Where 7 mpg is Good

[GeneK]

I saw on an older topic where someone mentioned they wished their was a real truck driver on site for feedback. It got me to thinking you might be interested in my "Work Drive".

Im a contractor with a large transportation company who runs the same terminal to terminal run every night. I thought you might be interested.

Vehicle - 2006 Freightliner Columbia with Double 28' Pups.
Weight - 20x typical car (53,000-80,000 lb)
Rolling Resistance - 22x typical auto (22 tires with 4x the rolling resistance each)
Air Drag - 12x typical auto (115 sq ft x 0.68 cd estimate)
Powertrain - 470/515 bhp - 1650 torque (Increased output in cruise) 14.0L S60 Detroit with 13-Spd Double OD Trans (0.73 Top Gear).
Drive Tires - Michelin XDA3 275/80R24.5 (493 rpm)
Engine Rpm - 1403 rpm @ 65 mph (Factory Spec with brand new tires)

Run - 202 mi x 2 over mixed roads with a maximum of 4 hours run time for each leg. Primarily rural non-limited access 4 lanes with a 65 mph limit.

Driving Technique - Get it over the top at minimum rpm in top gear and give minimum throttle (Sometimes none) to reach 61 mph by next flat. I use moderate throttle in lower gears leaving redlights in deference to the drivetrain (20,000 lb of torque is hard on ujoints). 1400-1600 rpm shifts in lower gears. 1300-1500 shifts in uppers (I can split these gears).

Note: In a 13 speed you have - A low you only use in extreme situations, You normally use 1-4 and then change ranges and go through 5-8 (Which are the same locations 1-4). You farther have a seperate OD on the back of the trans that only works in the "upper" gears which allows you to "split" the upper gears.

Cruise Speed - 1350 rpm (61 mph with current tire wear) where legal.
Average Speed - 58 mph
Note: The truck averages the best at 61 mph due to forced downshifts on hills at lower speeds.

Mpg - Average 7.2 mpg (6.8-7.3)

Mpg Various Cruise Speeds -
1200 rpm (54 mph) - 7.5 mpg
1250 rpm (56 mph) - 7.8 mpg
1300 rpm (59 mph) - 8.0 mpg
1350 rpm (61 mph) - 7.8 mpg
1400 rpm (64 mph) - 7.5 mpg
1450 rpm (66 mph) - 7.2 mpg
1500 rpm (68 mph) - 6.9 mpg
1550 rpm (71 mph) - 6.5 mpg


Note: this includes an average of 10 stops per trip (Stop Signs, Lights, Scales) and approximately 1 gal used at each end for assembly and disassembly of sets.

[GasSavers_RoadWarrior]

Nice info.

Why do you peg the rolling resistance so high though? Large diameter high pressure tires with a hard compound should have less RR than a car tire.

[imzjustplayin]

Yeah rolling resistance is kind of irrelevant anyways, the main point should be that per the weight, a semi truck is far more efficient than MOST cars and light trucks.

[GeneK]

Originally Posted by RoadWarrior

Nice info.

Why do you peg the rolling resistance so high though? Large diameter high pressure tires with a hard compound should have less RR than a car tire.

It could be inaccurate. I once read that a typical loaded truck tire had 4x the rolling resistance of a car tire. It might be because the truck tire might have up to 6000 lb on it versus 900 lb for a typical passenger car tire.

[civic94]

nice info... if a truck company can crack 10mpg on those things, they will make a ton of money

[markweatherill]

Somehow it's still more cost-effective by truck than by railway transportation!

[theholycow]

Great compilation of data! Thanks!

[R.I.D.E.]

Imagine the mileage you could get in a car if it was very aero and driven like the trucks, especially one with all of the high mileage tweaks used by truckers.

regards
gary

[bowtieguy]

i'd say 7 is good. i drive a straight truck w/ a refer(cold) unit. our trucks get from ~4 to 6mpg depending on the route, but being local delivery, we have many more stops than you do. i've often wondered what we could achieve w/out the refer running.

[GasSavers_RoadWarrior]

Originally Posted by Tightwad

It could be inaccurate. I once read that a typical loaded truck tire had 4x the rolling resistance of a car tire. It might be because the truck tire might have up to 6000 lb on it versus 900 lb for a typical passenger car tire.

Ah right, I'm thinking of the RR coefficient being lower, something like 0.006 vs 0.015 for typical truck vs typical car, but that's multiplied by the weight on them for the total RR. So actual rolling resistance would be 0.006x6000=36 vs 0.015x900=13.5, so yup, could be 2-4x higher depending on what weights and tires you pick for the example.