Recently did a 3908 mile trip from Austin, TX to Las Vegas, a 2 week tour to the Grand Canyon, Bryce Canyon, 2 Utah state parks, Zion and back to Las Vegas. After that we went to Cedar Breaks, via Panguitch to Escalante, Capitol Reef, Blanding, Hovenweep, Cortez, Durango, Pagosa Springs, Chama, Santa Fe, Clines Corners, Santa Rosa, Ft. Sumner, Clovis, Lubbock, Abilene, Austin. 8.3 mpg average for the whole trip with no deduction for generator run time as I forgot to log the time when I filled up. From Austin to Las Vegas, mostly on the interstate with the generator and 2 AC running at about 62 mph most of the time -7.43 mpg. For the 2 weeks visiting the Grand Canyon, etc. 8.54 mpg. From Las Vegas to Austin - 9.13 mpg. I used economy mode all the time for the entire trip. The one thing that was really different on the return was that a lot of it was driven at about 40 to 45 mph in 5th gear as a large amount of it was on scenic roads without much traffic. About half of the return we didn't have to run the generator. Still there was a climb to 10,000 ft at Cedar Breaks, and another climb after Escalante and a lot of just up and down. Towing a 3000 lb toad too, but no aqua hot to use diesel. At one time I thought I got better than 11 mpg between Las Vegas and Santa Fe, but decided that I didn't get the tank really full in Santa Fe, still it was probably close to 10 mpg on that portion. After Cortez we were in 6th gear most of the time.
I always thought from comments on this forum that the best mpg was in 6th gear at about 62 mph. Apparently one can get even better mpg in 5th at about 40 - 45 mph. Can anyone with a fancy program on their computer with all the engine parameters, mpg, etc. verify this? I'm talking about the bus style coach, I know the Grand Villas have better mileage.
Two years ago I got 12 mpg going slow East To West on MI-41 in Michigan's Upper Peninsula.
I often travel at 62 mph, too.
But better mpg is at slower speeds. Wind resistance is the key.
All measured by SilverLeaf's VMS 240 CL
best, paul
Best mileage is top gear at rpm that produces maximum torque for your engine. On My ISM that is 1500 rpm, which happens to be geared to produce 70 mph in my 40 foot U-320
Tim.... do you have data to back that up? I'll bet mileage would be better at max torque in 5th gear. For that matter the slower the better at max torque RPM. I think wind resistance goes up with the square of speed. See http://en.wikipedia.org/wiki/Drag_ (http://en.wikipedia.org/wiki/Drag_)(physics)
George
Yup, sure you are correct, meant best mileage at Max torque. Don't intend to travel at 45 to minimize wind resistance. Drive as fast as your budget allows, when the money runs out park it and enjoy it where you stop!
According to this link, there is an optimal speed for each vehicle and the slowest is not the best. The question is what is best for a FT.
Mpg For Speed - Fuel Efficiency Vs. Speed (http://www.mpgforspeed.com/)
Any ideas? We normally cruise at 55mph. That may not give optimal mileage, but Pat is "optimized" so to speak!
For those that like reading, take a snoop at what Cummins sez.
Enjoy
Dave M
Yes, I suspect that peak torque RPM in 5th gear would give better MPG, but unless you are about to run out of fuel, probably not worth it (frustration factor> MPG improvement).
Brett
Excellent Cummins reference! I need to read it every year. Thanks!
And here is another excellent one, by Caterpillar Corp: http://www.catrvclub.org/PDF_Docs/Understanding_Perf.pdf (http://www.catrvclub.org/PDF_Docs/Understanding_Perf.pdf)
Both documents apply to all heavy vehicles, irrespective of engine manufacturer.
