I don't recall my truck ever downshifting while in tow haul to 5k. Frankly I don't think it downshifts far enough at hwy speeds during deep descends to maintain desired speed. Example, if I have crested a hill and it is 5-6% down for the next mile plus and I am at 65mph the truck only downshifts to 4th gear in an attempt to hold speed. Often speed increases and a downshift to 3rd is no longer an option even on manual mode. If I am concerned about the length and steepness of the decline I typically slow to about 50 or 55 and manual go to 3rd gear beginning descent. Rpm's are higher but truck is in control of trailer and brakes are cool if needed.
I drive a little slower most of the time, around 62 mph. I do not use cruise control in anything but predominatly flat terrain for that exact reason. I pickup speed on downhills, maybe up to about 67- 70 mph, whatever existing steady pedal pressure achieves within reason, anticipating bleeding speed on uphills. Much more efficient than losing 3 or 4 mph while climbing before downshift and then attempting to regain lost speed while climbing.
Ray Pickle replied to C/K Man's topic in 6.6L Gas V8 & HD Transmission Powertrain (L8T/MYD)Couple of comments hear desiring 4.10 gear along with 10 speed. Frankly the need for 4.10 is greater now with 6 speed. 10 speed has deeper gearing and the benefits of lower axle ratio are greatly reduced, not to mention poorer mpg of lower gear offsetting potential gains from trans.
I agree to consider non- tow haul at your weight the truck will pull in 6th gear far more often which may improve your mpg. I would be curious of your results, please let us know if you try it. I'm such a frugal bastard I will turn tow-haul off whenever conditions allow to maximize mpg with a 12k fiver! Any sustained downhill grades or hills that it can safely be executed, or on level ground- slight inclines with a tailwind!
Is your truck pulling in 5th and occassionaly in 6th gear on flats to 2% grades? If not something isn't right. Do you lock out top 2 gears and pull in 4th to keep revs up? If so that would account for poor mpg. Iam at a loss otherwise for poor towi ng mpg. I can maintain speed on level ground up to 2% grades in most instances in wither 6th or 5th gear. Rarely do I downshift to 4th and rev to 3k range unless terrain is more hilly or headwinds. Check trailer brake adjustment, tire pressures on both?
Was that stop and go in town for a significant % of the tow? Possibly high headwinds or front quartering winds? That is low mpg for what you are pulling if your speeds are 65 or less. I average that pulling a 12k 13.5 ft tall fiver in the west and a little over 9 in the midwest. I acknowlege a fiver has better aerodynamics than a travel trailer but your shorter length, less height, and weight should more than make up for that.
Ray Pickle replied to OH Camperpuller's topic in 6.6L Gas V8 & HD Transmission Powertrain (L8T/MYD)Late answer- they clearly did not reset your o.l.m. I personally would do 5k on that oul and then follow o.l.m. moving forward. Obviously and unfortunately you need to check that they reset it!
Thats sound about right for DIC accuracy. I consistently see .3 mpg lower hand calculated vs DIC. Although it varies. Just to add to the mpg discussion I average 16.5 on the highway I have done better than 17 and as bad as 15.5..weather-terrain dependant. In town 12 is pretty typical. I typically run about 60 hwy 40 town per tank and hit mid 14s most tanks.
Ray Pickle replied to jeffd's topic in 6.6L Duramax Diesel & Allison Powertrain (L5P/MGM)If the brakes are not larger that could possibly be a point of minor worry.
Ray Pickle replied to jeffd's topic in 6.6L Duramax Diesel & Allison Powertrain (L5P/MGM)I would guess the obvious- more payload- more tires- higher towing max capacity- more width and weight. If
Without experiencing your truck's specific symptoms its hard to ascertain if it is normal drivetrain "slop". I will say that the length of driveline and rear diff setup will cause a minor clunk under certain downshift parameters. I notice it is most consistent slowing and turning as someone else has already mentioned.
Engine rpm / lack thereof, due to tuning by Chevy, is darn near all the difference.
I have towed that route and had no problem maintaining speed far above what TFL accomplished. Clearly the truck will perform much better than their representation. No one that knows anything about towing would allow their truck to lug like they did in their test. Understanding the desire to test uniformly, thus leaving the truck in drive, the ultra conservative tuning by GM caused the poor result. Another poster alluded to thermodynamic characteristics of small block being the reason for this tuning. My own experience indicates normal operating temps for coolant and trans temps ascending that grade at higher rpm. Further I see no significant differences in cooling capacity between the big three that would require the GM trucks to limit engine speed under those conditions more so than the competition. I would sure be interested in a more in depth reason for the tuning parameters GM has chosen to use in this truck. I don't see where a 10 speed would change the result much with the tuning parameters GM is presently operating this engine under.
Who's Online 28 Members, 0 Anonymous, 1,169 Guests (See full list)
- Grumpy Bear
- Johnny Johnston
- Daryl Z71
- 2001 workhorse 5.7l vortec
- Mike Barber
- Salsa De Piña
- Alan Hu
- Camille Leblanc
- Dmitrii Lysenkov
- Trapusouras Rex