I love the 6.2L engine. The primary reason the 6.2L gets the same or better FE than the 5.3L is higher efficiency with premium fuel, a 3.23:1 axle ratio (lower RPM) and that it will remain in AFM at higher engine loads than the lower displacement 5.3L. This is if you can keep your foot out of it. Hard to beat simple and efficient power and torque.

The latest 5.3L and 6.2L engines have DFM where cylinders can be turned off to just a single firing cylinder.

Jim

I have no issues with running the premium fuel. My previous truck was a 2015 Chevy crew cab 4x4 with the 5.3. The new one (with the 6.2) gets better mileage (towing and not towing) than the 5.3. I imagine the 8-speed trans vs the 6-speed with the 5.3 makes a difference. Overall, I am totally satisfied with the 6.2......

Refclown,

This is what I would expect from the 6.2L engine where it will exhibit similar fuel economy to the 5.3L. You probably know this but your engine is premium recommended and not premium required. The means you can use fuel all the way down to 87 octane with no issues. So you could experiment with octane levels from premium fuel down to 87 octane and run the numbers on economy. Of course that 420 Hp will only be achieved on premium fuel. A friend I work with drives mostly highway (non towing) with the same truck as you where he achieves good all around drivability and economy on 89 octane.

Jim

I am towing a 2020 Imagine XLS 22MLE (about 6,200 lbs loaded) with a 2018 GMC Denali 4 x 4 Crew Cab with the 6.2L Engine (420 hp). I towed my trailer from central Calif to the Oregon Coast and back and averaged about 10.2 mpg round trip (at around 58 MPH). I ranged from 8.0 (going over some passes) to 11.5 on the flat land. I have the 8-speed transmission with 3.23 gears. The truck pulled the trailer quite well....never had a lack of power or any heating of the transmission. The only issue is that the 6.2 takes premium fuel....which can be a little pricey, but considering the performance....I'll accept the added cost.

Not towing I average between 14-15 mpg around town and about 22 mpg on the freeway at 70 MPH

Great Points Rob,

We are learning a bit more on jessearts truck where the all terrain tires will reduce FE. Sometimes there is a need for these aggressive tires where the trade off in not being stuck is easily worth the extra fuel usage. For toppers, again if there is a need, the benefit will outweigh the FE loss but toppers that have that high triangle shape that sits much higher than the cab will result in lower FE. The OP has not gotten back on the topper style but if this is the case, the FE with this and the all terrain tires could explain some of the losses. Also AC in warmer temps or seat heat in cold temps will reduce FE. Nothing is free.

Second to this is lower temperatures and winter grade fuel. I would need to look if Texas uses winter grade fuel in the northern region but if so, winter fuel is oxygenated and it will run lean. The reason for this fuel is for cold startability primarily. The downside is a 2-3% lean condition where the ECU will enrich to achieve what is called stoichiometric fueling. Other factors such as cold air that may be in the 40s in northern TX would also have an effect. But even after all this, I'm struggling with 6-7 mpg pulling a smaller rig at 60 mph so a visit to the dealer may still be needed to look into other areas.

Jim

I sometimes tow a flatbed trailer that weighs about 800 lbs empty and has an axle capacity of 2000 lbs. It has a (removable) flip up ramp tailgate that is a huge air drag. I lose a couple of MPG to the empty trailer without the ramp. Another MPG if the ramp is attached and maybe one more if the trailer is carrying cargo. Nothing like the 9 MPG that I lose when towing the 303, but it is not a linear relationship. Towing anything does immediately affect fuel consumption . . . probably mostly air drag.

Brian,

Not sure what to make of this. I tow my 20 ft boat at times and will achieve FE in the mid teens. I do not use tow haul with a light load in towing the boat buy always do while towing my RV.

Vehicles maintain equilibrium between load, mass and acceleration (F=ma), aero drag, and rolling resistance. Other factors such as RPM and a term called "unthrottled" comes into play for gas engines where high RPM and low loads results in additional pumping losses i.e. increase manifold vacuum will consume more energy.

Is this with your diesel truck?

Jim

I have always thought that the truck engineers put a code into the computer that said , "when a trailer is plugged in then get poor fuel mileage". I came to this conclusion when towing a small boat , around 1000 pounds or less and got the same mileage as pulling our RV.

