mileage question

[maverick_935]

I just completed a round trip with a flat deck fifth wheel on for 1200kms. Loaded a car and parts and drove back.

Trailer weighs 3500pounds + the car weighs about the same with about 5-600 pounds of parts.

I hand figured it out to be just shy of 13 mpg, hand calculated. Am I wrong in expecting more?

[Mark Rinker]

Sounds about right to me.

Gooseneck trailers and cars are not very aerodynamic...the faster you go, the worse it gets back there.

[maverick_935]

I stayed right around 106 km/h as much as possible, lots of hills as well. This was just under 2000 rpm. Is there any way to get it up higher?

[Edahall]

When I tow, I keep my speeds down to 55mph / 90 kph for best fuel economy. I also keep my rpm's down and slow down while ascending the hills.

[Mark Rinker]

I stayed right around 106 km/h as much as possible, lots of hills as well. This was just under 2000 rpm. Is there any way to get it up higher?

My thoughts, in order of their likelihood to increase mileage while towing, as described:

1) Slow down to whatever ground speed 1600-1800rpm will give you, with the gear selector in 'D', and tow/haul always engaged.
2) Pitch your Edge tune in favor of Kennedy Diesels tow/economy tune. You'll likely gain .5 to 1 mpg towing with this change, alone.
3) Use stock diameter, or alternatively 285 85/r16 load range E tires, inflated to MAX psi rating. (Same thing for trailer tires!)
4) Synthetic gear lubes front and rear diffs. Some products like Amsoil claim mileage increases, just by running their 'super-slippery' products. I like Royal Purple, but not really sure why.
5) Run a good fuel treatment to keep sprays spraying nicely.

[DmaxMaverick]

My thoughts, in order of their likelihood to increase mileage while towing, as described:

1) Slow down to whatever ground speed 1600-1800rpm will give you, with the gear selector in 'D', and tow/haul always engaged.
2) Pitch your Edge tune in favor of Kennedy Diesels tow/economy tune. You'll likely gain .5 to 1 mpg towing with this change, alone.
3) Use stock diameter, or alternatively 285 85/r16 load range E tires, inflated to MAX psi rating. (Same thing for trailer tires!)
4) Synthetic gear lubes front and rear diffs. Some products like Amsoil claim mileage increases, just by running their 'super-slippery' products. I like Royal Purple, but not really sure why.
5) Run a good fuel treatment to keep sprays spraying nicely.

Not sure about that one. Never heard of that size. A 285/75/16 however, is a common upgrade, but will be an immediate loss of 1-4 MPG across the board. 285/85/16 would be a 36" tall tire, and would change the final drive ratio, as if you had 3.08 gears with the OEM tires.

Did you mean 255/85/16? They are only slightly wider (1 cm) than OEM 245/75/16 (and really exist), but are 33" tall. Not recommended for heavy loading on a tow vehicle with single rear wheels, due to the tall sidewall and narrow tread. Not very stable with side-loading (cross winds, turns, etc). They are great in the snow.

Wider tires generally mean less economy. Changing tire height to move your cruising RPM to a more ideal (or economical) range may help in one respect, but will usually be counter productive in the end.

[Mark Rinker]

My mistake, 285/75/16 is the tire size favored for our mixed use of long distance towing and snow plowing.

Have upgraded all the 2500 and 3500 series SRW trucks with this size, after the stock set has been used up. ~6 sets of four have been installed over the years, all Michelin ATX's save the latest choice of Hancook Dynapro ATMs. (So far, the Hancooks are wearing nearly as well, for a much better price.)

Never seen a significant difference in mileage from the stock set to the next - and we measure these things pretty closely. However, I probably wouldn't notice any change under 1mpg average as we vary alot more than that due to seasonal temperatures, truck use, and winter blend fuel. Our results have not been 'an immediate loss of 1-4mpg less across the board' - or we clearly would have measured/experienced this, and wouldn't continue to run these tires on the four SRW trucks over ~300K towing snowplowing miles.