Brett
Maybe if the fuel mileage issue it too big, staying at home and watching "Wheel of Fortune" is always an out. ;D
Dave M
I remember a period from my past life when I was studying power requirements and streamlining. I found a lot of data at Drive55.org - Your Brain on Mathematics (http://drive55.org/content/view/88/25/)
There is some very good reading there. I set up a little spreadsheet into which I could put in a little data and see how the increased air drag affected the horsepower requirements. The key thing to remember is that the horsepower required to overcome air resistance is cubed as a function of speed. That means is takes 8 times as much power to double your speed. (2*2*2 = 8)
For example, consider the power requirements to run 50 mph versus 75 mph. Let's say we have a frontal area of 80 sq. ft on our coach, with a coefficient of drag of 1. (basically a flat plate) It will take about 69 horsepower to push our coach through the air at 50 mph. Now, the total requirements are much higher due to rolling resistance, etc., as the Cummins PDF explains. Right now I'm talking just air resistance.
If we decide to run at 75 mph instead of 50 mph, we are now running at 1.5 times the original 50 mph. So, 1.5^3 = 3.375 times as much power to run 75 mph. So at 75 mph it now requires 232 horsepower to overcome the air drag.
Basically what this means to me is to drive about 55 to 60 mph and enjoy the scenery.
Glen
A couple of questions concerning the Cummins Doc:
1. Graph on Pg. 27 states that the max time in the highest gear is required for max mpg. The ISM seems to go into 6th at 1600 rpm which is over 60 mph. Has wind resistance killed 6th gear savings ( which I guess was the orginal point of the topic)?
2. Aeodynamics: Do GV's get significant mpg improvement? How about a big clear pointy cowl on the front of a flat nose?
I've wondered this myself. Our 450's sweet spot for 6th gear is around 1350 rpm/62-63 mph. I have noticed, however (especially now that we have the VMSpc to hawk), that our mpg seems to be better when speed limits keep us between 55 and 60 mph, which means 5th gear.
Would be a fun "Mythbusters" segment. Or a lab project for a college engineering fluid dynamics class. Anyone got a kid/grandkid looking for a master's thesis project?
Michelle
at that speed- and that gear- would you be lugging your engine? I believe they like some rpms- to keep everything lubed ad cool
The Cummins guide recommends letting engine rpm's approach peak torque before downshifting. With ISM peak torque at 1200 rpm and Allison in econ mode and 6th gear, can you then back off the throttle and cruise at 50-55 in 6th gear? I've noticed when climbing grades, doesn't downshift until speed drops down to about 50. Would this maximize economy?
While this is contrary to everything I've ever heard, I recently discovered my Powerstroke pickup gets significantly better mpg running 65-70 than 55-60. I've always kept the rpm under 2K cruise but no more. F350 4x4 with 4:10 rear.
I wish I had a Floscan on the coach.
Yesterday we drove from Asheville, NC to Raleigh, NC, a distance of about 250 miles. One looses a fair amount of altitude (about 2100 feet to 300 feet) which may explain our mileage of 9.5 mpg on the Silverleaf. Our average over about 20,000 miles is 8.5mpg. Normally the cruise was on and the mode switch on. The route has a lot of ups and downs and I noticed on the Silverleaf, which shows the engine load (from 0-100%), that the transmission waits until it has reached 100% before it shifts down. If it can maintain close to the set cruise speed, it will stay at 100% and not shift down.
Nope. You cannot cruise in 6th gear with my 6 speed tran. You'll drop yo 5th.
best, paul
Erniee,
You are correct, for a Detroit Two Stroke engine-- lugging will kill them.
But, the opposite is true of all modern 4 stroke engines by DD, Caterpillar and Cummins. For all these, most economic way to climb is at or just above peak torque RPM. They can be run all day at peak torque RPM at WOT.
This was a VERY hard adjustment for most truckers used to the DD two strokes!
Brett
Drive it like you stole it is the old DD adage. I was always used to a 2300 or 2400 rpm fuel cutoff so the 2100 in the U300 seems like it's barely turning over. We were taught to shift down anytime the rpm dropped below 1900 if we had much of a load on the engine. Naturally, cruising down the boulevard with no load, we could ghost along not far above idle in fourth (manual trans).
The big exception to the rule was my old 4107 Buffalo bus with a 8V-71. It only had 225 hp configured for the bus but pulled well above idle all the way to a 1900 max rpm. The engine was so flexible that GM did not even put a tachometer in any of them. Just a 4 speed non-syncro Spicer trans (no CL). The afternoon I bought it, the bus company had just gotten an 86 mph speeding ticket with a full load of passengers outside of Boston. Got great mileage with 9.3 mpg through the Rockies with toad but was slow on the hills. Over 12 mpg at 55 on the flat.
