Gas mileage - hmmm?

Lately, I have been driving my son's 1984 F150 with 4.9L engine. Last year, I did some work on it as it was getting ~13 mpg on my normal weekly work and weekend driving [very repeatable and a decent test bed]. The drive is some rural @ 45 mph [14 miles each day] mixed with small town driving @ 25-35 mph with numerous stop signs and traffic lights. The round trip is ~28-30 miles.

Since the work, the mileage has averaged ~17 mpg.

Friday, I drove to another town which was a 91.3 mile round trip. The drive consisted of 45 mph sections [~10 miles] and the rest was 55 mph. There were sections where the speed limit is 65, but I maintained 55 mph. The temperature was 31-50F and I had a friend with me who weighs ~240# [I really do not believe that to be a big factor].

91.3 miles divided by 4.925 gallons = 18.53 mpg. At 55 mph, the tach was indicating 1650 RPMs.

Saturday, I repeated the trip. But, this time, I drove @ 65 mph when legal. The distance driven @ 45 mph was the same. I was alone and took a side trip which consisted of getting off the described road and driving 3 miles @ 45 mph and then going uphill in 3rd gear @ 25-35 mph for a little over a mile. The side trip added about 10 miles. The temperature was 30-55F.

101.7 miles divided by 5.027 gallons = 20.23 mpg. At 65 mph, the engine is turning ~1950 RPMs.

An engines most efficient speed is independent from the speed of the vehicle. Meaning, running the engine in it's "sweet" spot, will yield the best economy. You have found where that is.

Yes, I will second that, my 1966 GT-350 got around 18 mpg @ 55 mph, turning about 2750rpm, at 70 it got 22 turning 3500 rpm, this was with the original R3259 Holley with the dual Holleys it went a little better @ 55 most likely due to the 465 cfm primary carb being at a real good cruise on the primaries, 70 mph dropped a bit, but at the time we had the national 55 mph speed limit so it didn't matter.

According to my text book The Internal Combustion Engine an engine has the best Brake Specific Fuel Consumption (BSFC) at the torque peak.  But, that is just the engine and the vehicle has to be taken into account, which includes tire rolling resistance and wind drag - and it goes up by the square of the speed.

I'm sure that you could put an equation to each of those things and figure out where the sweet spot is.  But, the easier way is as you've done - drive it and record the results.  Sure enough, Daniel's truck gets better MPG at 65 MPH than 55 MPH.  

I would be happy with that MPG on mine. Even the lowest MPG is still better than my DD 02 Durango with 5.9 auto/3.92 gear.

BTW what trany, rear gear and tire size you running in that truck and is there any work done to the motor & systems to get that MPG?
Dave ----

FuzzFace2 wrote

I would be happy with that MPG on mine. Even the lowest MPG is still better than my DD 02 Durango with 5.9 auto/3.92 gear.

BTW what trany, rear gear and tire size you running in that truck and is there any work done to the motor & systems to get that MPG?
Dave ----

The engine is very stock with all the 1984 computer controlled feedback system intact. The transmission is the manual 4 speed OD and the rear gears are 3.55. The tire size is 235/75 X 15.