I think the 285/75/16 is a great tire choice for a person that works with their Duramax, (i.e. daily heavy towing, navigating jobsite mud or snow) - and they look great in the large wheel wells of the latest bodystyles as well. Again, the use of the vehicle must be considered, I do concur that varying greatly from stock OEM tire diameter can be a mileage killer.


"Individual results will vary."

[Kennedy]

Your "chip" department could use some improvement as Mark suggested. We can onlty do so much with the Boost Stick and generally speaking it is to decrease smoke and EGT while improving power.

Good fuel additive like FPPF Total Power will often pay for itself with MPG gains so the longevity/system maintenance benefits are essentially free.

I'd suggest some nice Michelin rubber in the 265-75-16E size.

My personal experience with Hankooks is none, but from what I have seen and heard from tuning customers:

1) They run short for their respective size (hence the low price for less rubber)

2) They wear fast

[Mark Rinker]

SORRY for muddying the water around tire size...agreed 265/75/16 is probably a better choice since we are specifically talking 'mileage' on this thread. (Seem to recall that I have done this 265 vs 285 confusion before, as tires are something I simply trust my local guy to set me up with, based on his knowledge and understanding of what my trucks do and how many miles / $$$ we experienced from the last set...)

Interesting observation on the Hancooks 'running short', maybe this is why the 285s seem to work fine (first on '06 LBZ, swapped at trade time to the '09 LMM) ??? I do get a little rub on the fronts against a 1/4" thick mudflap when backing and turning. This particular set is at 40K miles and appears to have 25-30% tread life remaining, so they will be close to the Michelins, 55-60K miles. Great snow traction, possibly a bit better, much more aggressive tread.

I suspect that the long towing miles favor the slightly larger tires, to a point of diminishing returns. It allows me to keep the RPMs in the 1600 range, while maintaining slightly higher ground speeds. I am also into mileage figures that don't vary as much as comparing unloaded mileage - but in those cases, a stiff headwind and/or another 5mph can eat (or make) a big difference, too!

As we all know - its mostly about your desired ground speed that will "make or break" diesel mileage.

For mileage, "drive by your tach" is my advice.

[JohnC]

Mark:

what sort of tread wear patterns do you see running them at max pressure? I'd expect them to wear excessively in the center unless you're loading them really heavy.

[More Power]

Wider tires generally mean less economy. Changing tire height to move your cruising RPM to a more ideal (or economical) range may help in one respect, but will usually be counter productive in the end.

I wish tires were rated in rolling resistance as well. You're right, bigger tires increase rolling resistance. Agressive tread increases rolling resistance. Below spec air pressure increases rolling resistance. Generally, the bigger the tires, the more air pressure one needs to maintain ride & handling and fuel economy.

Jim

[Mark Rinker]

Mark:

what sort of tread wear patterns do you see running them at max pressure? I'd expect them to wear excessively in the center unless you're loading them really heavy.

Never experienced any cupping or unusual center tread wear. I normally run them at 80psi cold - whatever the max printed on tire sidewall is. The tire pressure monitors on my 2009 DIC has taught me how much they heat up under load - sometimes as much as 5psi per tire in the winter!

If the season allows (spring/summer) at end of tire life, I run them down to where they look BAD, like maybe 1/8th inch tread depth remaining before putting on the new skins - many times late fall before hunting season for the muddy back roads.

I think you hit the nail on the head John - they are loaded heavy most of the time, and my wear comes trom the drivers in the back, getting scuffed off faster under acceleration. Its never the shoulders of the fronts rounding off, in my case.

Rears are my indicator to rotate, which I do often - usually at 10K oil change intervals - before the backs get well ahead of the fronts.

[Kennedy]

I don't subscribe to the drive by tach school of thought. If this was true the 6 speed trans would have ushered in considerable MPG gains.

I'm in the:

1) SLOW DOWN

2) Keep the nose low

3) Keep the tire size moderate and narrow with low rolling resistance.


Michelin LTX MS always treated me well. Wear like iron and quiet which are indicative of a hard rubber compound with low rolling resistance.

[maverick_935]

I will have to try the 90km/h thing and see what happens. This will put my truck in the 1700 rpm range. I run Michelin on the rears but have Good year on the front trying them out.

