasser is a more economical choice, despite loosing out in the mpg numbers.

Gas, typically a V10 Ford motor, maybe 8 MPG on a good day
Same basic MH with a diesel, typically a Cummins typically 10 on the same good day.
Higher maintenance cost?  Myth plane and simple.  My 6.7L Cummins takes 10 quarts of oil.  Using full synthetic thats about 40 dollars every 10,000 miles.  My wifes Suzuki takes 5 quarts for the same 10,000 miles.  Difference?  20 dollars.  BOY that  really a much higher maintenance cost!
If either breaks down your going to be paying a lot of money in repairs, so in reality you need to plan and have money on hand to pay for it.

All things being equal (which is rarely the case) a diesel engine will get better mileage - but not by a whole lot. A new diesel engine coach, however, will cost significantly more than a new gas engine coach to buy. When shopping for a motorhome, I believe that the coach's quality, condition and floorplan are far more important factors than the type of engine it has.

Over the past 25 years, we've had two gas coaches - a 28 foot Seabreeze with a Chevy 454, and a 32 foot Allegro with a Ford V-10. Our current coach is a diesel pusher. I've done pretty much all the house maintenance on all three coaches, but I've always left the annual service work (oil changes, filters etc.) to RV shops.

Over the past four years, it has cost us three times as much for annual service (scheduled maintenance) than it did for either of our gas coaches. A diesel engine has more filters, bigger filters and greater quantities of everything. Yes, shop rates have increased over the past four years, but not by that much. Diesel engines just cost more to maintain.

Kev

jymbee said:

No lack of discussion re. gas vs. diesel-- with strong opinions on both sides, but without getting into all that I'm just wondering about the respective fuel costs involved.

These days at least, diesel is more expensive than gas but is it true that diesel is more efficient? IOW, theoretically if there were two identical Class A RVs, both weighing the same and one with a gas motor and the other diesel, would the diesel's better fuel economy come close to offsetting the higher price per gallon?

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I had that exact situation many years back. I was driving a body load truck hauling bulk cement. Some of our trucks had an International 6 cylinder gasoline engine, while others had had the engine replaced with a Jimmy (GMC) 4-71 diesel. Performance was approximately the same in both kinds, and all of them were hauling either the bulk cement or bulk sand, usually around 20,000 lbs worth. But when I filled the tanks, the Jimmy got around 11 mph and the gas versions got about 6 mpg. This was over a period of time (not one shot), and in several different trucks with each engine. The trucks were otherwise identical -- same age, same chassis and body, same load, same transmission, etc. The above listed results were approximately the same in all of these cases.

Note that those old gas-burner engines were not the high revving engines so typical today -- they were truck engines with almost the same red line RPM as the diesels had, and I expect the torque was similar too. Those old gas engines had the long stroke and low RPM typical of diesels, and that long stroke is a lot of what gets you the extra torque in diesels -- they're all maxxed at 1800-2500 RPM, vs the 5,000 plus on today's gassers.

Or, my 26,000 lb. max Bounder F-53 (V-10 gas) got around 7-7.5 mpg. My 50,000 lb. Beaver (525 HP diesel) got 5.5-6 mpg. My current 36,000 lb. Ventana (360 HP diesel) gets 8.5-9.5 mpg, depending on conditions. This is all with a Jeep Wrangler as toad.

So sometimes, when prices are right, diesel can cost a lot less, but often the more expensive diesel fuel makes fuel costs somewhat more even. Oil changes on the diesel are more expensive (a lot more oil, among other things), but you don't have to do it as often (15K miles, or once a year) as on a gas engine -- depending on usage, though, too. And, of course, it depends on the specific rig also.

About the only apples-to-apples diesel vs gas fuel comparison would be the Tiffin Allegro Open Road, which was available as gas or diesel versions of essentially the same coach.  I think the 8 vs 10 mpg figure is a fair estimate for that rig.  However, you can run a substantially larger and heavier diesel coach and still get about 8 mpg.

