Since I'm an automotive engineer (retired) I ran some calculations for you that gives the respective HP required to move the following loads at different speeds.
First, assuming you are at sea level to 1000 feet altitude on flat ground with no wind. Your gross rig weight is 15,000 lbs for all calculations. Frontal width is 8' (96") and you have 12" of ground clearance.
With a height of 10' 6" (126") and traveling at the following noted speeds your HP requirement would be as follows for rolling resistance, air resistance, and total with tire/road friction and drive train friction loss etc:
55 MPH = Roll: 18.3 HP --- Air: 50.6 HP --- Total: 79 HP
60 MPH = Roll: 20.7 HP --- Air: 65.7 HP --- Total: 99 HP
65 MPH = Roll: 23.3 HP --- Air: 83.5 HP --- Total: 123 HP
70 MPH = Roll: 26.0 HP --- Air: 104.3 HP --- Total: 150 HP
Increase frontal height to 12' (144") with other factors remaining the same, HP required is as follows:
55 MPH = Roll: 18.3 HP --- Air: 58.6 HP --- Total: 88 HP
60 MPH = Roll: 20.7 HP --- Air: 70.0 HP --- Total: 111 HP
65 MPH = Roll: 23.3 HP --- Air: 96.7 HP --- Total: 138 HP
70 MPH = Roll: 26.0 HP --- Air: 120.7 HP --- Total: 169 HP
These are close estimated calculations based on standard normal configurations. It should be noted that slight differences may occur depending upon actual vehicle characteristics and aerodynamics employed etc. Tire inflation/footprint/etc will have a large effect on HP required.
2004 Chev 2500HD D/A crew cab LB 4X4 - Air Bags - Loaded
915 Lance Camper with 2'X 8' rear porch (my own design n build)
29 ft Carri-lite 5th wheel - 1 large slide - specially built
36 ft Carriage - 3 axle 5'er -NOW SOLD- Looking at some new 5'ers
Those numbers are about what I had figured out for my old truck and a few various setups. I had 15K and 70# of FA at 105 HP at 60 mph, 15K and 90# at 135, and 18K at 70# at 115HP.
I found with my truck, if I go 60 I can get about 17.5-18 empty, at 70 I am down to 15-16 mpg. I have not really pulled long enough distances with a trailer to figure that mpg out, and really, I have only done 4-5 total freeway trips with it to figure out that mileage. as most of my driving is mixed city and freeway, with a few miles pulling my bobcat etc.
I did do a number of trips from Seattle to Ellensburg and back over I90 thru the cascades with my old Truck and TT, along with a couple pulling my equiment trailer and a mini excavator. Typically 15K or there about, 90# with the TT, and 70# with the ET, I typically got 1-2 mpg more with the ET than the TT, pulled most hills about 5-8 mph faster, and 1 gear taller. So wind resistance IMHO is more of a factor for mpg, hp needed etc for towing than wt itself!
marty
05 Chev CC D/A LS Dooley
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
00 Chev C2500, V5700, 4L80E, 4.10, base truck, no options!
92 Red-e-haul 12K equipment trailer
3 Single axle utility trailers
Fast Mopar wrote: Considering your significant air resistance, I think 15.4 mpg is very impressive. As you stated, speed makes a huge difference. I would guess that you would have been at 10-11 mpg at 70-75 mph.
Do you really think dropping 10mph would help that much? I may have to try a trip at 55-60. It would be worth it if I could get up from 10 to 15mpg just sacrificing 10 mph.
Food for thought. An engineering thumbrule is it takes eight times the power for a given vehicle to double it's speed, eg 30 mph vs 60 mph.
Actually 4 times.....3600/900 = 4
Run Travelnutz's numbers above. Wind resistance increases with the square of the difference in speed. Power required to overcome it increases with the cube of the difference. It works the same as pump laws.
