Report on Chevy/GMC 1500 with 6.2 engine and Max tow pack

[DC Bruce]

OP here. Now having 84,000 miles on this truck about 50,000 of which are pulling my Airstream, this is my experience: (1) porpoising. When I first got my rig and was dialing in the amount of tension on my w/d hitch, I experienced the same effect. The solution was more w/d tension (not a whole lot, just a little). I have a ProPride hitch using “1,400 lb.” bars. It’s probably getting near time to replace the shocks. Assuming Bilstein makes a fitment for the max trailer tow package (which, as a minimum, includes different rear springs and axle), I’ll probably go for them as opposed to OEM. (2) As I have said before, the 8-speed is a little problematic at very low speeds. There have been several reflashes of the controller and after our last X-country and back a few months ago a super-abrupt lock-up just getting away from a dead stop, under load or not, along with a vibration at around 50 mph. A transmission flush seems to cured those recent problems. I suspect there’s a TSB about that; my dealer said to come back if the issue wasn’t resolved in a few hundred miles. Newer versions of this truck have a 10-speed, so owners of those models won’t have to deal with this problem. That would be my only reservation about getting this truck as a used vehicle. I think the 10 speed started being used in 2018 models but it could have been 2017. The 8-speed is in the 2015 and 2016 models for sure. The 2014 model has the 6.2 DI engine but uses a 6-speed with ratios very poorly matched to the engine. I would avoid that year.

[nvestysly]

Modern vehicles have computer control, algorithms, fuel management, etc. Many people have reported the MPG of the 6.2L, 8-speed, max trailering truck in a variety of situations on this particular thread. The driver information center on this truck (at least our 2018) indicates average fuel mileage between fill-ups. Or you can calculate the old fashioned way - miles driven divided by the number of gallons of fuel to top off the tank.

If you have a 6.2L, 8-speed, max trailering GM truck how do you determine MPG? Have you noticed much difference between the number the truck (computer) reports and the manual method of calculating fuel economy?

I'll go ahead and tell you why I ask... The computer number is much higher than the number I calculate. For instance I filled up the tank today, after combined highway and city driving, and the computer indicated 16.6 and I calculated 14.4. It's not uncommon for my calculated number to be 2 MPG less than the computer number.

What gives? Am I doing something wrong? Am I misunderstanding what I see on the driver information center? How do your numbers compare? If you've reported MPG numbers in a previous post on this thread which numbers where they - DIC or calculated?

[DC Bruce]

The truck does not calculate mileage between fill ups. Depending on settings, it calculates mileage over distance. The largest distance, if memory serves is 400 miles. The shortest, I believe, is 50. It’s a “rolling” average, so it’s always updating itself. The trip computer (there are 2) will calculate mileage for the trip. So, if you wanted to compare the a hand calculation to the truck’s, you could reset a trip computer at fill up to zero. Then, the next time you fill up, you could read out the truck’s calculation of mileage and compare it with your own. The problem with a hand calculation is that unless you fill up to the point where you spill fuel out of the fill pipe (not a good idea), it’s just about impossible to get a consistent fill. So the hand calculation isn’t going to be totally accurate. The truck calculates fuel running through the fuel system and it has to be very accurate because otherwise the engine management system would not achieve emissions targets. Both hand and truck calculations depend on the accuracy of the speedometer/odometer. If the speedometer reads a little fast (as is common with many vehicles), mileage will be overstated. On my truck, the indicated speed on the speedometer matches the speed shown on the GPS, so I assume it’s accurate. By contrast, my 2008 Honda Pilot speedometer reads 1 mph faster than what is shown on the GPS.

[nvestysly]

I've seen the rolling average best/worst MPG and noticed it has three or so "averaging" distances - I seem to recall 25, 50 and 400 miles but I'm not certain. Changing the distance does change the best/worst calculation shown on that screen.

The MPG I'm referring to is the Trip A parameter. I reset it every time I fill up with fuel. I've not recorded the average fuel economy shown on the Trip A screen but I know it's always been higher than my calculated MPG. Almost always 1 MPG higher and sometimes 2+ MPG higher than my calculated value. I realize filling up the tank is not a precise science but I typically go to the same gas station around town (it's pretty level) and I typically look for a relatively level area to fill-up when we're traveling.

