MPG at 40-45 MPH

Topic: MPG at 40-45 MPH (Read 1762 times) previous topic - next topic

Re: MPG at 40-45 MPH

Reply #25 – July 25, 2012, 10:06:11 am

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

Re: MPG at 40-45 MPH

Reply #26 – July 25, 2012, 10:08:04 am

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!

Re: MPG at 40-45 MPH

Reply #27 – July 25, 2012, 10:20:15 am

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

Re: MPG at 40-45 MPH

Reply #28 – July 25, 2012, 10:24:57 am

Quote from: wolfe10 – July 25, 2012, 10:06:11 am

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.

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.

Re: MPG at 40-45 MPH

Reply #29 – July 25, 2012, 10:42:42 am

Quote from: Dave M – July 25, 2012, 10:20:15 am

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

If I had an electronic engine with Silverleaf, I would certainly do that, but alas, I don't have the electronic engine.

Re: MPG at 40-45 MPH

Reply #30 – July 25, 2012, 10:42:56 am

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

Re: MPG at 40-45 MPH

Reply #31 – July 25, 2012, 10:52:23 am

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

Re: MPG at 40-45 MPH

Reply #32 – July 25, 2012, 10:54:35 am

Quote from: Mark Benko – July 25, 2012, 10:24:57 am

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.

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

Re: MPG at 40-45 MPH

Reply #33 – July 25, 2012, 10:59:51 am

Quote from: Pierce & Gaylie Stewart – July 25, 2012, 10:52:23 am

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

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.

Re: MPG at 40-45 MPH

Reply #34 – July 25, 2012, 11:04:20 am

Another trinket from Cummins on how to save fuel. Enjoy.
Dave M

Re: MPG at 40-45 MPH

Reply #35 – July 25, 2012, 12:05:15 pm

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.

Pierce

Re: MPG at 40-45 MPH

Reply #36 – July 25, 2012, 12:44:52 pm

Quote from: Dave M – July 25, 2012, 11:04:20 am

Another trinket from Cummins on how to save fuel. Enjoy.
Dave M

Nice link, Dave. Very interesting stuff. Thanks for posting. Try not to have too much fun in the dunes.

Craig

Re: MPG at 40-45 MPH

Reply #37 – July 25, 2012, 01:42:14 pm

Quote

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.

You're welcome, Jerry (running and ducking... ;o)

best, paul

Re: MPG at 40-45 MPH

Reply #38 – July 25, 2012, 01:55:24 pm

Quote from: Pierce & Gaylie Stewart – July 25, 2012, 12:05:15 pm

Dave,

Great reading!!! Provides excellent guidelines for any diesel manufacturer/type.

Hope others take the time to download the PDF and read.

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

Re: MPG at 40-45 MPH

Reply #39 – July 25, 2012, 05:21:21 pm

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.

FWIW
Dave M

Re: MPG at 40-45 MPH

Reply #40 – July 25, 2012, 09:55:32 pm

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

Re: MPG at 40-45 MPH

Reply #41 – July 26, 2012, 12:19:41 am

Quote from: Dave M – July 25, 2012, 05:21:21 pm

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.
FWIW
Dave M

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.

Re: MPG at 40-45 MPH

Reply #42 – July 26, 2012, 07:23:28 am

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.

Re: MPG at 40-45 MPH

Reply #43 – July 26, 2012, 10:02:41 am

Quote from: Caflashbob – July 26, 2012, 12:19:41 am

Used to do some customers in nitrogen as the pressure never varied. Some can feel the handling differences. I can.

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

Re: MPG at 40-45 MPH

Reply #44 – July 26, 2012, 11:23:42 am

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.

Re: MPG at 40-45 MPH

Reply #45 – July 26, 2012, 01:08:22 pm

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

It showed that the compatibility was "good.... Minor Effect, slight corrosion or discoloration."

Any thoughts?

Re: MPG at 40-45 MPH

Reply #46 – July 26, 2012, 01:18:08 pm

George,

Maybe of more concern would be the reaction w/ the aluminum wheels.

FWIW

John

Re: MPG at 40-45 MPH

Reply #47 – July 26, 2012, 01:53:24 pm

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

Re: MPG at 40-45 MPH

Reply #48 – July 26, 2012, 05:59:28 pm

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.

Re: MPG at 40-45 MPH

Reply #49 – July 26, 2012, 08:45:56 pm

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

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