Pierce
Pierce, My Knee still hurts from driving an Old BUFFALO on a city route with a convention in town. That clutch was a bear. Any one that can shift that tranny can shift anything.
Gary B
Gary,
We had Spicers in our Crowns but big tachs and with the engine right behind the driver, it was easy to listen to it so shifting was OK. Just had to time it exactly. The Buffalo with no tach and the engine in the back, was just a guessing game shifting. Non-power steering was tough if you couldn't roll it a couple of feet.
Sure was a super well built rig with excellent handling too. Sure miss the compartment doors.
Pierce
I do laugh a lot reading about the old DD and the Buffalo etc, why when I did the MCI, I removed the Spicer 4 speed and the push clutch that went with it, changed to the Road Ranger RTO910 with the pull clutch, worked the linkage for a easier clutch pedal, MCI had a air cylinder assist on clutch, on & on etc & etc.
Cheers
Dave M
Can someone provide a little clarity for me and my 2005 Cummins 400 ISL? I guess I'm looking for my engine's sweet-spot. In 6th gear for the best mpg, my tach should read...? How about for 5th gear?
(400 HP - Peak torque 1200 LB-FT - Governed speed 2200 RPM; Allison 3000)
Mark,
What is the peak torque RPM.
A hundred RPM or so above peak torque RPM in economy mode gives best MPG for most all modern diesel engines. Economy mode will help keep it in that gear/at that RPM.
Yes, many coaches are geared such that there really isn't any "good" economic speed. On many, by the time 6th gear is reached, aerodynamic drag is significant enough to negate the advantage of high gear. But, as most have posted, unless you are in danger of running out of fuel, driving at that RPM in 5th gear would not be worth it.
Brett
Well, I got one reply about my question (Thanks Paul) before the thread morphed into something totally different about pickup trucks, comments that if one wants to save fuel stay home and watch TV, and what a great time we had in the good ole days, etc. Surely someone with the Silverleaf can find a level stretch of lightly traveled road and drive at various speeds long enough to get a reasonably accurate mpg reading. Make up a chart before hand and have your DB (dearly beloved) record the results. Start at say 30 mph and do it for every 5 mph increase, 30, 35, 40, 45, etc and record which gear. If 10 people did this, I'm guessing there would be 10 different results, even with identical coaches. However, there should be a trend that would indicate the most economical speed. That should be good information to know, just for the time when one is low on fuel and needs to make it to the next station. In my previous life as an airline pilot, I spent a lot of time going fast and burning tons of fuel all paid for by someone else and getting paid to do it. On almost every flight though we were constantly trying to figure out how to use less fuel, so don't assume that I will be driving down the road at a slow speed. If the scenery is great and not much traffic, I will be going slow, what's the rush!
Jerry, I recommend you for the testing on the fuel mileage report, you could share the results here and since it is your idea, you know how to get the exact info your wanting.
Sounds like fun for you, let us know what works and what does not work.
Thanks
Dave M
Brett - this doc shows peak torque RPM at 1300? So, if that is the case then I have some investigation to do. On all my flatland travels I average 62-65 mph and my tach and driver info center (DIC) show 1600-1650. I'm guessing but will have to confirm that I would need to slow down to 55-58 mph for optimum mpg which makes sense according to info in the previous posts.
If I had an electronic engine with Silverleaf, I would certainly do that, but alas, I don't have the electronic engine.
My m11 seems to be geared for that 100 rpm over peak torque at 62 mph. Previous owner had a silverleaf on it and mentioned under optimal level conditions getting 10.2 mpg.
I go too fast for that. The cummins link matches my mileage loss. .1 mpg per mph over whatever. 55?
Bob
Bob,
The loss will be exponential rather than linear as the aerodynamic drag really increases with speed. Air resistance (drag) is the biggest factor in mpg. I go from 10-12 mpg at 50 mph to 7 mpg at 82 mph.
Pierce
The older diesel pushers I was used to had the horsepower to climb grades faster than gearing up for torque.
