Mileage Weirdness - Montana Owners Club

pbahlin · 09-01-2010, 04:11 PM

I've got a strange thing happening. Recently I've been on a long trip with my rig. It's taken me through the rolling hills of PA and the flats of Southern MN.

What seems odd to me is that I got over 1 mpg better mileage in the hard slog through PA than I got in the flatlands.

Could this all be due to wind resistance? My avg speed in the mountains of PA was probably 55-60 and in the flat my avg speed was 60-65. No appreciable weather was encountered in either place so headwinds don't seem to be a factor.

lasater · 09-01-2010, 04:23 PM

Did you use the manual shift button on the gear lever to downshift to 5th or 4th before the transmission called for it? Sometimes, I can get better mileage by controlling the shifts myself and sometimes the truck is smarter than I am. But, being the egotistical person that I am, I like to control the tranny myself.

One other aspect of you trip is weather. The difference in moisture in the air that the truck breathes can actually make a difference in your mileage. Where did you go through rain or high moisture areas?

pbahlin · 09-01-2010, 04:37 PM

Don't use the manual shift yet. The truck seems to be doing fine without my messing with it.

I had bone dry conditions over the PA mountains and in the MN flats. Could it be from 'playing' the hills a bit vs. putting the pedal down in the flat?

pbahlin · 09-01-2010, 04:45 PM

Another thing I've noticed is that at 65 the engine just purrs like a kitten, 1800 rpm and small changes up and down hills don't seem to make a bit of difference. In the same terrain though, 60 mph seems to make it work more. It will lug more on a small rise.

I think 60 is just a bit to low on rpms for the high gear it seems to find itself in. Actually 65-70 seems really quiet and smooth while 60-65 makes me want to 'help'. Unfortunately at 70 I can hear the fuel being sucked out of my tanks (sounds like the sound of burning money).

scductman · 09-01-2010, 05:25 PM

Pbahlin its the mph. my old truck with the 410 gears will get 13.5 empty on the interstate at 73mph and 18 around town and in hills at 60 or below. it just don't high rpms.
bobby

H. John Kohl · 09-01-2010, 05:38 PM

Quote:

quote:Originally posted by pbahlin

I've got a strange thing happening. Recently I've been on a long trip with my rig. It's taken me through the rolling hills of PA and the flats of Southern MN.

What seems odd to me is that I got over 1 mpg better mileage in the hard slog through PA than I got in the flatlands.

Could this all be due to wind resistance? My avg speed in the mountains of PA was probably 55-60 and in the flat my avg speed was 60-65. No appreciable weather was encountered in either place so headwinds don't seem to be a factor.

Remember in the mountains you also had some down hill runs.

I would say your are finding wind resistance. Cummings did a study on big trucks and found 55 was the transition from Rolling Resistance to Wind Resistance. Wind Resistance had a major impact of .1 mpg for every mph over 55. So 65 would drop a whole mile per gallon. I find that analogy is close on my truck. It likes 1700 and that is about 57 mph.

Trailer Trash 2 · 09-01-2010, 06:17 PM

Another thing to remember that the computer has a bladder that is used for alatude, and when the bladder tells the computer you are in high alatude, where the air is much thinner than sea level, it will regulate the air fuel mixture and lean out the fuel. so I was told by a ford mechanic on my old 96 f350. this might be the reason for the milage increese.

TLightning · 09-02-2010, 02:48 PM

I do better in the hills than against the wind.

pbahlin · 09-02-2010, 03:38 PM

Has anybody got experience/numbers for an improvement due to the 'open' tailgate replacements? I was told by someone that the open tailgate makes a big difference in towing, giving the down draft from the trailer nose a place to go.

bigmurf · 09-02-2010, 05:11 PM

How about a LOUVERED TAILGATE.

thor · 09-02-2010, 06:54 PM

the speed seams to be the differnce.55 to 60 in the mountains and 60 to 65 on the flats seams like 5 mph faster on the flats ucks more fuel.i bet if you need 65 in the moutains you would see a different story.just my wierd way of thinking

Tfthorp · 09-02-2010, 07:30 PM

Speed and wind resistance are the big ones find the sweet spot and go with it, for me it about 62 and no tail gate. Just make sure the tire pressure is correct and the e brake isn;t dragging. Lessoend learned here.

simonsrf · 09-04-2010, 07:30 AM

Quote:

quote:Originally posted by bigmurf

How about a LOUVERED TAILGATE.

That's EXACTLY what I was thinking!!

pbahlin · 09-04-2010, 12:39 PM

What about the louvered tailgate? Do you think they're effective?

sailer · 09-04-2010, 04:41 PM

I watch my RPM's more than the speed the 1800 is best and that is about 63 MPH ..if i get up to 65 or 70 i can see the fuel going out the rear ;;;sailer

PowellsMonty · 09-05-2010, 02:49 AM

I have read studies that say driving with the tail gate up give better milage than driving with it down, so I would question the louverd tail gates. My truck is a gas pig, 11mpg city driving no matter what I mix with the fuel or what kind of oil I buy.

mlh · 09-05-2010, 05:40 AM

All internal combustion engines get the most work done per unit of fuel at its peak torque RPMs. If your peak TK is at 2000 RPMs like my Ford or 1800 RPMs like your Chevy thats the RPMs your engine is running at its peak efficiency. Thats not to say this is wear you get the best fuel mileage, because other things come into play, things like friction but mostly air drag. If you are look ins to get to wear you going in the least amount of time with the least amount of fuel this is your compromise. You will use more fuel than you would at a lower speed but your engine will be running at peak efficiency. If you go over this RPM engine efficiency drops off, air drag increases and now we are really some fuel. Its all a compromise.
Lynwood

clutch · 09-06-2010, 07:06 PM

Quote:

quote:Originally posted by bigmurf

How about a LOUVERED TAILGATE.

Myth Busters did that one on tv and an open tail gate made more drag. They concluded the air flow behind the cab was stepped down from the cab to the bed with hi pressure to the ground behind the tail gate. With the tail gate down the flow was messed up and caused more drag.

padredw · 09-07-2010, 03:11 AM

WIND is the great enemy. I have just towed across Texas, NM, CO, UT, and WY. Mileage varied from less than 9 to over 11. Mountains made no difference--about as much down as up. The difference seemed almost entirely due to wind.

You can climb a hill, but you WILL ge to the top and likely come down. The wind is endless, and cross wind seems as bad as head. As one friend said, "it is like towing with your brakes on."

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