GSP

My truck doesnt have a fuel milage computer up by the rearview mirror and I think that isnt accurate anyway ,and read with electronic fueling boxes(edge)etc they gauge reads way off. I Met a guy with a comp box and all kinds of hop up stuff ,tell me his milage computer reads he is getting 32 mpg. Yea right.

I check my mileage by hand , miles driven and gallons at fuel up.

I get 20-21 mpg unloaded highway. I tried driveing slower 55-60 on a trip with my cabover camper ,checked milage. Then just put my foot in it and did 65-75 and hauled *** up hills. Got same and better mileage with my foot in it.

rebal

at 60-61 i get 26 below that 23-24 above 20-21...65mph + starts to drop real fast 75 = 17mpg I hand calc this .truck has stock tires 01 HO 6spd

the 01 5spd is getting 15 with a utility box on it but it drops like a rock above 60 mph

Marc Platz

About a month ago we did a round trip from Phoenix to Sacramento. I found that the best milage and performance was at the max cruise control and one bump down on coast. I got 19.4 round trip on the freeways to and from, hand checked. Not too bad for a 9000 lb. vehicle and a fair load? The big differance is changing the fuel filter and a well serviced truck, with attention to the details.

airaid, stock truck, so engine, auto trans, slt 4x4, ca emisions 37,000 miles

Marc

bentwings

Haulin...I think you are probably right. I have 4.10 gears in my dually and get consistant 20+ on the highway at 60 or 2000 rpm. 65 costs about 2-3. Also the relative wind makes a big difference. a 20 knot head wind will cost about 2 and a 20 knot tail wind is worth about 1 over the 20 mpg average.

It seems like the guys with tall tires and gears get less mpg as the motor is working a bit especially if going over 65 .

need to be geared right on for the load. My truck doesn't seem to change much if loaded in the bed. I've carried 4000 pounds in the bed in a concentrated load (heavy steel parts) and not seen much if any difference in mileage.

I've also noticed a bit better mileage in gently rolling hills of Wisc. as opposed to the flats of Ill. It may have to do with the quality of fuel however too.

bentwings

HOHN

OK, I'm hoping this post doesn't get me in hot water... That's not my intention.
Start by reading this:http://www.cummins.com/na/pdf/en/pro...on_for_web.pdf

There are many things that contribute to the overall load on the engine. Tire pressure, size, construction, etc all affect rolling resistance. Add to this the friction in your tranny, t-case, and differentials, and you have "road load." This can get pretty detailed. Alignment, type of road surface, etc-- there are a million little things that all add up.

Now, add the aerodynamic component, and you have the "aero load". This is a product of Coefficient of drag (Cd) frontal area, and speed. Cd tells you have efficient a shape is. Frontal area is pure size. Just for illustration, if you have a 100 sq ft frontal area, but a Cd of .4, then it will act like a flat panel (perpendicular to the wind direction) with an area of 40 sq ft. Super aerodynamic sports cars are around .32 Cd. Our trucks are closer to .5

Gearing plays a role, because it determines WHERE the engine's efficiency curve will plot out against the "road load" and "aero load" curves. The point where these curves cross is the point of best mpg.

For example, say your engine is most efficient at 2000 rpm (like the 24V is). OK, so just run 2K rpm for max mpg, right? NOPE! What if you run 2K rpm in neutral? ZERO MPG! How about 1st gear? VERY BAD MPG. Second gear is slightly better. 3rd is better than 2nd, etc.

This is because it takes a certain amount of fuel just to run the engine-- to overcome its own internal friction. Adding the load of 1st gear doesn't increase the load on the engine much at all, but is DRASTICALLY increases mpg. (after all, we've gone from ZERO mpg in neutral to some kind of mpg that's at least measurable). Shifting to 2nd gives still more load on the engine, but your speed increased more than the load on the engine does, so again, we get better mpg.

However, there comes a point where shifting to the next higher gear increases the load on the engine MUCH more than the gain in speed. This is a step backwards in mpg.

For example, at it's "best" rpm of 2K rpm, the NV5600 with a 3.54 axle gives 9mph in 1st gear, 15mph in 2nd, 25 mph in 3rd, 37mph in 4th, 51mph in 5th, and 70mph in 6th.