My m11 is geared for the torque although fully revved its horsepower will climb grades faster than the torque in higher gears. It's possible foretravel made a different decision on your isl.
Mine is 5-10 mph faster up grades revved than lugging in higher gears.
Older lower torque diesels like the 3208 and 8.2 needed revving in the hills. 743 pound feet of torque did not pull hills like the 300hp revved out.
Bob
What another racer like me? 82? I think we are past the tires speed rating but I have never heard of a speed rated failure. Drove an 8v92 CC concept way past 100mph for a long run just because it had that power to weight and gearing setup.
Another trinket from Cummins on how to save fuel. Enjoy.
Dave M
Dave,
Great reading!!! Provides excellent guidelines for any diesel manufacturer/type. Often thought how much fuel could be saved with radiators up front in the earlier Foretravels and less dependency on the fans in the back.
Interesting about new vs older tires. My tires are very stiff sidewall plus I always run every tire I own at maximum recommended cold pressure. I sacrifice a little comfort for mileage, wear, handling and safety. Always able to maximize mpg this way. Tegas Coach Company seems to also agree here.
Driver factor is really the key with a huge difference between the worst and best.
Hope others take the time to download the PDF and read.
You get a headband full of Seminole feathers for this. :D
Pierce
Nice link, Dave. Very interesting stuff. Thanks for posting. Try not to have too much fun in the dunes. :D
Craig
You're welcome, Jerry (running and ducking... ;o)
best, paul
My coaches were driven the 1369 miles from Nac to irvine by delivery drivers who knew me. Most asked if it were ok to max air pressure the tires for the better mpg. Saved them $20 dollars when gas was $1.40? Sure, I always told them
No one does dumb things on purpose, mostly from lack of knowledge. For me, I learned many years ago at Bear Alignment school, that running a higher air pressure in the tires give you many advantages, the reason companies recommend lower pressure is purely for the soft ride. Higher pressure give you better fuel mileage, better handling and a few other advantages. If a soft sponger ride are your main concern, do it. You need to do what makes you the happiest.
Having been in the mechanical world my entire life from my early days to current age 70, sooner or later you figure out the difference between book smarts and the real world. For me the real world is where it is mostly.
Me, I go for the performance side of things, so sometimes I swap between the econ mode to geter done mode. ;D
FWIW
Dave M
And the softer ride translates to sales for a tire or car company. Who wants to buy a firm riding tire, car or RV? With lots of competition, companies try to find a compromise between a cushy soft ride and adequate handling.
Pierce
I did not like the steering on my u320 until I went from 95 to 105 on the front. Next step is nitrogen. Used to do some customers in nitrogen as the pressure never varied. Some can feel the handling differences. I can.
The information placard next to the driver's left elbow in our GV states: 80#PSI for front tires and 70#PSI for the rear duals. I run 100# cold on the front tires and 90# on the rear duals. Just what it says on the sidewalls of the Michilens. Our front tires are 275s and the rears are 235s. Placard calls for the old 19--something size. Don't know why the different sizes but it came that way and it does sit level.
Mileage went up from 7.2 to 8.55 average (towing our Geo) calculated over 3600 miles each trip.
Sorry, but even sellers of Nitrogen for tires can not overcome physics. Nitrogen, like DRY air follow the Ideal Gas Law and change PSI with temperature change exactly the same. Now, WET air, like out of those coin op machines at some stations do not, as water and water vapor are not ideal gasses.
There are some advantages to Nitrogen, but not changing PSI with temperature change is not one of them.
Brett
What Brett says is true regards Nitrogen's thermal stability. Having said that, I've used 100% Nitrogen since day-one in our coach (and car / motorcycle / trailer) tires, partly due to habit/experience I'm a retired heavy-jet pilot and all such aircraft use 100% Nitrogen in the tires, as it is "inert"; it is a "dry-gas" that doesn't support moisture (oxygen combined with hydrogen makes water) or combustion. Also, Nitrogen molecules have a more difficult time escaping through the microscopic spaces that exist between a tire's rubber molecules and its nonreactive nature with many materials. Finally, I also like the convenience and ease-of-use associated with carrying a small cylinder with regulator and hose; when I do need to service a tire having 1,000 psi available (regulated, of course) it's quick and simple. The down-side, thou minor, is filling/exchanging the cylinder; I have my own aluminum cylinder, but most welding shops will do an "exchange" for not much money.
