Rules 1.
Fuel consumption drops in proportion to frontal area.
Rule 2:
The formula for fuel useage on a level road when weight is added is
(the scale factor of weight gain X 0.2806)+0.7194
A car optimised to reach 75 mph at just 1.25 tons all up will see up to 60 mpg with a super low drag body and a small cross sectional area.
When you go to a much wider, taller vehicile , and have a blunter shape, you cannot expect to get more than 10 mpg at 75 mph.
This graph is one complied with all the road load (transmission losses, rolling resistance, air drag, weight, and sea level air density, but no gradient or road surface factors)
Generally, a good , well tuned engine under full load will use a 0.45 pounds of fuel, per road hp, per hour. It's pretty easy to find out how much fuel you'll use.
At 75 mph, an Econoline won't give better than 10 mph with a truck having a 44 sq ft frontal area and a 0.7 drag coefficent. cdA is 44 x 0.7, or 31. Reading off the 31 figure, you can't expect to get 10 mpg, even with only 1.25 tons.
Each time you add weight, the fuel consumption drops by a factor.
Having a tare mass of 5500 pounds ( 2.5 tons) means you have a wieght increase factor or twice the curve above.
At 75 mph, the fuel consumption falls off at the rate of (2 x 0.2806)+07194, or 1.28 times the fuel use.
That 10 mpg with 2750 pounds will drop to 7.8 mpg in a 5500 pound truck.
Rule 3.
Power Loss due to axle gearing rise is very small. You take the gearing rise as a fraction, and multiply it by 0.1346, and then add 0.8654. That gives you the amount of power you loose with a gearing increase. in some cases, a gearing rise improves power due to a reducton in wheel spin or gear changes over a standing 1/4 mile.
Rule 4.
Adding an Overdrive or making top Underdriven.
Adding or subtracting a gear generally results in a proportinal rise or fall in economy.
A 1.294 growth in gearing by having a 0.77 over drive results in a 1.08 times greater mileage figure at 60 mph.
That is (Overall rise as a fraction x 0.2721)+0.7279
Rule 5.
The hp required for an given speed is calculated below but you must know the tire size in mm, the cd and FA.
Formulae for power due to drag and drivetrain loss is then able to be worked out with ease.
cd is the drag factor. A Pinto is about 0.48, an intermediate Falcon (66-70) about 0.48, a Thunderbird 0.35, a Fox Mustang 0.44 or 0.36 if its got an SVO body kit. Early 60's XK Falcons are quite slippery before the T-bird roof get things messed up...more like 0.40 than the 0.48 or so of a 65 Falcon. Early Sprints were likely to have the least drag.
FA is the frontal area. Then multiply cd by Frontal Area Note that a Pinto is about 20.2 ft2, an early (late 60's) intermediate Torino/Fairlane is 24.2 ft2, and a fat bodied Mustang (71-73) about the same. A Fox Mustang is around 20.8 ft2. A Maverick could be as low as 21 ft2 for an early tudoor, or over 21.5 ft2 for a post 74 dodgem bumper number.
cd*FA *mph*mph*mph * 1.27
. 147733
Then add tire loss
mm*lb*mph*8
. 58 036 680
Rule 6:
The Hp loss or gain due to grade is
perpendicular rise/ horizontal run x lb x (mph/375)
This is the amount of power to add or subtract from the speed formula above if going up hill or down dale.