Marty the Mutt Mom wrote: This topic brings up something I've been wondering about for some time. I've been told by a couple people (one a professional trucker) that diesel engines get their optimal MPG at 55-60 and going slower than that will not help. Anyone have any definitive information about that? And if it's true, is there an optimal speed/mpg factor for gasoline engines? I've been wondering about this for months and I could get a good night's sleep if I find out!
Marty
Most 18 wheelers are geared for the best mileage they can get at a freeway cruise speed. Change the gearing, you change the RPM the engine turns at for a given speed. Rigs meant for rough service (cement trucks and the like) are geared pretty deep, won't go very fast because of that, and get awful highway mileage as well. A cross-country runner would spec a completely different set of ratios. The last highway rig I drove had a 3406 Cat that developed around 1600 ft/lbs of torque at 1100 RPM. It would cruise 65 at 1350 RPM, and averaged 7.42 MPG. In the 55 MPH states, the mileage could get close to 9.
Going slower will help if you don't mind the extra time.
Marty the Mutt Mom wrote: This topic brings up something I've been wondering about for some time. I've been told by a couple people (one a professional trucker) that diesel engines get their optimal MPG at 55-60 and going slower than that will not help. Anyone have any definitive information about that? And if it's true, is there an optimal speed/mpg factor for gasoline engines? I've been wondering about this for months and I could get a good night's sleep if I find out!
Marty
Most 18 wheelers are geared for the best mileage they can get at a freeway cruise speed. Change the gearing, you change the RPM the engine turns at for a given speed. Rigs meant for rough service (cement trucks and the like) are geared pretty deep, won't go very fast because of that, and get awful highway mileage as well. A cross-country runner would spec a completely different set of ratios. The last highway rig I drove had a 3406 Cat that developed around 1600 ft/lbs of torque at 1100 RPM. It would cruise 65 at 1350 RPM, and averaged 7.42 MPG. In the 55 MPH states, the mileage could get close to 9.
Going slower will help if you don't mind the extra time.
At 55 MPH the RPM would be about 1142. It appears that the truck was geared for 55 MPH. It would be interesting to know what the MPG would be at 1150 RPM and down one gear. I would bet it may have been about 10 or 11 MPG. The change from 7.42 to 9 is a 21% improvement. That is a significant improvement for a 10 MPH change in speed. I have found my 5th wheel will get better mileage at 45 than 55 even though I have to go down one gear. Not sure where the best mileage would be but probably around 35 to 40 MPH. Wind is the most significant item at speeds greater than 45. Most hills are only a small percentage of the miles driven.
RCMAN46 wrote: At 55 MPH the RPM would be about 1142. It appears that the truck was geared for 55 MPH. It would be interesting to know what the MPG would be at 1150 RPM and down one gear. I would bet it may have been about 10 or 11 MPG. The change from 7.42 to 9 is a 21% improvement. That is a significant improvement for a 10 MPH change in speed. I have found my 5th wheel will get better mileage at 45 than 55 even though I have to go down one gear. Not sure where the best mileage would be but probably around 35 to 40 MPH. Wind is the most significant item at speeds greater than 45. Most hills are only a small percentage of the miles driven.
If I left it in 9th/over (or 13th gear) is would chug right along at 55 at 1100 - 1150 RPM. Had to grab a gear to run over a beer can, but the mileage was sure better.
I don't remember the precise mileage at 55, which is why I said it "could get close to 9." It was still a nice jump, and is mostly attributed to a reduction in RPM and shoving less air out of the way.
If you are not able to get 23 MPG at 60, you must have a Bee with a gasoline V-8 with a design heritage that leads straight to 1954.
In our '07 LTV Free Spirit ('06 Sprinter chassis w/ 2.7L I-5 Mercedes Turbo-Diesel) 23MPG is no sweat in the 55-60 range. Speed up to 65-70 and mileage drops to 22MPG
Slow down to 50-55 (why does one have an RV if not to enjoy the scenery of back roads) and the mileage jumps to 26MPG!!! Have also gotten 26MPG on tanks where speed was 45-50 in mountains. In our Sprinter speed definitely makes a difference; a bigger factor than grade.
Additional test runs on our van showed that the best mileage was at 50 mph (level road, constant speed). If we try to drive below that speed (level road, constant speed), it shifts down to 3rd gear and MPGs get worse. (at 50 mph it was about 20 mpg, at 45 mph, it was about 16 mpg (3rd gear))
This was a surprise to me - I would have guessed that 45 mph would have been better MPGs - but because it cannot hold 4th gear anymore, it's worse.
It also surprised me that the engine could handle small rises in the road at 1250 rpm (~50 mph/4th gear) without shifting down.
Anyway - it's been fun to experiment with it and see what effects it has. Maybe on the Moab trip we'll be able to drive 55 mph on long stretches of road and see if we can confirm what the scanguage says.
All so true, speed, the higher is a big killer of mpg. Just got a scanguage recently (more to use in my Escape Hybrid), but took it along on a recent 4,000 km trip in the LTV van (2000 ol' gas generation). I must admit I was surprised at how much better the mpg was if I drove 90 (about 55). It's a trip I've made often and there's one leg that is always a bit of a knuckle biter for making a particular gas station (ponts!). Going 90, it was no sweat, there was plenty of gas in the tank.
I think where it made the biggest difference was going up hills, where I would monitor the scan guage and think, "Hmm, do I really need to push it to get up this hill - Not!"
I found it interesting that truckers are constantly complaining about fuel prices, but when they know the road is clear, clear of 'smokies' that is (which I assume they know from other truckers ahead of them), man they blast along, no doubt just guzzling the fuel!" I think the whole industry should relax and slow down.
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