I was on a road trip in semi-hilly mid Missouri last weekend, and along the way I had the opportunity to ponder a fuel efficiency question. I was trying to optimize the fuel efficiency of my 190D 2.5T 5-speed car. ( I averaged 40.2 mpg for the trip, which pleases me). With the 5-speed and the 2.65 rearend, I am sometimes faced with a choice of using either 4th or 5th gear when going up a long hill at 60-ish miles per hour.

Here's the question, and what I percieve to be the correct answer. Is it better to stay in 5th gear and to apply 2/3 throttle to crest the hill at 60 mph, or to downshift to 4th gear, keep 2/3 throttle pressure, and crest the hill at 60 mph? Going up some hills, it seems that the car will go the same speed or faster with the same throttle pressure in 4th gear, as oppposed to 5th gear.

I think that 5th should still be more efficient, as long as I don't have to increase the pedal pressure by more than 20% to maintain the same speed. My logic is that with 20% fewer rpms using overdrive 5th gear, that I will still use less fuel, even if I have to increase pedal pressure by up to 20%.

I hope to spark some debate about the merits of either technique (i.e. use the highest gear possible, or let the rpm's stay closer to the power peak).

SteveM.

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner

will yield better mileage. but dont lug the engine. run the higher gear as long as the engine doesnt struggle and shake.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.