transmission for fuel efficiency and hauling heavy loads

[Toiaat]

So I drive a scania s650, and I have the most expensive transmission which is meant for hauling heavy loads, but now I started to want more fuel efficiency. and the transmission I'm using goes to like 1.4k rmp or more when cruising on 12th at 80kmh, which I think is using quite a bit of fuel, so I was wondering if I should take the transmission that is worse for heavy hauling, but might be better for fuel efficiency considering it has longer gears. so going from a 3.96 diff, to a 3.08 diff, with the same last gear ratio. how much would this hurt my accerelation with heavy loads, and how much would it improve my fuel efficiency?

[Interstate Nomad]

Interesting question.

 

In general, hauling heavy cargo decreases fuel economy. The same applies to speeding, rapid acceleration and braking.

 

Fuel economy isn't something that comes to my mind if I haul heavy cargo.

For me, it is more important to get the heavy cargo moved, even under extreme (road) conditions.

[Foobrother]

I'm not a truck driver or mechanics in real life so basically not an expert!

But my understanding is that to have low fuel consumption you need to:

1. Cruise at the highest gear possible.
2. Always remain low on revs (less then 2K RPM on a truck)

 

That said, to haul heavy cargos you often need to :

1. Drive in low gears to get traction
2. Go high on revs on hills to keep the momentum during gear change (or simply because it's the only way to keep a constant speed) and avoid losing speed

 

So depending on the road and the cargo you will take, you might be able to use one gear box or another. But if you are on a steep hill with an heavy cargo you will need the gear box with the more gears for sure and fuel consumption will go 2nd.

The problem with gearbox with many gears gearboxes is that you reach high RPM very easily. The problem with few gears gearboxes is that you might not get enough traction/power to climb a hill or even start (even in 1st gear!).

 

To conclude, once you have the minimum number of gears for the road & cargo, I would say that the consumption really depends on the driver rather than the gearbox itself.

[Granite [SVK]]

As far as cruising goes, GRSO 925R is fine, cause the highest gear is overdrive (GRSO 905R is too), which lowers RPM and saves fuel when cruising on highways, just don't exceed 80km/h.

I don't know what you are doing, but for me it sits around 1330 RPM at 80km/h and 1500 RPM at 90km/h in 12th gear, on flat road, with 315/70 tyres, loaded or not. With larger 315/80 tyres it goes lower by around 70 RPM, so you may want to try taller tyres, at the expense of some torque.

 

The limiting factor is the drive axle differential ratio, as higher ratio (lower number) is better for fuel economy at highway speeds, but lower ratio (higher number) is better for towing power anytime you need to accelerate or climb a hill. If you swap for GRSO 905R, which comes with higher differential ratio and has much taller lower gears, you will sacrifice towing power at low speeds and on hills, and also two extra crawler gears.

 

Depending on what trailer/cargo you want to haul, you should test it yourself, but from my experience, I would stick with GRSO 925R for heavy trailers (low bed or low loader) and loads above 50-55 tons gross, and GRSO 905R for single trailers up to up to 40 - 44 tons gross, or doubles up to 50-55 tons. 

You probably won't achieve better average consumption than around 50L / 100km in the long run with low differental ratio and really heavy loads.

 

[Davina]

Good data above on the different gearboxes, albeit Scania-focussed only. My heavy-load trucks are around that eco, if not worse. Achieving best economy however is mostly down to driving style. Anticipate as much as possible, roll in-gear as much as much as possible & try to use the services brakes as little as possible.