Regardless, if I'm consistently 1 - 2 MPG different than the trip value that means I'm pumping in more gasoline than the tank can hold over time. Even with some measure of inconsistency in filling the tank it should average out. I've put fuel in this truck over 20 times and it's always more gallons than the Trip A screen would lead you to believe.

I'm not complaining about the MPG - I like the truck and its fuel economy is higher than the previous tow vehicle although with the premium fuel requirement the $$ are probably a wash.

Let me get at this issue another way... Let's consider doing a science experiment with our 6.2L, 8-speed, max trailering trucks. Consider checking your MPG the old-fashioned way: miles driven divided by gallons put into the tank and compare that to the fuel economy shown on the Trip A screen.

I'll do this for the next several tanks of fuel and report back on this thread.

P.S. I've done some preliminary validation of the speedometer and odometer using a separate GPS and the numbers correspond very well. There doesn't seem to be enough variation to account for this odd MPG variance.

[jaybauman]

Originally Posted by nvestysly

Even with some measure of inconsistency in filling the tank it should average out. I've put fuel in this truck over 20 times and it's always more gallons than the Trip A screen would lead you to believe.

How vigorously do you fill your tank? Do you keep topping it off after the pump has automatically shut off? Modern gas pumps have a vapor recovery system with an ill-fated feature that pushes all "extra" fuel back into the pump reservoir tank. In simple terms, topping off your tank simply raises your fuel bill without any additional liquids entering your vehicle's tank. This would serve to artificially lower the value of any manual fuel efficiency calculations.

[DewTheDew]

The variance in fill will not make a significant difference in the calculated mpg. It is, at most, 1/4 gallon variation in a 30+ gallon tank and, as noted, it would average out over a couple of fills.

[dznf0g]

Originally Posted by DewTheDew

The variance in fill will not make a significant difference in the calculated mpg. It is, at most, 1/4 gallon variation in a 30+ gallon tank and, as noted, it would average out over a couple of fills.

I believe you would find it it be more than that since returnless fuel delivery systems came about.

[gypsydad]

Originally Posted by dznf0g

I believe you would find it it be more than that since returnless fuel delivery systems came about.

Are we splitting hairs yet?? Waiting for the "science guy" and "engineer" to weigh in next!

[dznf0g]

Originally Posted by gypsydad

Are we splitting hairs yet?? Waiting for the "science guy" and "engineer" to weigh in next!

Nope, up to to 2 gallon variation, depending upon truck attitude, particular pump shutoff variation, and a much larger expansion volume provision in the returnless fuel delivery system instituted after a pretty aggressive evaporative emissions standard change. That change necessitated the returnless system and larger expansion volume design tank. (on GM vehicles...my only area of detailed knowledge)

[white laker]

Hi again folks! Since my last post we have managed to jump over the 100,000 mile/ 160,000 km mark on our 2015 6.2 max tow. This truck has pulled our 31 and now our 34 slide from the east and west coasts in Canada as well as the east and west coasts of the southern U.S. as well as a short run down the Baja, and has never given us a moment of trouble while on the road. We have had a few minor warranty issues but a wonderful network of G.M. dealers have all taken care of us as needed so no issues there. As far as gas usage goes , I use the information provided from the system as a guide. Is the number going up or down? The actual # isn't important to me. Just is it changing and if it is,why? My big decision was to keep it or trade it after the warranty expired. We that will happen this week and after looking at the new trucks I decided to keep it and run it for a few more years, which is a departure for me. That is my testament to reliability and confidence that I have with this vehicle, not any statement about the new product. I believe that about half of the mileage on the truck was with it hitched up and the rest was daily driving which is very comfortable. When I do tow, I am pulling the heaviest trailer that Airstream ever built, a 34 slide. I am yet to have any pulling, stopping or control issues with this combination and fell safe at all times doing so. This is and has been a great tow vehicle and will recommend it to anyone pulling any Airstream.
My working life was with two wonderful manufacturers of diesel engines,Cat and John Deere and having worked with them for 40 years I do not want to go diesel if I don't need to. With the 6.2 Max Tow I do not need or want to.
thanks
Pete

[Andrew T]

Thanks for the update

[Steve Bryant]

I want to post an update of my prior comments on the occasional mildly harsh 1-2 up shift of the 8L90 transmission. This would only happen intermittently under very light throttle application.

