This should help explain powerband ratio...
A diesel that makes 500/1000 and a gasser that makes 700/500.
Gear 'em so the diesel is at it's TQ peak, and the gasser is at it's HP peak.
Put 'em head to head on the hill with the trailer that Jake mentioned, roll WOT and watch the gasser eat the diesel for lunch.
Now, if that's a very long hill the gasser will most likely kill itself before reaching the top, and even if it survives. Gasser's don't live long at that power level.
This is why diesel's are the kings of heavy hauling. In addition to their incredible ability to take abuse for mile after mile, they have excellent powerband ratio's that allow a driver to make use of his engines potential at a wide RPM range. I.E. powerband ratio.
Now, powerband ratio is quite literally that. Today, we'll talk about the 90% ratio of a couple engines making similar power. The 90% ratio is simply the ratio of RPM's that it makes at or above 90% of peak HP.
First, we have my old PMR motor's dyno sheet. 361/770. Not that impressive, right? But, lets take a look at the 90% powerband ratio. Max HP is 361, so 325 would be 90%. 325 HP begins at 2200 RPM's, and the pull ends at 3800+ with it still making 325 HP. So, the 90% powerband ratio we'll use for this motor is 3800 (rpm's) divided by 2200 (rpm's) = 1.72 ratio.
Now, we have a lightning motor. I pulled this off google, so I have no idea the history or application or mods. Just that it makes about the same max HP. We'll use the same 325 HP from my old motor, even though this gasser is making just a touch more. 325 HP begins at 4000 RPM's, and is still there at the end of the run at 5400 RPM's. So, once again, divide the upper end of the powerband by the lower end, and 5400 / 4000 = 1.35
Let's translate the difference there...
Put both motor's in the same truck, with a 3:73 rear end and a ZF6 trans. Run 'em down the road and go WOT at 50 MPH. Just so the gasser has a sporting chance, we'll put the diesel in direct drive, 5th @ 1900 RPM's, and the gasser in 4th @ 2500 RPM's.
Roll wide open.
At 57.9 MPH the diesel gets into it's 90% (2200 RPM's) and begins to pull away, it'll continue to pull in that powerband until it reaches 100 MPH. Incidentally, the upper MPH divided by the lower MPH is 1.72, ring any bells?
Lets get back to the gasser... In 4th at 50 MPH he goes WOT, but doesn't get to his 90% until 80.3 MPH, which lasts until he hits 5400 RPM's at 108.5 MPH. And, you guessed it, 108.5 / 80.3 = 1.35.
So, while he's luggin it up the hill hoping to get to 80 MPH so he can use his powerband to make a decent time of it in the gasser, you've been comfortably floating along right there in the powerband, lettin off the throttle for corners, and rollin right back into it on your way up that mountain.
Now, he could downshift to 3rd gear, and he'd be right at 4000 RPM's and the bottom of his powerband at 50 MPH. In which case he'd jump out on you while you got up to 57 and came into yours. But, at 67 MPH he'd be at his redline, and he'd have to shift to 4th, which would drop him back under his powerband to 3350 RPM's. And you'd come walking right on by him in fifth gear as he struggled to get his speed back.
Anybody remember Grandpa or Dad luggin his truck up a mountain? He'd lug out a gear, and downshift. The downshift would speed the truck up, and he'd shift again, only to repeat the process each time he fell out of his powerband.
Well, if Gramps had the powerband modern diesels do, he'd have never had to downshift.
So, hopefully I've done a fair job illustrating why diesels pull better.
It's not the torque, it's the powerband. Power = Horsepower.
Took a very long time to beat it into my head, and the above was the illustration that finally got it done.
On EDIT: If we started up that hill at 50 MPH in 4th on the diesel, we'd have been around 2500 RPM's. In our powerband. When we got to redline at 76 MPH and shifted, guess what? Yup, still in the powerband in 5th, at about 2900 RPM's.
Just a little icing on the cake there.
Diesels get loads moving because the wide powerband ratio is available in every gear, at every shift. And from the stop, in first gear, it's available damn near off idle.