most efficient speed for heavy trucks

[farmfromkansas]

Was wondering if anyone has tested their 3/4 ton and heavier trucks to find the most efficient speeds.  With this 4$ plus gas price, need to figure how to use trucks the most efficient way.  When I was in college 50 years ago, the shop teacher had this "gas per mile per gallon gauge" that we could put on our cars to find the best speed per mile of gas, and found that 30 to 40 mph my buddy's old Ford did best on gas.  Thinking of driving 40 to reduce fuel use.

[sawguy21]

I hear ya, fuel prices are brutal and won't change anytime soon. My Ram 2500HD with 5.9 ctd does best at 1700-1800 rpm which is about 60mph, any faster fuel consumption goes up in a hurry. Same at a lower speed especially when towing, it doesn't like being lugged. I once had a Dodge hal ton with a 360 and oversize tires that came with it. I discovered it got better mileage at 75 than the legal 60 because the engine was now working in its sweet spot. Slowing to 40 won't help and will annoy everyone behind you.

[Dave Shepard]

My '99 Cummins was most fuel efficient at 1,700 according to Cummins. I think that's why the newer six speed autos have two overdrive ratios. 

[red]

Use a Vacuum Gauge for best MPG 

[barbender]

It depends on a lot of stuff. I have an 03 Dodge 3500 Cummins 4x4. It has 4.10's and the mileage is poor. It could easily pull another gear. We like to pull our 30' 5th wheel out West, you get in western states with a 75-89 mph speed limit and run that, it will be down to 7-8 mpg. I can nearly double that staying down around 60.

[jimbarry]

We bought a new 2021 International 6500, it gets better fuel economy 14.3L/100km at 80 km/hr than our 2011 2500HD Silverado (14.9L/100km). Both Duramax 6.6L diesel engines. Both 4x4.
Once you get into 100km/hr speeds the fuel economy of the International climbs to 15.7L/100km.
That's hauling empty. Both trucks hauling 10-12,000lbs will get up into the 19-20L/100km.

[Southside]

63 with my Duramax dually gets me 20 MPG

[barbender]

20mpg calculated, or on the computer?

[Southside]

Calculated.  LBZ engine, light EFI Live tune, 420K+ on the clock, drive it like there is an egg under the pedal.  If you stand on it the results are different.  When I can get B99 for it she does even better. 

[barbender]

My 97 Dodge dually 12 valve Cummins hit a calculated 23.6 once, the last time we had $5 a gallon diesel I was trying to see just how many miles I could squeeze out of a gallon. Same thing, drove like there was an egg under the pedal and never faster than 55. That truck would routinely get 21 just normal driving. Another victim of MN road salt, gone too soon👎

[newoodguy78]

Drove an 01 dodge single rear wheel with the Cummins and manual 6 speed for years. It weighed around 9,000 with what I was carrying for daily use. Never put my foot into it,Keeping right around 60 mph it consistently got 20-21. Once you climbed above that the mileage drop was noticeable. 
Unfortunately the salt ate that one, got to the point the only thing between my feet and the road was the floor mat. 
Have a little Chevy Colorado with the 5 cylinder and manual transmission now. Anywhere between 55-70 on the highway mileage seems to stay the same. 

[Ianab]

Have a little Chevy Colorado with the 5 cylinder and manual transmission now. Anywhere between 55-70 on the highway mileage seems to stay the same.

With modern vehicles that's pretty much true. The top gear is generally selected to give the best economy at the local speed limit. Car companies have issues when selling in different markets, like cars that only select top gear over 60mph, in a country with a 55 mph limit. The cars with  an 8 speed never got into top gear (legally).

Testing a friend did years back (when car computers were a new thing) found he got the same  mpg at 80km/h and 118.  Somewhere In between was obviously the sweet spot, and my Dad's old Audi was about 113? (70mph).

Basically no matter the gearing, as you get over about 70mph the aerodynamic drag starts going up exponentially, no matter how "slippery" the vehicle, it becomes more and more of a "drag", and that's what kills your economy. 

Interestingly my Toyota with it's fancy cruise control will drop gears to maintain 60mph, to keep the engine in it's "economy" mode. Driving in auto, it usually stays in 6th for a slight hill, but the "eco" light goes off. Dropping to 5th gear, about 300 more rpm, and the car maintains speed while staying in the "lean" eco mode. But that also relates to the "drive with an eggshell on the pedal" comments.

[B.C.C. Lapp]

My F250 and my GMC 2500 get better mileage by far when Im towing the loaded dump trailer or the flatbed trailer than if my son drives.   Remarkably better.  My son puts it right down the road at 55 or 60 mph on the two lanes.   I rarely get over 45 MPH on the same trips.   Yeah, I drive slow and get past a lot.  I don't care I'm careful and aint in a hurry.      And that saves quite a bit on fuel every month.

[doc henderson]

I have things to do and people to see.  I drive the speed and poss. 5 over.  In Ks, it is a non moving violation if you get a ticket under 10 mph over.  Our top speed is 75 mph on I 35.  I do watch the pedal.  merging onto a highway in traffic, and all bets are off.  getting up to speed in the right lane I take my time, but also consider the flow of traffic.  It is not only the acceleration, but I take my foot off the pedal as soon as I see a speed decrease ahead sign.  I find I coast and at about the time of the new speed, I have decelerated to the proper speed or maybe slightly above.  same for exits ect.  If you use your brakes, you are wasting fuel that you used to accelerate in the first place.  also wearing brakes.  If you see black smoke coming from the exhaust, I am told that is unburnt fuel.  obvious waste. I get about 18 empty and 11 to 12 pulling 32 foot gooseneck (5k empty, up to 24k), or the 41 foot camper (16k).  I do not use cruise with the trailers and allow some drop in speed topping steep inclines.  lots of those in Ks.  Min speed on most highways is 40 here.  but with top speeds of 80+ it creates a little potential for accidents, not keeping with the flow.  My son in his little Jeep, will still accelerate toward a red light half a block ahead.  I find myself stepping on the floorboard, and occasionally ask him if he plans to stop.