My old Dodge Ram with Cummins diesel...

[7milesout]

3.54545454545454545454545454545454

[Wallythacker]

there must be compatible rear ends in the 2.25-2.5:1 ratio. I think your Cummins would happily use a 2.25:1.

Heck, OTR rigs are running 1.8:1. on a GVW of 45 t. Mind, they have 1800 ft.lb. torque. your engine is 800 ft.lb. against only 7200 lbs. truck weight. I really think you could half the ratio and bump your mpg up by 75% or more. Imagine 35mpg from your rig

[Fummins]

Heck, OTR rigs are running 1.8:1. on a GVW of 45 t.

Examples? Low rpm doesn't always equal higher fuel economy. There's that whole engine load thing to contend with. If you remove rolling and air resistance, I agree that an impossibly lower ratio would be great.

[Basic]

there must be compatible rear ends in the 2.25-2.5:1 ratio. I think your Cummins would happily use a 2.25:1.
Imagine 35mpg from your rig

I have been toying with this idea for a long time in an old classic car for a bulletproof cruiser with enough torque to do burnouts and 3X the fuel efficiency of a comparable big block.

I had a '57 Dodge Custom Royal and a wrecked CTD pickup for just this purpose, but then I needed some quick cash one day and ended up selling the '57. Still want to do it at some point.

[7milesout]

It won't work nearly as well as you think. In comparison to big rig diesels that make 1,800 - 2,400 lb-ft of torque, mine will make yes about 800 lb-ft. That's at its max, which is from maybe 2,000 rpm to 2,700 rpm if I were to guess.

OTR trucks make their max from (some as low as) 900 rpm, but most from maybe 1,150 rpm to about 1,600 rpm.

Mine will toodle around town at 1,200 rpm and hold like 50 mph on flat level ground. But if you throttle it to pull a hill it will lug. And this is one engine you don't want to lug. With the huge internal rotating mass, you don't want to throw off ignition timing and piston speed and direction. Things would get messy.

At minimum, to produce decent working power, I'd have to run maybe 1,750 rpm, to be able to pull hills decently empty. I've towed heavy a lot. And I've experienced the fact that the turbo is tuned to work nice N juicy from about 2,000 rpm to probably 3,000 rpm. When towing heavy and pulling anything more than a bunny hill, it would struggle below 2,000 rpm. Struggle because it wouldn't make much boost. Low boost = low fuel = low power.

Can what you suggest be done? Sure. The drivability would be crapola. I'd be shifting down out of 6th pulling bunny hills, empty. Forget towing in 6th gear at all. I'd have to tow in 5th (which wouldn't be too bad actually, because 5th gear is 1:1 and as such there is less drivetrain friction). As it is, if I were content to run 50 - 55 mph while towing, and leave it in 5th gear, I'd probably get about 20 mpg towing its max rated capacity.

In summary, all (well let me change that to most), OEMs tune their gearing to provide good fuel economy with maximum drivability. If they didn't, buyers would be all pissed and go buy a competitor's vehicle because it drives better. OEMs do a good job to make their cars very drivable. Hyundai-Kia doesn't tune their gearing however, they just put whatever gear in their car that they found in the competitor Toyota car. A gearing change like what you suggest would make me want to get rid of ole blue, and buy some crap Duradud.


7milesout

[Wallythacker]

Examples? Low rpm doesn't always equal higher fuel economy. There's that whole engine load thing to contend with. If you remove rolling and air resistance, I agree that an impossibly lower ratio would be great.

Can't find the precise article in my history but this one: https://www.fleetequipmentmag.com/in...-downspeeding/ refs running 2.2:1 and lower

I have been toying with this idea for a long time in an old classic car for a bulletproof cruiser with enough torque to do burnouts and 3X the fuel efficiency of a comparable big block.

I had a '57 Dodge Custom Royal and a wrecked CTD pickup for just this purpose, but then I needed some quick cash one day and ended up selling the '57. Still want to do it at some point.

I can't recall for sure but wasn't one of the 60's Caddy mills torque to the moon @ very low rpms? Maybe the 472 cu?

It won't work nearly as well as you think. In comparison to big rig diesels that make 1,800 - 2,400 lb-ft of torque, mine will make yes about 800 lb-ft. That's at its max, which is from maybe 2,000 rpm to 2,700 rpm if I were to guess.

OTR trucks make their max from (some as low as) 900 rpm, but most from maybe 1,150 rpm to about 1,600 rpm.

Mine will toodle around town at 1,200 rpm and hold like 50 mph on flat level ground. But if you throttle it to pull a hill it will lug. And this is one engine you don't want to lug. With the huge internal rotating mass, you don't want to throw off ignition timing and piston speed and direction. Things would get messy.