I just thought easy friving and only 13mpg was a little low. Camping season is coming and I pull a 32' Salem all over the maritimes.

Thanks for the input.

[Mark Rinker]

Today's stats, as an example:

• Total miles: 635 roundtrip miles, 13hrs, 30 minutes = 47mph average for the day.
• Load: trailer (3500# empty) + 7000# cargo 317 of roundtrip, empty trailer on return
• 1600-1700 rpm at cruise, never exceeding 2300 even on accelerations from dead stop. (~62-64mph loaded, 68-70mph 'empty' with trailer on return)
• 13.6mpg trip average on DIC, 13.4mpg hand calculated. DIC indicated 12.4mpg when the 7000# cargo was delivered.
• 285/75/16s rubber, otherwise a bone stock LMM with DPF intact.

[Mark Rinker]

I will have to try the 90km/h thing and see what happens. This will put my truck in the 1700 rpm range. I run Michelin on the rears but have Good year on the front trying them out.

I just thought easy friving and only 13mpg was a little low. Camping season is coming and I pull a 32' Salem all over the maritimes.

Thanks for the input.

Your biggest enemy is the wind - the one nature makes, and the one you make with your right foot. Add them together, and you have a bad mileage day. Get the wind at your tail, and have a good mileage day.

Slower speeds will always help. Good luck!

[Mark Rinker]

>>> I don't subscribe to the drive by tach school of thought. If this was true the 6 speed trans would have ushered in considerable MPG gains. >>>

Actually, the 6spd did help...but most of us don't measure or record what it potentially gave us back...TIME!!!

It still takes the same amount of work (fuel) to propel the same mass down the road at the same speed, with the same 6.6L pump. Resistance is resistance, regardless of final gear ratio. In that regard, a 2002 LB7 5spd will be similar to a 2006 LBZ 6spd in fuel consumption, towing the same load, if you remove or factor out emmissions equipment inefficiencies.

BUT! If both trucks left on the same trip with the same load, and burned the same amount of fuel doing that same work - who got back to the shop quicker?

Ta-DA! The 2006 with the 6spd did! Time = money. There is more to efficiency than MPG can measure.

All the 6th gear (or 5th or 4th for that matter) does is give you more OPTIONS of keeping your engine at or near peak torque (read: peak efficiency) while you perform a certain task, at a certain speed.

With another taller gear available, we can do that same work, at the same efficiency, with slightly higher ground speeds.

[Kennedy]

Actually, the 6spd did help...but most of us don't measure or record what it potentially gave us back...TIME!!!

It still takes the same amount of work (fuel) to propel the same mass down the road at the same speed, with the same 6.6L pump. Resistance is resistance, regardless of final gear ratio. In that regard, a 2002 LB7 5spd will be similar to a 2006 LBZ 6spd in fuel consumption, towing the same load, if you remove or factor out emmissions equipment inefficiencies.

BUT! If both trucks left on the same trip with the same load, and burned the same amount of fuel doing that same work - who got back to the shop quicker?

Ta-DA! The 2006 with the 6spd did! Time = money. There is more to efficiency than MPG can measure.


All the 6th gear (or 5th or 4th for that matter) does is give you more OPTIONS of keeping your engine at or near peak torque (read: peak efficiency) while you perform a certain task, at a certain speed.

With another taller gear available, we can do that same work, at the same efficiency, with slightly higher ground speeds.

If both trucks were doing the same amount of work that would assume that EVERYTHING was equal. This would include speed as teh faster you go the more work you do to buck the wind and down goes the MPG.

Or did this one fly completely over my head.

[Mark Rinker]

You are correct, there would be additional wind resistance encountered by the faster truck.

Consider this scenario: Both trucks from above example leave the shop with the same load, and run 'side by side' (same wind resistance) all day...i.e same load, same speeds, gear selector in "D".

When they get back, which one used less fuel - the '02 5spd, or the '06 6spd?

[Kennedy]

If you believe all the cries: I wish my LLY,LBZ,LMM would get as good of mileage as my old LB7...