A different view of fuel economy/efficiency

[rbattelle]

I was thinking about how to indicate efficiency recently, and came up with a sort of specific fuel consumption: fuel economy normalized by unladen weight. So I looked at a few vehicles, took the manufacturer-advertised maximum fuel economy and divided that by the unladen weight. Specifically, the formula I used was (1/MPG)/weight*10000 (the factor 10000 is just to make the result a little easier to look at). Here's what I found:



Vehicle - MPG - Weight - SFC



2003 Ram 3500 HO Cummins - 18. 6 - 6945 - 0. 0774

2003 Ram 1500 Hemi 4x4 SWB - 17. 0 - 5270 - 0. 1116

Honda Civic 2dr VTEC - 38 - 2657 - 0. 0990

Honda Insight - 66 - 1975 - 0. 0767

Toyota Prius - 60 - 2890 - 0. 0577



Lower SFC is better. The 3500 Cummins is my own personal vehicle, and instead of using the best possible economy, I used my lifetime average after 15000 miles (note my best economy ever was 20. 6 for a full tank). This handicaps the Cummins a bit. Notice that the 3500 gets about the same SFC as a Honda Insight (a fancy Hybrid).

 

[klenger]

Now refigure the numbers with a 10,000 lb trailer behind them.

 

[Josparkz]

Very good

 

[bighammer]

So are you going to argue with a tree hugger that your truck is as efficient as their Honda?

 

[RustyJC]

I believe one weight-compensated measure of fuel efficiency is fuel consumed per ton-mile. For instance, when pulling our 5th wheel, my rig's GCW is 21,180 lbs, or 10. 59 tons. It gets 10 MPG. In 10 miles, the rig generates 10 miles x 10. 59 tons = 105. 9 ton-miles. Fuel consumption is 1 gallon / 105. 9 ton-miles = 0. 00944 gallons per ton-mile.



By contrast, my Acura 3. 2 TL-S weighs probably 3800 lbs, or 1. 9 tons. It gets around 28 MPG on the Interstate. Therefore, for each gallon consumed, it generates 28 miles x 1. 9 tons = 53. 2 ton-miles. Fuel consumption is 1 gallon / 53. 2 ton-miles = 0. 01880 gallons per ton-mile.



Therefore, the diesel-powered truck & 5th wheel is almost twice as fuel-efficient as the Acura (0. 01880 / 0. 00944 = 1. 99) in terms of gallons of fuel consumed per ton-mile.



To be absolutely correct, we should really look at BTU/gallon to reflect the different energy content of #2 diesel versus 93 octane gasoline and express this measurement in terms of BTU/ton-mile, but the gallons/ton-mile measurement is close enough to make the point.



Rusty

 

[klenger]

On the other hand, I wouldn't suggest towing the 5er everywhere you go because it's more efficient than driving the sedan. An 18 wheeler is even more efficient than our trucks.

 

[RustyJC]

Originally posted by klenger

On the other hand, I wouldn't suggest towing the 5er everywhere you go because it's more efficient than driving the sedan. An 18 wheeler is even more efficient than our trucks.

Ken,



Yep, all this is saying is that the 3500/5th wheel is doing more work per gallon of fuel consumed than the Acura - thus, is more fuel efficient. It certainly isn't saying that the truck/RV combo is necessarily cheaper to operate than the Acura to transport my wife and me 500 miles down the road when we don't need the RV. In that case, we don't need to generate the ton-miles that we do when we're pulling the RV.



And, yes, a Class 8 rig will be more fuel efficient (in terms of fuel consumed per ton-mile) than the 3500/5th wheel.



Rusty

 

[onestackdram]

I need to start hanging out with you guys in person, maybe some of your smarts will 'rub' off on me... good lord I will never understand how you guys come up with some of that stuff... LOL



But I do rather enjoy it, it gives me something to think about on boring days. .





Russell

 

[rbattelle]

Rusty,



I like your ton-mile method!

 

[klenger]

onestackdram



Sorry, it an occupational hazard. Engineers, can't live with them, can't kill them.

