Driving for fuel mileage

I'm not sure if that's a fair comparison. With the exception of those running e logs most owner operators are going to get the load there and make it right on paper. I don't run 55 but very rarely does my truck go over 70. Like stated above, if you're trying to beat closing then go for it. But I think more often than not in the real work it would be the difference between rolling into town at 8pm versus 10pm, which to many would mean getting a parking spot I guess. That's not something I have to deal with.

 

The "average" truck (according to Cummins whitepaper) uses 320 horsepower at 70mph vs 180 horsepower at 55mph:



Horsepower and fuel consumption (per unit of time) are correlated, so:

70mph truck uses 78% more fuel per same time ( (320-180)/180 )
70mph truck goes 27% farther in same time ( (70-55)/55 )

So in 1 hour, your 6.2mpg 70mph truck would burn 11.3 gallons (70/6.2) and go 70 miles. In 1 hour, the 55mph truck would have burned 6.34 gallons ( 11.3/ (1 + 78%) ) and gone 55 miles (55/6.34= 8.7 mpg)


16,100 gallons/year 70mph
11,500 gallons/year 55mph
---------
4600 gallons/year ($9,300-$23,000 per year fuel savings at $2-$5 per gallon)


In addition to fuel, maintenance costs are also correlated to power (just think of drive tires alone -- going 1 mile putting down 180 horsepower vs 1 mile putting down 320 horsepower)



That said, if you need to get there by a tight deadline, by all means, drive as fast as is required (within reason). Likewise if you know you can get reloaded in the same day if you hurry, hammer down. But racing to get a front-row spot at the truck stop where you'll sit for 12 hours just doesn't make much sense to me...

 

A truck geared to run optimally at 70mph is, at worst, going to be maybe 200rpm off of optimal at 55 mph. That means instead of a BSFC of 0.320 you might be at 0.325 -- a whopping 1.6% inefficiency compared the the 40% gain in efficiency at the slower speed.

 

Roughly equal. Look again at the BSFC graph:



So for that 1994 12.7 detroit at 320 horsepower, the BSFC is ~0.310 at peak efficiency (1400 rpm). At 180 horsepower, the BSFC is ~0.325 at peak efficiency (1400 rpm)

320 horsepower * 0.310 lb/bhp-hr = 99.2 lb of diesel burned in an hour (~7lb/gal ~= 14.2 gallons)
180 horsepower * 0.325 lb/bhp-hr = 58.5 lb of diesel (~=8.4 gallons)

(14.2-8.4)/8.4 = 69% -- which isn't terribly far off from the 78% we'd estimate if we assumed perfectly equal BSFC.

On perfectly flat ground, my truck does ~9.3 mpg at 55mph vs ~6.8 at 70. The more aerodynamic your truck, the less variance you'll see. Here's what that cummins power graph would look like with just a trailer tail (~10% aero improvement, no change in other resistance):

 

Hey Danny, if you get clucks Swedish meatball, it means no more driving slow... your gonna have to race from one service center to the next [emoji2]

 

My first thought was maintenance costs - drive train stress travelling at higher speed, to offset the cost in your first post Danny. Have to be tracked consistently over a long period, but I'm thinking it might narrow the gap.

Funny, after so long speed governed, I kinda feel a bit naughty busting the 65 barrier - but it is nice to get around much slower traffic without causing a fuss. Usually I'm at about 63 - 66, and watching the scan guage.