The alternator really doesn't produce much drag. Fully charging it's only around 2 hp. Something like turbocompounding or exhaust heat recapture can produce up to 50 "free" horsepower.

Some states, yes, but I'm talking about across the board

No and no. Take a 10 spd overdrive with 3.58 gears, and a 10 spd direct with 2.64 gears. Both behave exactly the same, same startability and shift points. But in the final gear the torque goes right through the direct drive, and has to go through a gearset for the overdrive. This results in a 2-3% or so gain in fuel mileage. Schneider has known this for years, and they always spec direct drive trucks. It's the way of the future. Unfortunately 2.64's are the lowest numerical gearset currently availabe. If one wants to run efficient 22 LP tires, the top speed is very limited. Much like 3.58's with an overdrive transmission with 22 LP tires.

At 70 mph you'd be screaming at 1600 rpms.

With direct drive and fuel efficient tires(22.5 LP's or super singles) you need to run slow. That's why you don't see direct drive transmissions very much.

 

Allan5oh........great posts with good information. I also read that the alternator only accounts for about 2 hp.

I know this isn't related to aerodynamics but your gearing example brings me to my question. Where can I find an article explaining gear ratios? Or can someone here explain it to me? I ask this as I can't find an explanation of it anywhere, and most drivers I've talked to don't understand it beyond saying that one is for higher speeds versus another for higher weight. Any info would be greatly appreciated.

Oh, and Mandilon........As Ron White said, "you can't fix stupid." One can't even begin to figure out where to start in correcting such a statement as yours.

"Even if THEY did, we'd B paying the American terrorists (domestic oil co.'s) INSTEAD of THOSE islamic (et al foreigners, like the idiotic Shov-ASS [chavez?]) terrorists , since we wouldn't B importing as much oil."

 

Flying W... I did an explanation on here somewhere, I'll try to find it and give you the link.

The quick and dirty is that the # is how many times the driveshaft spins vs. tires.

For example, 3.90's the driveshaft would spin 3.9 times every time the wheels spin.

This gives torque multiplication, but reduces the speed that the truck can run.

Consequently, a truck with 3.55's can run much faster, but torque is multiplied less.

If anyone is interested, I got my greasy hands on an EIGHTY page aerodynamics report from TMA. Very good stuff. There's 12 pages on mirrors alone! Trailer gap, deck plates, trailer rear and side fairings, truck belly fairings, very cool stuff!

 

I'd much rather have an areodynamic truck over a large car.

 

I appreciate the response, and look forward to seeing the link. My understanding of torque is that torque helps you accelerate. So the 3.90 would get you up to speed (highway) quicker but not be as efficient/smooth/etc once there as the 3.55 gear ratio?

I understand this is an aerodynamics forum but ultimately we are talking about fuel mileage. Well a few of us might just want better aerodynamics so that we can have the window down with less noise from the howling wind when driving. 8) So picking the right gear setting for the application should improve fuel mileage (or could)? There has to be a buyers guide or something addressing this. I'll keep looking.

I can't speak for the others but I'd be interested in that TMA report (mirrors, deck plates, and truck belly fairings particularly).

 

Maniac,

30 years ago, mpg's may have been in the 5's, like today. However, todays engines are burning cleaner than back then. EGR, for example....

 

Anyone hear that guy on XM Trucking channel on the weekends? He's pretty sharp until he says something stupid like "We don't need this horsepower we're making today. We used to have 235 to 290 and we made it up the hills just fine" Well, I remember those days, and I DON'T remember getting up the hills just fine.

 

Why would anyone want to waste evan TWO HP when it can be gotten 4 FREE from the wasted heat from the exhaust :?:

Alternators are in the upper half as problamatic (tires are #1) as far as breakdowns. What's THE COST of an alternator breakdown, lost revenue et al :?:

God Bless all

 

I remember those engines. The first truck I bought had a 238. The truck just kept on going. It would pull up the steepest mountains, but take much longer to get there than the newer more horsepower engines. On flat ground they would do as well as the big hp engines. Unfortunately, there are a lot of big hills in this country. We could still get along with the smaller engines, but would spend more time climbing the hills. That truck only got about 5 mpg as I recall.

 

I'd like to see that report allan5oh. I'm looking for info that covers mileage increases on just the tractor to get a feel for how much aerodynamics would play if pulling a flatbed or tanker. Most always describe a tractor-van combo and cumulate the results rather than saying "the slanted windshield resulted in 1.2% gain, the single wide tires on the tractor .8%..." etc.