* This post was
edited 03/04/08 04:42am by airduds *
Send this data to the government officials. Instead of mandating slower speeds on all roads they let the states increase speed limits but then don't even patrol or enforce those!! Kentucky just raised it's speed limit on interstates to 70 and now a great number of drivers are hauling a** at 80. Too many folks will now push "10 over the speedlimit" and think they are legal. Of course with the lack of enforcement they probably will get by with it!!
I just purchased a Dodge with the 5.9 and record MPG of over 20 non towing if I drive 60 or less. The overhead lie o meter is fairly accurate and if I kick the speed up to 75 I can watch the mileage fall from the low 20's very quickly.
It would seem logical that if we are going to reduce consumption of fuel by mandating higher CAFE REGULATIONS (combined average fuel economy) for auto manufacturers, we should take a look at driving them slower. Can you imagine what engineers go through trying to choke down an engine to get two more miles per gallon?? Can you imagine how much weight they cut from the vehicle? All we have to do is slow down 10 to 15 miles per hour and those higher mpg figures are here today!!! I'm not saying run 45 mph. Just drive 60!! Far too many people run 80 consistently and get used to it. They go nuts when slowing back to 65. Speeding is just a bad habit
Sorry, you got me! I committed a typo on the average price per gal of the diesel. It was supposed to have been $3.4666/gal and I typed $346.66/gal. I had missed the mistake when I re-read my post. Must be these old eyes. I was trying to be very precise and probably should have rounded the average fuel price to $3.47/gal. I think most people know what I had meant. Just for kicks, how long before my mistaken posted average price might be not so fictitious? It's inching that way. Kind of scary!!!
When we left, the price going south was in the $3.20-$3.30 range. Large increases occurred while we were in Florida and the return trip north had per gallon prices in the $3.58-$3.69 range. Diesel in South Florida was a min of $3.69 a gallon when we left. We left home with a totally full fuel tank having slowly filled to a couple inches of the top of the filler tube and did the same when we got back at the same station 1.7 miles from our home. Yes, I added the 1.7 miles times 2 and the fuel used for the 3.4 miles into the total figures so the numbers are very accurate. The $733.51 cost figure came directly from our pump receipts with .22 gallons added at $3.4666/gal for a $.76 add on.
You are spot on. Simply put, IF you are driving your 10' 6" high 15,000 lb rig into a 10 mile per hour head wind, your air resistance is as if you are moving at 65 MPH instead of the indicated 55 MPH speedometer reading. This increases the hp required from 51 HP to 83 HP for the air resistance in this average sized rig. Therefore, it requires more fuel to be burned to produce the needed extra HP. Down shifting eases the load on the engine by letting it rev higher but it still requires approximately the same amount of fuel burned to do the actual work required. No increase in rolling resistance and little added mechanical efficiency loss, just air resistance.
Even a slight 1% uphill grade greatly increases the HP required to maintain rig speed. 22 HP more is required for each 1% of grade climbed even at 55 MPH for a 15,000 lb rig with a 10' 6" height. At 70 MPH, the factor is 28 HP additional for each 1% grade. A 6% grade requires an additional 168 HP just to maintain 70 MPH. Numbers broken down are as follows:
70 MPH in a rig weighing 15,000 lbs total climbing a 6% grade requires 26 HP for rolling resistance, 104 HP for air resistance in still air conditions, and 168 HP for grade resistance. This combo totals 343 HP required with tire friction and mechanical efficiency losses. More than "pedal to the metal" for most tow vehicles running at their highest OEM output. Better hope there's no head wind!
Same rig/conditions climbing a 6% grade at 55 MPH requires 18 HP rolling resistance, 51 HP air resistance, and 132 HP for grade resistance making the total power need at 231 HP. Most tow vehicles can do this.
Maybe you can see why an engine producing it's highest "torque output" (Turning force of the drive shaft) at low RPM's allows it to pull and climb much easier and at a lower fuel consumption. Higher RPM's require more fuel flow to do so. Notice the 18 wheelers have high torque low RPM engines that pull 80,000 lbs up a hill many times every day. Try pulling that 80,000 lbs with your tow vehicle up the same hill and you'll get the drift. Can you say about 5 MPH max with your engine screaming if at all?
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