Rusty Rockers

[JimT]

It's not about the fuel consumed by the engine while coasting in or out of gear. It's about the net gains in momentum from coasting.

Based on my testing I've seen the difference between coasting in gear compared to neutral. It's most noticeable when traveling down hill. When in neural you gain more speed and momentum compared to the exact same scenario except in gear. When in gear the engine is actually holding back the speed of the truck. That's why you downshift going down grades because the engine will slow your decent to a certain degree even without jake/engine braking. Remember, when in neutral the spinning mass of the engine is no longer affecting the movement of the vehicle. Simply put there's less friction leaching energy from your momentum.

If you haven't put 2+2 together (and got the sum of 4) then you should know that more speed/momentum (energy gain from gravity) gained downhill translates to less energy needed to climb the other side.

There's one situation where it actually benefits you to stay in gear downhill (beyond the safety aspect of controlling decent). It's when you are running VERY light and the downgrade is too slight to maintain your speed. In this case staying in gear with very minimal throttle will maintain your speed without using much fuel. For me this has become common when hauling 8,000lbs of ISO (roofing insulation boards). Between the lack of weight and the drag from the feather light ISO I've found that I will actually slow going down some grades if I take it out of gear. I average about .5-1.0 mpg LESS when hauling ISO at a gross weight of 38,000 lbs compared to grossing 70,000 lbs on the same lane (I-81 & I-84 between PA and MA).

Also, as I understand it, all the newer automated manual transmissions will (or can) automatically shift into neutral when coasting down hill as a means to increase fuel efficiency. The programming controls if, when and how much this will occur.