So I was playing around with some fuel mileage calculations the other day and came up with some interesting finds. One of the things that is published on the Cummins engine is specific fuel consumption. Knowing my average fuel mileage I am able to calculate a few things.
Using an average highway mileage of 19.0 MPG at an average speed of 72 MPH yields 3.79 GPH of fuel consumed.
Turning 2300 RPM, and using a BSFC of 0.37 #/hp-hr from this chart gives 72.2 HP to drive down the highway (with 7.05 # fuel per gallon). This accounts for drag, driveline loss, and all inefficiencies. (On a side note you can see what the fuel consumption will do if I regear to a lower RPM)
So given 72.2 HP @ 2300 RPM, 164.8 ft-lbs required to keep me moving - on level ground.
If we add even a moderate 5% grade for an overpass is 5%*5280 = 264 ft/mile or 316.8 ft/min vertical rise at 72 MPH.
With a 5200 lb vehicle at 316.8 ft/min = 1.64e6 ft-lbs/min. Divide by 33,000 ft-lb/min per horsepower and we require another 49.9 HP to maintain speed up the grade.
So 72.2 HP + 49.9 HP = 122.12 HP required to maintain speed up the grade. At 2300 RPM that's 278 ft-lbs required to deliver that power. Neither the 3.5 or the 3.7 can deliver that torque at that speed. I think the 5.3 can. The only choices for the I5 are to a) lose speed b) downshift or unlock the torque converter to get the RPM up to deliver the power.
That's why the transmissions hunt trying to cruise at highway speeds especially on hills. The engine just can't deliver the necessary power at cruise RPM.
Using an average highway mileage of 19.0 MPG at an average speed of 72 MPH yields 3.79 GPH of fuel consumed.
Turning 2300 RPM, and using a BSFC of 0.37 #/hp-hr from this chart gives 72.2 HP to drive down the highway (with 7.05 # fuel per gallon). This accounts for drag, driveline loss, and all inefficiencies. (On a side note you can see what the fuel consumption will do if I regear to a lower RPM)
So given 72.2 HP @ 2300 RPM, 164.8 ft-lbs required to keep me moving - on level ground.
If we add even a moderate 5% grade for an overpass is 5%*5280 = 264 ft/mile or 316.8 ft/min vertical rise at 72 MPH.
With a 5200 lb vehicle at 316.8 ft/min = 1.64e6 ft-lbs/min. Divide by 33,000 ft-lb/min per horsepower and we require another 49.9 HP to maintain speed up the grade.
So 72.2 HP + 49.9 HP = 122.12 HP required to maintain speed up the grade. At 2300 RPM that's 278 ft-lbs required to deliver that power. Neither the 3.5 or the 3.7 can deliver that torque at that speed. I think the 5.3 can. The only choices for the I5 are to a) lose speed b) downshift or unlock the torque converter to get the RPM up to deliver the power.
That's why the transmissions hunt trying to cruise at highway speeds especially on hills. The engine just can't deliver the necessary power at cruise RPM.
while soul-sucking Camrys get better mileage, I'll keep my soul and my smile in a Hummer.
