Gas Vs Diesel
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Should I buy gas or diesel? This question has fueled many a heated debate over the years. A universal response you could expect to hear to this question goes something like this; diesels cost more than gas, don’t buy a diesel unless you plan to put a bunch of miles on it and keep it for a long time. That’s the only way you can justify the extra cost of a diesel.
Several years ago there was some truth to this somewhat typical response. But things are changing, especially when you are talking about motorhomes. Today, the cost of some gasoline powered motorhomes is about the same as many of the entry level diesel motorhomes available on the market. This higher cost is associated with advancements in gasoline RV chassis’ design; with greater Gross Vehicle Weight Ratings (GVWR) to accommodate coach designs offering 3 and 4 slideouts on the RV. And don’t forget to factor in new technology in gasoline engine and transmission design over the past several years too.
The old response, of diesels costing more, does hold true with some of the specialized diesel chassis manufacturers, but it’s not just the engine that accounts for the higher price tag. You are moving to a higher plateau across the board. For the most part gasoline RV chassis’ are heavy duty truck chassis’ modified for Recreation Vehicle use, whereas many diesel chassis’ are a bus chassis. These bus chassis’ have even greater GVWR’s, air brakes, air ride suspension systems, larger transmissions, rear mounted diesel engines and more. They handle better, ride better and cost more. I always say you get what you pay for.
Other than the cost factor, what are we really talking about when it comes down to gas versus diesel? Say for example, in a truck you plan to use for towing a trailer. This is where we need to factor in the engine itself. This is also where torque and horsepower come into the equation.
Explaining torque and horsepower can get extremely technical, and we want to keep this simple and easy to understand. With that said, torque is basically the force or energy required to move something. Torque is the measurement of force, and force is measured in reference to a twisting or rotating shaft. In English terms torque is measured in pounds-feet, but is more commonly referred to as foot-pounds. So in keeping it simple let’s just say that torque can be thought of as the amount of turning force it takes to move one pound of weight the distance of one foot.
Horsepower on the other hand is torque X RPM’s. Torque is how much work is being done, and horsepower is how fast you get the actual work done. What’s interesting is, an engine rated at 350 horsepower only produces that horsepower at a rated peak power RPM. This RPM range, for a gasoline engine, is often between 5,000 and 6,000 RPM’s. When an engine is idling the horsepower is significantly lower, and as the RPM’s increase so does the horsepower. When you are towing a trailer the engine speed is more likely to be in a lower RPM range, which means you have much less available horsepower than the engines rated horsepower. Horsepower is measured by a dynamometer. A dynamometer puts a load on the engine and measures the amount of power the engine produces against the load at various speeds. Even at the rated peak power RPM you really won’t get the rated horsepower, because a percentage is lost through auxiliary equipment on the engine and the process of getting it back to the wheels.
In a diesel engine the horsepower peaks at a lower RPM, and there is more torque at a lower RPM compared to a gasoline engine. This results in a diesel engine having much more power at a lower RPM, around the RPM range you will be towing at. This higher torque and higher horsepower at a lower RPM equates to better towing.
There are many other factors involved in the question of gas versus diesel that you will need to consider. What are the maintenance costs involved, cost difference between fuel types, fuel economy, your budget, and the resale value? Whether it’s for a tow vehicle or a motorhome, take your time and make a well informed decision when comparing gas to diesel.
If the question is which truck will tow more or which motorhome has more torque the diesel will win hands down, but I honestly have no complaints with our gas powered motorhome either. Especially when considering the price!
Happy Camping
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Very good comparison and information! Gas or diesel??? The battle goes on…..hopefully we will be able to afford to drive either one in the next few years.
This is very easy. Gas engines cost less period. To buy, maintain, operate and repair. There is no debate about this at all. Other than that the diesel is great. Diesel engines will last much longer than gas engines but you could buy several gas engines with what it would cost to replace a diesel engine. We like our Diesel a LOT mostly because it is in the back of the motorhome and we don’t have to listen to it. Also we like the diesel chassis better than our gas was. Darrel, TVNAV.com
Every gasoline engine person I have spoken with in our travels admits that the diesel engine gets better gas mileage. My 2001 Ford F250 7.3 diesel has got as much as 15.7 mpg pulling my 32′ fifth wheel. That mpg was on fairly level highways, the mountain areas it averages 12.5 according to my factory computer readout. The better gas mileage and the longevity of the engine makes the diesel my personal choice.
Good article but it omits the factor of the rear axle ratio which has a large effect on pulling power and fuel economy. I have a diesel so I don’t know how this figures in for gas.
I would dearly like to find a power vs fuel economy curve (substitute RPM or speed for power) but have been unable to locate such through Google or Ford (I have an F-550 DRW, 6.0 L diesel with a 4.88:1 axle ratio.Got any suggestions?
You’ve forgotten to mention one a very important point. I think in most cases there is a good deal of difference between the tow rating, Most diesel chassis are set up to tow 10,000 lbs. An awful lot of gas OWNERS IGNORE the rating and pull what they want. I consider this the same as pulling a 6500 lb truck or hummer with a 5000 lb tow bar.
