Ram 3500 MPG test

4x4ord wrote:

ShinerBock wrote:

4x4ord wrote:


To get an idea as to what the 10 speed would do for the Cummins all you'd have to do is drive your Ram down the road towing a heavy load at 1700 rpm in high gear and reset your milage meter...... drive a few miles and see what your average trip mileage was. Then go back and drive the exact same stretch of road at the same speed in 5th gear (about 2095 rpm) and see what your trip meter says for mpg. There is always people claiming they lock out high gear while towing and don't see any loss in mileage. I've never believed these claims, but at the same time I don't think the increased fuel usage is going to be anywhere near 30%.

The same way a shorter rear gear ratio will provide better fuel economy when loaded, so will will a shorter trans gear when loaded. Don't believe me then ask a Ford engineer.

"Also consider that the axle ratio is only a portion of the overall pickup gearing. Manufacturers spend a lot of effort on powertrain matching. "Powertrain matching is really a systems approach," says Dennis Slevin, vehicle engineering manager, Ford Commercial Vehicles. "It is the engine, transmission, axles and tires working together. It is the effective ratio of engine power transmitted all of the way through the drive system to the ground."

He adds, "From our perspective there are interconnected reasons to choose optional axle ratios -- if you are hauling heavy loads with large trailers in hilly terrain, and you do a high frequency of towing." Consider a PowerStroke 6.7-liter diesel that offers a choice of a 3.31, 3.55, 3.73 or 4.30 axle ratio. "The 3.31 might be capable of towing the trailer. But if you are towing the majority of the time, you probably want the 3.55 because it will tow better and provide better fuel economy loaded.""


How to Pick Pickup Truck Axle Gear Ratios to Improve Performance

If you are in to high of a gear and "lugging" the engine to overcome the load, then the engine is not operating efficiently. The most efficient rpm is always changing based on speed and load. Towing 7-14k at low speeds of 60-70 mph, then it is best to lockout 6th to keep you at more efficient rpms. At speeds above 70, the 6th becomes more efficient.

I understand this stuff very well. As a rule for any speed you are travelling the highest gear the engine will pull the load in is the gear that will give you the best fuel economy. There might be some isolated segments in an engine's lower rpm range under certain power demands where running the engine a little faster might gain you 3 or 4% better fuel economy. The Cummins engine might like 1900 rpm better than 1700 pulling a load like TFL was pulling. If one rpm is going to be better than the other it's not going to be better by a very significant margin.

I was commenting on this "There is always people claiming they lock out high gear while towing and don't see any loss in mileage. I've never believed these claims..." Believe it or not, you can gain fuel economy by using a shorter gear ratio depending on your speed, load, and the engine's sweet spot. Even the Ford engineer agrees.

And yes, per Cummins, the 6.7L's sweet spot is 1,900-2,100 rpm, not 1,700.