I decided to take the truck in about a month ago and left it over night with the dealer so that they could see what was happening from a cold start. The technician found no diagnostic codes, but when he drove the truck, he experienced what he considered to be mildly harsh 1-2 and 3-4 up shifts and down shifts when cold. The dealer replaced the valve body under warranty (truck is barely two years old) and set up the fast re-learn mode for the transmission. Now, after one month of driving, I think that this is the smoothest shifting transmission that I can remember (other than the old Buick Dynaflow transmissions of the 1950's) and I'm 70 years old. It's simply excellent!

[Countryboy59]

Smooth is not always good when it comes to an automatic transmission, especially for towing. Smooth shifts can cause increased wear of clutches.

[jcl]

Originally Posted by Countryboy59

Smooth is not always good when it comes to an automatic transmission, especially for towing. Smooth shifts can cause increased wear of clutches.

More primitive transmission designs accomplished smoother shifts with clutch slippage. That is not the case with modern designs. By integrating the engine control strategy with the shift control computer, manufacturers can accomplish smoother shifts by reducing engine power momentarily during the shift event. This reduces clutch wear, but more importantly it reduces heat and improves efficiency. It has been taken to such an extent that lock up torque converters are being kept locked up for much more of the time, further improving efficiency. Also, more gears means smaller steps between gears, so less slippage per shift.

Clutch wear is not a factor as it once was. The reduced heat associated with that has a lot to do with the extended change intervals specified for transmission fluid.

[SSquared]

A comment on the discrepancy between gas mileage calculated by the truck via the trip computer, vs. the gas pump. I met a retired Big Three emissions engineer recently. He said the computer calculates the mileage based on the fuel being delivered by the injectors. What it overlooks is the fuel being sucked out of the charcoal canister. They have no way of measuring that. So told me to always expect the computer to be optimistic.

This engineer did not work for GM, but I suspect the same rule applies to GM vehicles.

[nvestysly]

We've had this truck for nearly a year and there are certainly many things to like about it. However, the 1st to 2nd shift and 2nd to 1st shift can exhibit a bit of a lurch in some situations. That subject has been mentioned a number of times on this thread with some owners finding improvement by having the dealer to reprogram the computer. I think DCBruce (the OP) went as far as having the dealer replace the valve body to provide improved shift patterns.

I've recently experimented with putting the truck in tow haul mode for day to day driving. My anecdotal results indicate the lurch in upshifts and downshifts is completely eliminated. A trade-off is each gear is held a little longer so shifts occur in the 2000 - 3000 RPM range rather than 1500 - 2000 range we normally experience during day to day driving. I'm planning to use tow haul mode for the next few tanks of gasoline to see the results on MPG.

Anyone else use tow haul for day to day driving?

[subfan1]

I knew about putting the transmission in tow haul mode and I gave up on our truck. We went with the lemon law on the transmission and GM bought it back. We are picking up our 2019 Sierra Denali next week equipped with the new 10 speed transmission. I have driven it and it feels very smooth. Nothing compared to the 8 speed. I’ll break it in through December and start towing next month.

[twbucksr]

I too occasionally experience the same lurching on my 2 year old GMC Sierra 6.2 with the max tow package. I will start the tow mode daily driving experiment tomorrow and report back on the mpg.

[pappy19]

Originally Posted by subfan1

I knew about putting the transmission in tow haul mode and I gave up on our truck. We went with the lemon law on the transmission and GM bought it back. We are picking up our 2019 Sierra Denali next week equipped with the new 10 speed transmission. I have driven it and it feels very smooth. Nothing compared to the 8 speed. I’ll break it in through December and start towing next month.

Next time rent a Ford F250 and see the difference between a GovernmentMotorsCompany and a real truck. You will be pleasantly surprised.

[gandttimes]

Originally Posted by pappy19

Next time rent a Ford F250 and see the difference between a GovernmentMotorsCompany and a real truck. You will be pleasantly surprised.

You do know that this is a discussion about real trucks, not F trucks don't you?