 

[onestackdram]

oh i understand, my step father is an electrical engineer, and my two 'uncle in-laws' are both engineers, one for Texas Instruments, so i have to listen to them all the time, it usually sounds like giberish, thats why im just a mechanic... although it wont be to long and i will probably have to be an engineer, just to work on engines... LOL

 

[JohnE]

Yep, many ways to skin a CAT. (Sorry, couldn't resist the TIC comment. )



If you go by ton-mile a regular semi is ok, but rail is better, and barge is best.

If you look at total fuel consumption those hyrids are in the same range as passenger car diesel and our pickups are outclassed, especially you over valvers.



Now, jump to my niche in the world, diesel powered hybrid transit buses. Up to &frac12' the fuel consumption along with reduced exhaust and audible emissions.

person-mile is our measurement. If a bus can average 10 MPG carrying 60 people that means they reduced the total fuel comsumption by a lot.



An old saying from my former defense days.

Engineers: Civil (Engineers) build targets for the Mechanical and Electrical (Engineers).

 

[mhenon]

Along the same lines......

I was thinking of putting an extra fuel tank in the truck bed... maybe 30 to 50 gallons or something. I´ve also thought about adding propane injection..... not for huge HP gains by more for efficiency and MPG.

How would the two compare... given the cheaper cost of propane over the energy output of diesel. If we used a 100 gallon tank to compare the two, which would be the better value assuming the tanks and installation cost are identical.

Where´s them engineers????



Mike

 

[LarryB]

To be fair shouldn't you also factor in the vehicle purchase price/mile over the life of the vehicle? How about insurance differences? Seems like the best vehicle would be one purchased for the lowest price, cost the least to insure, and the cheapest to repair/maintain, in addition to the best mpg. The vehicle must also be the most versatile and able to accomplish a wide variety of tasks.

Enter the VW diesel pickup

ARRGGGgggg!!!!

"What?? You wouldn't be caught dead in one of those?"



OOps! We forgot to include the most important criteria for purchasing/driving vehicles in America. At the very top of the list should be Ego!! One must "feel" good about what one drives. I guess this brings us back to Cummins powered Dodge trucks. Hard to beat the "proud" factor of a Cummins diesel engine.



Ah! What do I know?

 

[mgonske]

Originally posted by LarryB

To be fair shouldn't you also factor in the vehicle purchase price/mile over the life of the vehicle?

Indeed, operating cost is the correct numerator of the equation. That would include all the things LarryB mentioned, along with the effective cost (savings) of any tax incentives and residual value (or lack of depreciation) of the capital asset.



BTW, here in Florida, where diesel costs more than regular gas, our trucks would have to get better mileage than a gasser to compete, all those other factors being equal (which they're not).



What most of the above discussions have missed, however, is that the denominator should include only payload . Just because our mighty Cummins gets great mileage for hauling around it's own weight doesn't matter, if the driver is the only payload. (That's the normal case for me on my daily commute--and I'll defer to the cool factor, not to mention that it means I have the capability when I need it to haul more--it's hard to put a price on the options that capability represents, but it would involve the capital cost of two vehicles instead of just one). In any case, the CTD is better than most performance cars when hauling just a driver, and in another league altogether than even the best economy car when hauling a big payload, like my 5th-wheel.



Hmmm, see what happens when an engineer posts after the better part of a bottle of wine with dinner?

 

[klenger]

If overall operating cost was my primary consideration in purchasing a veheicle, I sure as heck wouldn't get a $40,000 truck that gets 18 MPG.

 

[Peter Campbell]

I justified my purchases the easy way- Is it what I want? can I afford it? Does it make any sense? (last question not too high up ) Of course having one of the best engines in the best truck is a good ego boost.

Although if I wanted to know a lot more about all the internal parts of a modern diesel I would buy a FORD 6. 0. I would be paying to have those parts fixed on an almost constant basis, the names of the expensive internal parts would roll right out.

I also really do not want to drive a vehicle that anyone can put a shopping cart handle on. ( Chicken slides dont count).