Brian

Jesseart,

All terrain tires will result in lower fuel economy. If the all terrain tires are a larger diameter than the factory tires, this will result in a double hit on FE.

If your topper is of the large wedge shaped type, this will also cost you on fuel economy.

Jim

Great input, i purchase this truck new in Idaho almost two years ago. the final gear ratio 3.42. with standard box. i have install all terrain
tires and have a fiberglass topper. if i am not towing the RV i get around 18-20 mpg. I agree, it’s not a good idea to go with after market mods for all the good reasons mentioned.

Well stated Rob,

As a powertrain engineer I can concur with these statements. We must calibrate to meet emissions but also to allow for peak performance while being able to allow the powertrain to live well beyond the warranty period. Engines by all makers are calibrated to the same limits which are.

1) Fuel to stoichiometric fueling. This is the perfect air/fuel ratio to allow for complete and clean combustion.

2) Spark to MBT (minimum spark for best torque) spark or knock limited spark at high loads. Adding spark in the MBT region of the table adds no torque and if in excess can reduce torque. Adding spark in the knock limited region will result in spark advance being pulled back by the knock system.

3) Increase fueling at WOT or heavy loads to increase torque and power but to also cool engine components as well as the catalyst system. The higher the airflow, the higher the cylinder pressure and at some point, more fuel is needed to keep components cool and within their design parameters. Yes everyone will cool components at some point with enrichment.

4) On boosted applications, optimum boost is delivered to allow for maximum longevity of the powertrain. Believe it or not, analysis is done where the life of an engine can be determined by how many WOT events take place at each firing event and what cyclic stresses are placed on components such as pistons, rods, bearings and so on. The calibration engineer will use combustion analysis tools to ensure these peak limits are not reached. So add that tuner, to increase boost on gas or diesel? Better sell it before the warranty is up and a smart buyer should have the vehicle inspected to see if an aftermarket calibration was ever used.

I should also add...this is a gas engine topic. Diesels require a different approach but also should not be tampered with by using power adders as listed in 4).

Jim

The "myth" that OEM powertrain engineers would purposely design and calibrate to less than maximum performance is fundamentally mistaken. Emissions are controlled in a very narrow range of part throttle operation. WOT (wide open throttle) performance is all about maximum power and your powertrain already has this, constrained only by what is necessary to protect the engine from destroying itself. The aftermarket will push into those limits of potential engine damage. Again . . . don't go there.

Rob

Well said Rob and Brian! The combination of these two posts should be put out for everyone to see. I have been looking at some of these aftermarket products for quite awhile, but I have always wondered why the big companies don't pick them up and use them in their products.

Norm

And so dies another "hold my beer and watch this" moment...




You're rarely that blunt, Rob. Any thought (not that I had any) of doing anything to my truck just went buh bye...

jesseart,

A few comments that may help.

1) In your manual GM will stress to use fuel from Top Tier suppliers. This does make a difference. Top Tier would be the major brands and can also be found online.

2) Moving up to high octane 93 octane fuel should not make a difference (actually will be less efficient on engines calibrated to 87 octane) where moving from 87 to 89 can help in some circumstances. Especially if not using top tier fuels.

3) A K&N air filter is a waste of money where the oils from the filter could contaminate the mass air flow sensor. Any claimed gains from them are small and at WOT which we do not spend so much time at.

4) A cat back system provides no benefit since the highest restrictor in the exhaust system is the catalytic converters. The only gain is noise and a loud system while running active AFM will not sound so good.

5) Tuners advance spark in order to burn premium fuel which will increase torque and power. The tune will however not increase torque and power on the intended 87 octane as the engine is already sparked for this fuel. GM can tell if a tuner has been used (even if removed) where the factory warranty will not be honored should an engine failure occur.

6) You may want to visit the dealer with a complaint on low FE. They will be able to check 02 sensors, knock sensors and other items such as brakes. Is this a TX truck and was it purchased new? If the truck has seen some time in the rust belt, an inspection for front brake rotor drag would not hurt. The GM caliper brackets can build up rust between the bracket and the stainless guide plates which can cause pad binding and increased friction.

Jim

9 mpg is within normal limits but your rig is smaller where I believe you should be seeing higher numbers. Most everyone here will see MPG at around 9-10 while towing larger rigs. What final drive ratio do you have?