You asked about fuel costs, so the cost/mile is more important than mpg alone.If the diesel version gets 25% more mpg but the fuel costs 25% more, it's a wash.  Prices for both gas and diesel vary all the time, so it's difficult to make an assessment, but I would say that more often than not, diesel at the pump in the USA runs about 20% more than gas. That may give a very slight advantage in cost/mile, but nowhere near enough to justify the higher upfront cost on the basis of fuel alone.

Kev gave the best answer: a combination of other factors are probably more important in the overall choice.

The fuel mileage/cost issue is pretty much irrelevant when the biggest mileage factor is your right foot. In one test conducted by a university several years ago with virtually identical trucks hauling identical loads on identical trips with rotating drivers over the course of a year, there was as much as a 30% difference in fuel mileage from driver to driver. The trucks were hauling empty aluminum cans from a can manufacturing plant to a bottling plant, then hauling full cans to a distribution warehouse, and finally empty trailers back to the can plant. Rinse and repeat daily...

Diesel engines get better mpg than gasoline engines because diesel engines product their power at lower rpms (fewer turns, less fuel consumed).  Diesel engines operate at a much higher compression ratio meaning heavier duty components (to start). 

You have to do ALL the math to determine which one is right for YOUR application.  Diesel fuel now cost more per gallon than gas.  New engines require DEF.  Typically more expensive oil changes (more filters and more oil).  More expensive to buy the equipment (engine and transmission).

One has to determine the break even point considering cost to purchase, cost to operate and cost to maintain / miles driven.  So, fuel economy isn't just a matter of comparing mpg to mpg.  You need to have the whole equation.

Sometimes, you just can't find a gasoline engine with the towing/hauling capacity you require (regardless of cost). 

I don't think that such blanket statements like 'diesel is cheaper than gas' or 'diesel is more efficient' hold any water. Gas and diesel are two different technologies for two different jobs. I have seen diesel selling for less than gas and I have seen diesels not being as efficient. I don't even think they can be compared fairly. It is a case of different technologies for different jobs. Gas is more efficient at long distance hauling at a steady speed. Diesels excel because of their tremendous torque for jobs like hauling up a mountain or hauling a heavy load. I feel you need to figure out what one works best for you in your situation and not base it on how much you think one system or the other will cost. Safety is more important than saving.

Diesel engines exist because they are indeed more efficient.  Gas engines have closed the gap somewhat, but there is no getting around the fact that diesel fuel has more BTU's per gallon.  Diesel engines have higher compression ratios and that adds even more efficiency.  A gas engine has a throttle plate that restricts air flow and that causes pumping loss as the pistons pull against a vacuum.

All the talk about torque is just talk and the advertisers are happy to supply large numbers to those that need to talk about those numbers. 

Efficiency is one thing.  Cost per mile is another.  And it is regionally dependent.  And isn't cost per mile really the important thing?

For instance, in my area, I can fill my 2015 Chevy 2500 6.0 with E85 at $1.60 a gallon currently.  The pickup averages 11 mpg for all miles on the stuff.... city, rural, highway, hauling, etc.  That equates to 14.5  cents a mile at current E85 fuel price. 

For a equivalent diesel in a 2500 to match the 14.5 cents a mile, it would have to get.... for all miles, not just highway.... an average 20 mpg at the current $3 a gallon price for diesel in my area.  Might be doable in the right hands, but that is just a break even.  In this situation,  a diesel equivalent offers no advantage to the gas engine in my 2500.... on fuel cost alone, which the OP started this thread about.

On regular E10 stuff, my 2500 averages about 14 mpg for all miles.  That plays out of be about 17.5 cents a mile fuel cost at current $2.45 for regular.  Diesel might be more cost effective in this situation.  A diesel 2500 would only have to get 17 mpg average for all miles at $3 a gallon to match the fuel cost per mile of my 2500 on regular gas.  That is probably doable most any day of the week in the right hands.

Folks really focus in on mpg alone, but fail to use that to figure what their actual fuel cost per mile is so they can evaluate which fuel is better.  Higher mpg numbers do indeed feel good, but mean very little if it is costing more per mile to do it.  And each area of the country is different.  What plays out in one area likely will not be the same in another.

