Since I track my mileage every tanks via my phone I decided to do an experiment. I had read a number of posts by people with similar vehicles that stated the best way to increase mileage is to slow down. I did this and here are the numbers I found....
Vehicle: 1988 E250 7.3 IDI NA, 3.54 gears, c6+Doug Nash O/D
Procedure: I tracked my mileage for 4 tanks during my normal weekly commute. My commute is 60% highway and 40% city. Terrain is slow rolling hills. For each tank I set my cruise when I was on the highway for a different speed. I started at 75-80mph and dropped each tank in 5mph increments. I filled up at the same gas station on the same pump for each tank. For consistency I also put the DN in over drive and just left it. This made it pretty doggy around town since it gave me the equivalent of having 2.83 gears.
Tank #1, 75-80mph: I drove 387 miles using 27.851 gallons of fuel. Netting 13.90mpg
Tank #2, 70mph: I drove 425 miles using 27.784 gallons of fuel. Netting 15.30mpg
Tank #3, 65mph: I drove 452 miles using 28.733 gallons of fuel. Netting 15.73mpg. This tank could be slightly skewed. I was having charging issues so I let it idle during stops instead of risking it not starting due to a low battery. Idle time was about 1 hr total. Some brief searching said I would use about .5 gallons an hour at idle. Removing this would give a mileage number of 16.01mpg. It was also raining consistently throughout this week.
Tank #4, 60mph: I drove 464 miles using 26.898 gallons of fuel. Netting 17.25mpg. I did accidentally set the cruise to 65mph for one 25 mile highway trip, but this would only effect the results by about .05mpg.
Conclusion: While there are a ton of variables and one tank sampling per speed does not provide the most accurate results, it does seem to show that reducing your speed does seem to have a fairly large positive affect on mileage. Mileage showed a 24% increase between tank #1 and tank #4.
Other thoughts: I would be curious to see if dropping to 55mph would continue the trend, unfortunately I couldn't bring myself to slow down that much. Our speed limit in the area is 70mph with most people going 75-80mph. Going that much slower than the traffic flow would drive me crazy. I also wonder if having the OD engaged during city driving had a negative affect on my numbers. It definitely took more throttle input to get up to speed and my EGT seemed to be higher doing it vs with the OD off. Once cruising my EGT were significantly lower with the OD on. I also did not check my tire pressures, but since I just had them rotated and balanced I would assume they set them to the recommended stock pressure. Increasing them to 80psi may reduce rolling resistance and net a small gain.
Vehicle: 1988 E250 7.3 IDI NA, 3.54 gears, c6+Doug Nash O/D
Procedure: I tracked my mileage for 4 tanks during my normal weekly commute. My commute is 60% highway and 40% city. Terrain is slow rolling hills. For each tank I set my cruise when I was on the highway for a different speed. I started at 75-80mph and dropped each tank in 5mph increments. I filled up at the same gas station on the same pump for each tank. For consistency I also put the DN in over drive and just left it. This made it pretty doggy around town since it gave me the equivalent of having 2.83 gears.
Tank #1, 75-80mph: I drove 387 miles using 27.851 gallons of fuel. Netting 13.90mpg
Tank #2, 70mph: I drove 425 miles using 27.784 gallons of fuel. Netting 15.30mpg
Tank #3, 65mph: I drove 452 miles using 28.733 gallons of fuel. Netting 15.73mpg. This tank could be slightly skewed. I was having charging issues so I let it idle during stops instead of risking it not starting due to a low battery. Idle time was about 1 hr total. Some brief searching said I would use about .5 gallons an hour at idle. Removing this would give a mileage number of 16.01mpg. It was also raining consistently throughout this week.
Tank #4, 60mph: I drove 464 miles using 26.898 gallons of fuel. Netting 17.25mpg. I did accidentally set the cruise to 65mph for one 25 mile highway trip, but this would only effect the results by about .05mpg.
Conclusion: While there are a ton of variables and one tank sampling per speed does not provide the most accurate results, it does seem to show that reducing your speed does seem to have a fairly large positive affect on mileage. Mileage showed a 24% increase between tank #1 and tank #4.
Other thoughts: I would be curious to see if dropping to 55mph would continue the trend, unfortunately I couldn't bring myself to slow down that much. Our speed limit in the area is 70mph with most people going 75-80mph. Going that much slower than the traffic flow would drive me crazy. I also wonder if having the OD engaged during city driving had a negative affect on my numbers. It definitely took more throttle input to get up to speed and my EGT seemed to be higher doing it vs with the OD off. Once cruising my EGT were significantly lower with the OD on. I also did not check my tire pressures, but since I just had them rotated and balanced I would assume they set them to the recommended stock pressure. Increasing them to 80psi may reduce rolling resistance and net a small gain.