As gearing gets taller (you upshift), you also travel faster. In the lower gears, this means you get very little increase in aerodynamic drag. C'mon, we're talking the difference between 9 and 15mph!

But look at the shift from 5th to 6th! Now, we're talking a difference of 51 to 70mph. There's a LOT more aerodyamic load at 70 than at 51. OD on the 5600 puts you past the point of optimum mpg, because to operate at the "best" rpm puts you going too fast for good mpg.

So, Haulin is 100% correct. For a 5600 tranny with 3.54 gears, the best mpg speed is around 51mph in 5th gear. At this speed, you could alternatively shift into 6th and turn just over 1400rpm. But we see from the BSFC curve graph that the engine is much less efficient at making power at 1400rpm than at 2000 rpm. (This chart is in the TDR mag 2 issues ago).

Here's the hot water part: you guys who think you get the best mileage at 80mph are violating all the laws of science to get the best mpg at that speed. I'd suggest that it APPEARS to give you better mileage because of other variables, like pumps that click off at different fuel levels or somesuch. Aero loads increase with the SQUARE of velocity. IOW, when you TWICE as fast, it takes FOUR TIMES more power to overcome aerodynamic drag.

I'd bet that all you guys who get better mpg at 80 or so have auto trannies. You SHOULD be getting best mpg at 50 or so, but the tranny is locking and unlocking all the time, and that hurts mpg. So with an auto trans, best MPG speed is the slowest speed that keep the converter locked in OD 100% of the time.

If you're getting 20 @ 80mph, a properly setup trans would get you about 25 at 50mph.

Rule of thumb: the less aerodynamic a vehicle is, the slower is the speed of optimum MPG.

HERE'S A GREAT ARTICLE THAT DOES A BETTER JOB OF SAYING WHAT I WAS TRYING TO
http://auto.howstuffworks.com/question477.htm

JMO

BigBlue

All I know is I've driven several different speeds and I get my best mileage at 72 mph with the comp on 2x2 and the cruise control set. 20mpg on the highway with this beast is impressive considering the wind resistance.

bentwings

I think that Hohn and Haulin.. are knocking on the door of what is really important and that is that the BSFC at the optimum level is where you want to be. Now if it turns out for ex. at 52 mph in 5th and 2000 rpm is optimum BSFC, above or below this = mpg going down hill assuming gears ratio is not changed. We sort of set 60 mph as an artificial speed level. The truck needs to be geared either by running in say 5th or 6th to run 60 mph with BSFC at the optimum at this speed and rpm. What ever it takes. Speed is adjustable by gears, BSFC is not.. it is RPM related. If you must run 80 mph you add a very large aerodynamic load plus greater tire rolling resistance and a few other factors all much higher than at 60 mph. Now if you gear for BSFC at this speed (mph) then you will get the best you can at this particular level. If you go slower you will get less if you faster still you get less. However your load is greater so you are going to get less anyway than say someone lightly loaded at 60 mph BSFC at 60 mph (2000 rpm).
We can only change a couple variables. The transmission ratio...1st thru 5th or 6th and rear end. 3.55 or 4.10 early and 3.73 or 4.10 later. And tires. Turns out where the rear end is not very changeable so the only real factor is transmission and minor factor is tires. So it may be that if 2000 rpm is the best BSFC and you have 3.55 gears with tall tires you may be better off running 5th at 60 mph. Exactly what Hohn and Haulin are saying.

My own situation is 60 mph is 2000 rpm 5th gear unloaded or what ever it takes to maintain minimum throttle and 2000 rpm (other than neutral) with a load will be the optimum gas mileage I can get with the given combination and it may not be 20 mpg. The boost gage is the gage to use. Keep the boost down as much as possible to stay in the traffic flow. again 60 mph is the arbitrary speed level.

This is why there are so many gears in the diesel transmission. We have a very narrow speed band to use.

Hope I said this correctly. I did read the cummins articles before and again.

bentwings

John Faughn

I was kinda supprized that knowbodys talken about RPMs , it seems to me that there are so many veryables with mods and everything els that RPMs around 1750-1950 is a good avg. , but the next is I've found that if you listen and feal for a resonace in the eng. drivetrain, that that is good for any vehical.

HOHN

The best way I am aware of to find the "sweet spot" in your engine's RPM range is to put it on a dyno.