I agree with Brett. Consider the fact that air is 78% nitrogen. How could pure nitrogen be retained siginificantly better than normal air? Air and nitrogen are gases that follow the same physical laws as to expansion or contraction with temperature. As for stability, I really don't think one has to worry about hydrogen and oxygen in air combining to form water. This reaction takes a lot of energy to start (like a spark or a catalyst in a fuel cell) as I recall. It just does not happen to any extent at normal temps.
Now I do have a question that has been bugging me for some time. I use a Powertank filled with carbon dioxide for topping off our tires. If there is any moisture in the tire, some would combine with the CO2 to form H2CO3 (carbonic acid) which is a weak acid in chemical terms. Could this harm the tire over time? I checked the compatibility of wet carbon dioxide with neoprene using this source:
Chemical Compatibility Database from Cole-Parmer (http://www.coleparmer.com/Chemical-Resistance)
It showed that the compatibility was "good.... Minor Effect, slight corrosion or discoloration."
Any thoughts?
George,
Maybe of more concern would be the reaction w/ the aluminum wheels.
FWIW
John
The only real need for nitrogen in tires is for aircraft that frequently fly in sub zero temps. Since nitrogen is a dry gas, no water will condense inside the tire and freeze in a puddle causing a very unbalanced tire upon landing. Other than that it's really not needed inside tires....... My 2 cents
Steve
Well, there is one other reason for nitrogen in high speed large aircraft tires, and that is to help prevent a tire explosion from excessive heat. Rubber when heated enough out gasses and forms a combustible/explosive mixture with air in a tire. Nitrogen is inert, so no explosive mixture. This could happen during a high speed rejected takeoff which requires a max brake effort. Typically in this situation the brake rotors become red hot. Each wheel has several rotors, except the nose gear wheels usually don't have brakes. An additional safety device is a thermal plug in the wheel rim that melts and deflates the tire. That generally happens too a few minutes after the high speed rejected takeoff. The tire can still explode just from the large increase in tire pressure due to the heat. Jet aircraft cold tire pressure is generally around 200 psi. A dragging brake on takeoff can create a dangerous situation when the gear is retracted, hence the reason for fire and heat detectors in the gear well. A tire exploding in the gear well is not a good thing as fuel tanks, flight control cables or wiring, hydraulic and fuel lines are generally close by. If you ever happen to experience a high speed rejected takeoff expect to sit on a taxiway for awhile until the mechanics can round up and replace the wheels and tires. It won't be as fast as NASCAR. Doesn't happen very often, I didn't have a true high speed rejected takeoff in 30 plus years. Got tested every six months in the simulator though.
I meant to post it did not loose air pressure as quickly through the casing itself.
My fault for saying none should have said less.
Quote:
First is that nitrogen is less likely to migrate through tire rubber than is oxygen, which means that your tire pressures will remain more stable over the long term. Racers figured out pretty quickly that tires filled with nitrogen rather than air also exhibit less pressure change with temperature swings. That means more consistent inflation pressures during a race as the tires heat up. And when you're tweaking a race car's handling with half-psi changes, that's important.
Read more: Nitrogen vs Air In Tires - Why Nitrogen in Tires - Popular Mechanics
Here is an answer (sort of) to my question about using CO2 for tire inflation. The question was.... does the carbonic acid which forms from CO2 and water have the potential to harm tires and the aluminum wheels on FT coaches? I contacted Power Tank who is a major supplier of CO2 tire inflation systems (POWERTANK - CO2 Air Systems (http://powertank.com/category.rv)) and this is the response I got tonight.
"Hi George- Yes, this questions has come up before. Carbonic acid is a mild alkalyd and nothing to worry about. Dog pee is more corrosive than this. In the 15 year history of Power Tank we have had no reported incidents of tire or wheel failure by a Power Tank user. Steve Sasaki, Power Tank"
Not sure what an "alkalyd" is (it is weak acid in chemical terms), but he does have a point. No more dog pee in the tires!