When we were thinking of buying a diesel motorhome a LOT of people said we would get double or more the mileage with the diesel. That did not happen. We got 3 mpg with our 8.1L gas motorhome and we are getting 5 with our diesel. Yes we pull a big trailer with those numbers. Without the trailer we got about 6 and 8 respectively. So at the current fuel prices it costs us $30.65 less per hundred miles with our diesel. Hmmm, now that I actually figure it even though we only put on about 6000 miles per year now we are saving $1839.00 per year with the diesel. I would doubt that it costs that much more to operate and maintain the diesel vs. gas so I’ll retract that statement. Darrel, TVNAV.com
Hmmm… A lot of commentary each way, though apparently biased towards Gas. I would like to respond to each, if I may.
Larry: I personally think we need to start looking in a different direction. Yes, I can see both gas and diesel running for a long time yet, but there are people out there right now that are proving that electric is really far more efficient in many ways. A reciprocating engine literally wastes a lot of the power the fuel itself generates by the incessant starting, stopping and reversing of each cylinder. In a gas engine, one shot of fuel has to drive that cylinder through four complete strokes. In a diesel engine, that same shot only has to drive the cylinder through two, realizing more push at lower speeds. However, electric does not require any stop-and-start motions at all, being completely rotary, thus providing power full-time and requiring as a result less than a quarter of the horsepower that a fuel engine produces.
Darrel 1st: Yes, you’re right. A gasoline engine is much cheaper. But its average lifespan is only a little over 100,000 miles, maybe as much as 125,000 when taking all engines into account. At an average of 60mph, This equates to approximately 70 days of steady running. I will acknowledge however, that maintaining a steady speed for a long time is much less wearing than the stop-and-go environment most gasolines live in.
On the other hand, the average diesel engine is designed for 10,000 hours or more before requiring a rebuild, roughly 6x as long as a gasoline engine. This makes part of the economy of a diesel a given. In fact, this is why the railroads started the switch from Steam to diesel back in the 1930s. The other part is that they realized that electric motors provided maximum torque at 0mph, where the power is usually needed the most. A diesel locomotive rated at 1500 horsepower can pull a load equivalent to over 100 tractor-trailer rigs running 350hp diesel engines each. Of course, today’s locomotives are rated at 4500hp or more. The economy is there, but you can’t see it in the same way you do gasoline.
Bruce: Essentially, you’re right, diesel economy is better, depending on how you drive it. Darrel’s second comment tries to dispute this, but when you figure his diesel coach is also pulling roughly 5,000 pounds (or more) behind a 10,000 pound coach with an engine little bigger than the one in your F250, you can see where he’s losing out. He doesn’t point out that he’s getting 60% better fuel mileage with his diesel, but rather that he’s only getting 5mpg. His numbers without the trailer are slightly suspect because I don’t know his driving style. If (for example) he tends to drive the coach unloaded at 75mph, then he’s not going to do as well as, say, 65mph. There are too many variables in his comparison. Even so, he proves that the savings he does get is worth almost $2000 per year over the gasoline engine.
Ian: You bring up a good point, but you also point out one of the drawbacks as well. At 4.88 final drive ratio (what you call rear-axle ratio) it takes less torque to start a load rolling but a lot more rpm to maintain speed through the average automatic transmission. As you shift gears, it takes more power to realize the same torque on the ground. At that rate, high gear would have to be a much, much lower ratio to the driveshaft, possibly as low .8:1 engine-to-driveshaft. It is very possible that transmission ratios need to be changed in order to have the engine running at its best economy at road speeds. Changing the final drive ratio alone changes the rest of the power curve more drastically than any one gear in the transmission.
Wayne: Your comment impacts directly on Darrel’s comment after yours. My guess is that Darrel is edging or even exceeding the GVWR when pulling his trailer, which is why he only gets 5mpg instead of the 7 to 9 I expect he should be getting.
Darrel2: I’m happy to see that you are realizing the potential savings of Diesel. But I think you can do more by modifying your driving and towing habits. I don’t know your particular circumstances, but I wonder if you can’t reduce the load somewhat in your trailer or find a way to reduce drag on the coach/trailer/combination. I’ve read of and seen different air-spoiling techniques designed for larger vehicles. I’d recommend analyzing some and perhaps adopting one to improve your mileage.
I commented above that I suspect your driving habits may be one reason your mileage isn’t average; I do not mean to imply insult. Instead, experiment while you are on the road. Measure the gas mileage between points A and B driving your normal way, then try reducing your speed (or maybe increasing) by 5mph. Over a longer run of 200 miles or more, the mileage should become obvious whether its higher or lower and you can further adjust accordingly.
The fact that you tow such a large trailer leads me to believe that you A: live out of your coach and B: frequent the antique/comics/baseball card shows in shopping malls around the country. As such, an experiment as I outline should be easy for you. If I’m wrong, considering the mileage you report, you should be able to work out a variation of the experiment easily enough. This alone could help you realize even more savings from your diesel.