Gizmo said:

Diesel is typically more efficient, IE. better mpg over gas but whether the difference is worth it depends.  While the mpg is better, diesel tends to be more expensive per gallon, the initial purchase is considerably more and maintenance costs are somewhat higher.  For me it comes down what is the right tool for the job, if you are towing a large TT, 5th wheel or carrying a heavy load particularly in the mountains while a gasser can in many cases do the job, a diesel is the better choice.  If one is not towing a large trailer or carrying a heavy load, I would argue a gasser is a more economical choice, despite loosing out in the mpg numbers.

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this is the best comparison I've read by far - its not really about the mileage per say so much as the power that the diesel gives you when towing, yes the mileage is better especially in the mountains where my old gasser would do 5-6 mpg the diesel will do 11-13 mpg but the power pulling is worth the extra upfront cost

Another vote for Gizmo?s response. We just ordered a 37? diesel pusher. It won?t be less expensive than a gasser, due to the initial cost of the engine and components, maintenance, and the fuel itself. However we travel almost all the time in the mountain west, and we really wanted the climbing ability going up and the exhaust brake going down. In addition, the bigger coaches with the nicer amenities were diesels because they can tow heavier loads.

steveblonde said:

this is the best comparison I've read by far - its not really about the mileage per say so much as the power that the diesel gives you when towing, yes the mileage is better especially in the mountains where my old gasser would do 5-6 mpg the diesel will do 11-13 mpg but the power pulling is worth the extra upfront cost

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To keep things in perspective, power is Torque x RPM.  A Ford V-10 with 424 lb-ft of torque at 3500 RPM can deliver almost as much power as a Cummins 8.3L diesel with 1100 lb-ft of torque at 1300 RPM.

The V-10 puts out a bit less power than 2018 version of the Ford 6.7L diesel with 935 lb-ft at 1800 RPM.  Prior to 2018 the two were about evenly matched.

In either case, the difference is when you're ascending a hill the diesel will keep chugging along close to it's cruising RPM after you downshift, while you'll have to let the V-10 rev up from it's cruising speed of 2500 RPM to 3500 RPM.

As Lou implied horsepower is horsepower. The reason we think diesel is superior lies in the fact that we are often reluctant to run the V10 at the rpm necessary to to develope the required horsepower due to the noise and the perceived potential to damage the engine. In actuality the V10 will maintain climbing ability just as well for a given power to weight ratio.

Ernie

All this torque and horsepower ratings is meaningless. Just remember there are no 18 wheelers that use gasoline. There must be a reason.

My 305 HP V10 does just fine out in the mountains and valleys of the Western USA. It revs going up a grade, and she'll scream on the way down if your not using good sensible driving methods. I maintain 40-45 mph up a 6% grade towing a 4,650# Jeep. I maintain 40-45 mph going down a 6% grade using the transmission and good braking patterns.

I am moving into a diesel rig with a higher tow rating, because my equipment has gained weight. I started off with a 3,800# TJ that now weighs 4,650#, and my JKU weighs around 5,600#. I need a higher towing capacity that comes with a diesel coach on a Freightliner M2 106 chassis.

SeilerBird said:

All this torque and horsepower ratings is meaningless. Just remember there are no 18 wheelers that use gasoline. There must be a reason.

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There is:  there is no gasoline engine of the same displacement that will produce the same level of torque at the same RPM level as will a diesel.  18 wheelers need torque to get that weight moving. The same applies to diesel-electric locomotives, etc. 

SeilerBird said:

All this torque and horsepower ratings is meaningless. Just remember there are no 18 wheelers that use gasoline. There must be a reason.

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One can't a million miles out of a gasser.

SeilerBird said:

All this torque and horsepower ratings is meaningless. Just remember there are no 18 wheelers that use gasoline. There must be a reason.

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Yep, the reason is that it costs less.  They would use skunk pee if it would save a nickel.