The RPM at which peak TQ occurs is almost always the point of most efficient BSFC.

It just so happens that when i dynoed my truck, peak tq occurs at 2000 rpm, JUST LIKE CUMMINS SAID IT WOULD (based on looking at the BSFC chart).

Now, I have susbstantially uprated the fueling of my truck. But I have NOT changed anything that would change BSFC. I haven't changed the cam or ported the head or lowered compression or installed a larger turbo.

BTW-- A larger turbo should shift the BSFC curve to the right in terms of powerband. For example, if the charger is very laggy, it might not spool until the engins is past 2K rpm-- in this case, you have a a very mismatched turbo. The engine will be past its point of optimum efficiency before the turbo is online, and the overall performance will be horrible.

While the BSFC is best at 2K rpm for a 24V, it's actually pretty decent in BSFC from across a wider rpm range. Here's the numbers for a 235hp (ETC) 24V:

RPM BSFC
1400 .339
1600 .337
1800 .336
2000 .334
2300 .344
2500 .354
2700 .365
2800 .371
3200 .393

See how it stays under .34 from 1400 to 2000 rpm? (Sorry I don't have more specific rpm values, as these are the only RPMs listed on the Cummins spec sheet, so 2100 and 2200 rpm are missing, as are many others). By the graph, the BSFC goes over .34 at about 2150 rpm or so. (the graph is a about a sq inch in the TDR mag, so pardon the approximation).

So, now we have some scientific basis to the old rule of thumb of "keep in under 2000 rpm or so."

We can also conclude from this that if you tow at 62mph or so, you can expect better mileage in OD with 4.10s than you could with 3.54 axle in direct (5th gear). This is because the 4.10s would put you right at 2K rpm, while downshifting with the 3.54s will put the RPM up to 2300-2400 or so, and the BSFC is no longer very good.

I don't have the BSFC curve for the HO 24V (wasn't in TDR mag), so I can't really comment on thie difference it may have vs the ETC other than to say that since my truck dynoed peak tq at 2k rpm, I'd expect the BSFC curves of the ETH and ETC to be very similar, and I think it's safe to say that what applies to one applies to the other in terms of generalities.

PS-- It's interesting to see how much different the 12V BSFC curve is compared to the 24V, and explains the better mpg. Anyone want the 12V numbers?

JLH

Behr

I have found with my 2001 to run about 1600 - 1700 Rpm's

HiJenks

Hohn
That was some great info.
I hadn't found any reference to RPM and TQ/HP on these engines, Dealer doesn't know much about them.

I run just under 2500@ 70 and get 17mpg empty, with or without the shell. I'll have to try it at lower RPM and see what happens.

I have a stock 98.5 3500,Auto Dually Qcab long wheel base at the moment.....I just realized that by the time I get it back on the road it won't be stock.
Guess I'll have to start all over on the test.

BigBlue

Well, I've done 400 miles on 3/4 of a tank running about 80% highway at 72mph and the other 20% of the time was spent hot rodding it.

Clunk

I averaged 21.4 mpg on my US Nationals trip pulling a fair heavy Apache pop-up camper.
I kept the speed around 60 mph on cruise.

darrenG

this will boost everyone's moral.

I get 18.5 @ around 70mph. I have a stock truck outside of the 4" exhaust. I have 31x10 ( I think x10 ) BFG All Terrains inflated to 60lbs w/ 3.54's. No idea what's in the diff's, tcase or tranny and in need of an oil change ( mobil 1 or rotella...hmm ). No shell / camper, tailgate up, turbo from 6 to 8 lbs.

When I go through I'll add good oil to the boxes, but I doubt that'll account for much. and I probably should clean the AIT (whichever gets gunked up).

supr

My truck is just getting broken in. I am normally always towing a heavy 5er, so wind is a factor, as is height; the 5er is 12' high. I have tried slowing down to 55, about 1750rpm with 4.10's/6 speed HO. I have broken 13 mpg on this last trip, about a 14k trailer. I think 55-60 is optimal, with or without trailer. Have K&N, may help a little. I also keep tires aired, 75 front, 70 back. Also have 5th wheel tailgate, also may help a little with wind resistance. I think 17 is about normal for me,solo, at 60-65, 2k+ rpm. Many guys have said 1800 rpm's works well for them.