My own views on the matter are simple: different technology is needed if RVers want to stay on the road in today’s high fuel prices. While others are trying to create new and different technologies that in the short run will be more expensive and less reliable, I believe that the most efficient technology–at least for now–is already riding on roads of steel. It shouldn’t be that hard to convert it to highway use. In a way, GM is; but the Volt is essentially a prototype. As that technology gains experience on the highways, I think ways will be found to make it more powerful and less expensive at the same time.
David, your assumptions about my driving habits and what we do couldn’t be further from fact. We have never, ever went to an antique/comics/baseball card show nor do we ever intend to do that. We have a slightly more interesting life that that
Darrel, TVNAV.com
Picture this. 2004 Ford F250 V-10 30! fiver 8500 ft Selected low gear foot to the floor and 15 miles an hour. Two years later 2007 Chev duramax allison same fiver ,same road , as fast and as easy as I cared to drive.My overall trip of 8500 miles came to the diesal costing me .03 cents less not 3cents but 3 tenths miles per gal. So when and if some major service comes the diesel will cost more but the added performance and the built in “jake brake” is worth every penny.
How does the coming availability of bio-diesel affect the equation? Would buying a diesel motorhome now looking forward to running bio-diesel without having to make any modifications to the engine tilt the decision over to the diesel, since with a gasoline engine you are stuck with petroleum up to only a 10% ethanol mix? To convert a gas engine to accept flex fuels (up to 85% ethanol) requires expensive modifications. And doesn’t the resale value of a diesel offset the initially higher price? I currently drive a gas motorhome, but am seriously considering moving to diesel.
Darrel:
Apologies if you took my statements as an insult, they weren’t intended as such. I did clearly state that I didn’t know how you drove or what you did, but made guesses based on what you described. I’m trying to be supportive here, not antagonistic.
Thomas, I have a question for you. You had a V-10 gas engine and a diesel on the same hill and clearly demonstrated that the diesel had more overall pulling power. The question is: was the diesel a Turbo? High altitude is notorious for sapping power from a normally-aspirated engine. Direct injection helps to reduce this effect, but Turbo almost completely eliminates it. Of course, Turbo also means it uses more fuel when you are demanding that power.
As you say, the added performance and “jake brake” are definitely worth the added cost.
What happened to the price of diesel these last few years?? It was always cheaper to buy than gas for many years. According to a friend ithat works in a refinery it takes less refining than gas. Are we getting taken advantage of?
David, did I miss something on diesel engines? If memory serves me the two strokes of the diesel you mention would be for two stroke Detroit engines(maybe others). Most current diesel engines are 4 stroke. Emissions and stuff. I never even realized they made a 2 stroke diesel until I started looking at coaches for m/h conversion. Then it made sense on the difference in sound. Good info in you post. I own a 2005 Duramax Dually and a 37′ Holiday Rambler. Can’t imagine trying to pull it with a big block gasser as I ran across the scales at 25,700lbs. Mileage wasn’t real high but then I was in a hurry. Still got 10-11mpg in the mountains so I’m pleased.
Tim,
To the best of my knowledge and what I’ve been told, the higher cost of deisel as compared to gas has mostly to do with government taxes being higher on deisel because of trucks using our highway. So 1.truckers have to charge more to cover their costs; 2. We have to pay more for everything they deliver if we want or need it; 3. The government keeps taking what they make from every gallon and mostly spend it wastefully; 4. We all complain but can’t drill for our own oil so we can protect the caribou! It all makes perfect sense, don’t you think?????????
I love the debate between gas or diesel. I can only speak from my experience with my diesel motorhome. At 65 miles an hour I get 10.8 miles per gallon. If I drive 55 I get 12.4 miles per gallon. We have traveled with friends that have gas engined motorhomes and they get between 5 to 6 miles per gallon. I probably paid more for a diesel engine but given the economy and longer engine life I feel I made the right choice.
When we first bought this motorhome, a Rockwood on a Sparten chassis, it had a differential of 3:73. I regeared it to 3:23. I think I did the right thing in doing that. My price for diesel is higher here in California than other states so feel I am really being ripped off. When on vacation last May of 2007 we paid about the same for diesel as those using unleaded.
Bob
Gas vs diesel is a very interesting subject and need more though. So I will appreciate some guidances. I will buy a Gulfstream Bounty Hunter probably next year. Reason is for the built in garage of course. It is offered with gas engines, Ford V-10 or 8.1 liters Workhorse, or a diesel made by Cummins on a Freightliner chassis. All are having nearly the same horsepower. It will be very interesting to see a comparison test of the three in the exact same conditions. I hope Gulfstream will have the guts to propose it. I will appreciate constructive recommendations and your evaluation of what gas milleage I could expect from the different versions. My intend is to do approximately 5 to 10 thousands miles per year and to keep it for more than ten years. If you know of any good road test with documented fuel consumption I will appreciate any references.
Regards to all haapy rvers.
Daniel.