I have just purchased a 1989 F250 7.3 liter with a C6 automatic. What kind of fuel mileage should I be getting out of it. Supposedly it only had 36831 miles on it. I just replaced the oil which was filthy and replaced the transmission filter and fluid. Dont ask what the transmission oil looked liked ( it was pathetically dirty). What is the formula for figuring out fuel mileage? I live in a rural part of south Louisiana. Since purchasing the truck I filled up the two tanks till the needle was way beyond the full mark (pegged to the right side of the guage) and once I started driving it slowly moved back to the fuel mark on the guage. I have driven 73 miles on that one tank and it is almost to a quarter of a tank (empty side). Is this about right or am i getting low fuel mileage, or siply is the guage faulty??? This may sound like a crazy question but I am new to the world of Diesel. I have two 1966 F100's so I am mainly a gasoline guy. Plus it seems like it takes a long time for my transmission to shift gears when driving. Sometimes I can get up to 45 miles per hour at about 3200 rpm before it shifts into second ( thats when I really Get into the accelerator), but if i ease into accelertionit will shift a little sooner. Does the vaccum modulator need to be adjusted, if so what way do I turn the screw to correct its operation??? Any help would be greatly appreciated.

 

To figure fuel mileage, full both tanks and write down you mileage on the odometer.
Drive normally till you need fuel again.
Fill both tanks again and write down the mileage on the odometer again.

Now subtract the first mileage from the second mileage, this gives you miles driven on that fuel.
Now divide the miles driven by the gallons of fuel it took to refill the tanks.
Your answer to that division is your MPG number for that tank of fuel.

73 miles on 3/4 tank of fuel is not very good.
3/4 tank should be about 12 gallons, 73/12= 6.08 MPG
Fuel gauges fail, the only true way to figure MPG is by miles traveled divided by gallons used.

 

Sounds like to much driving rpms due to slow shifting. By 45 you should be wayyy past second gear. Anybody out there that can fill in on about how many mph each shift should occur at with the C6??

 

With 3.55 gears and a C 6 the MPH at 3200 should be 33 MPH.
57 should be the shift to third at the same RPM.
And a top speed of 83 MPH at the same RPM.

To go 45 in first would take a gear ratio close to 2.65 at 3200 RPM.

What size tires are on the truck rear axle?

 

Sounds like the kick down lever is stuck down giving you a high throttle pressure which will delay shifts and make them hard as well..