At minimum, to produce decent working power, I'd have to run maybe 1,750 rpm, to be able to pull hills decently empty. I've towed heavy a lot. And I've experienced the fact that the turbo is tuned to work nice N juicy from about 2,000 rpm to probably 3,000 rpm. When towing heavy and pulling anything more than a bunny hill, it would struggle below 2,000 rpm. Struggle because it wouldn't make much boost. Low boost = low fuel = low power.

Can what you suggest be done? Sure. The drivability would be crapola. I'd be shifting down out of 6th pulling bunny hills, empty. Forget towing in 6th gear at all. I'd have to tow in 5th (which wouldn't be too bad actually, because 5th gear is 1:1 and as such there is less drivetrain friction). As it is, if I were content to run 50 - 55 mph while towing, and leave it in 5th gear, I'd probably get about 20 mpg towing its max rated capacity.

In summary, all (well let me change that to most), OEMs tune their gearing to provide good fuel economy with maximum drivability. If they didn't, buyers would be all pissed and go buy a competitor's vehicle because it drives better. OEMs do a good job to make their cars very drivable. Hyundai-Kia doesn't tune their gearing however, they just put whatever gear in their car that they found in the competitor Toyota car. A gearing change like what you suggest would make me want to get rid of ole blue, and buy some crap Duradud.


7milesout

Good points, all of them. But I think with tech all of the issues can be solved. how about a multi ratio rear end that will shift up and down to prevent the engine from lugging when towing or hills. Ya, it's a secondary transmission behind the primary. Even if that route isn't used there's still porting all the tricks the OTR rigs use to run 1200 rpm @ 70mph. Cam grinds, timing, porting, yyadda. The OTR guys, granted, don't place drivability first (maybe they do, I dunno?) but it must rate up there. They can't be dinking around totally loathing their ride, that will produce terrible driver productivity. We will see CVT drivelines behind the 2000 ft. lb. OTR rigs, eventually. It will happen, then final ratios of .8 and perhaps lower will be a common thing.

Till then it looks like 2.2:1 is an in thing on OTR with 1.8:1 hot on it's heels. And if it works to vastly improve fleet mileage it WILL be adopted by Detroit for his Joe 6pack F150 and quickly because the CAFE (is that it? the acronym for the combined fleet mpg a manufacturer must meet federally) standards won't stand still. Interesting times ahead boys!

[Fummins]

Looks like lower ratio's are good if you don't have overdrive in your transmission. At least that's what I took away from that article?

There is an overdrive(maybe underdrive too?) that's made by gear vendors for my truck, there might be one for 7milesout year too? They're pricey though.

[Fummins]

This one is a 5speed but should be pretty close enough of a final o/d ratio.

[Wallythacker]

Great video!

[7milesout]

As I recall (no expert, just recalling some things I've read), the 2.2:1 and other low gear ratios on BIG TRUCKS, are done for what they call DOWNSPED drivetrains. And it is as Fummins describes. As I recall, they forgo overdrive gears. Top gear is a 1:1, for the reason I mentioned previously. It has less drivetrain friction / drag to run 1:1. The input shaft and output shaft are "locked" together. One less place to loose energy. So it is more fuel efficient. And BIG TRUCK fleets are all about mpg. The drivers may not be, but the Fleet Managers are. As I recall also (can't remember this for sure), most or all of these "downsped" drivetrains are part of an automatic transmission, or automated manual (with 1:1 top gear).

The smarter / more successful owner-operators are simulating that drivetrain, within reason. One of the ways they do it (again, as I understand) is when they require a differential gear swap (just from wear, as yes a million miles towing heavy wears gears out), they will use a lower gear ratio for lower cruising rpm. They can go fairly radical with it. Especially with an Eaton-Fuller 13 or 18 speed manual transmission. In an 18 speed, gear 16 (range selection on the high side (gear #7), splitter on high) it is running 1:1. With a 13 speed it is the same, (range high / 7th gear, splitter on high) it is running 1:1. A 10 speed may be running 1:1 in 9th, I'd have to look it up.

In any event, the Eaton-Fuller manual transmission owner-operators can run 1:1 manually ... easily, and run low rpm at their lowest drivetrain loss position. Then when running empty they can use the top 2 overdrive gears. Very interesting stuff.

In my case, the case of my 6BT, I'm not tearing apart the engine chasing mpg or performance. And money I saved due to mpg gains, wouldn't be worth the cost of tearing the engine apart to get it. Plus, there's the risk that it would never be put back together as well as the factory put it together. Old blue truck runs just fine how it's set up. I'm just going to drive it as-is and enjoy it.