Well I've been driving my highboy around for a couple weeks now, and I have to say, I'm slightly dismayed by the mpg I've been getting.

I read all around that people with the 300/np435 combination were getting anywhere from 16-20 mpg based on driving habits and front/rear gearing, even in the highboys. So far, I've gotten 12.7, 13.4, and 11.5 mpg. Most of my driving is highway, with an average of 55 mph. Twice a week I go off island for college, where I see brief periods on the freeway around 70 mph. Now the most recent mpg might have been affected by a short (~5 miles) trip with a 600lb compressor loaded in the bed. I'm not expecting for this to get super high mpg, but I at least hoped it would be a little better..

As far as measuring, I fill up the cab tank (19 gal), mark the OD reading, and drive until I run out of gas and mark the OD reading again. Then I simply take the difference in the OD readings and divide by 19.


-Jameson

 

Don't try to calculate the mileage like that.
Fill it up, record the mileage. Drive it until you have a 1/4 tank or less. Fill it up again and record the gallons it took. Record the mileage.
Deduct the original mileage from the last mileage.
Take the mileage driven and divide by number of gallons used, that's your MPG.

Example:
You fill up and the mileage is 59,763.4 you drive it and fill up again, this time the mileage is 60,024.8 and you filled up with 21.6 gallons.
60,024.8
-59,763.4= 261.4
You drove 261.4 miles and used 21.6 gallons so that's 261.4 divided by 21.6=12.1 MPG.

 

I'm doing the math the same way, just running it down. All the time that I've measured I haven't ran it down on purpose. I try not to buy gas on island whenever possible, and try to fill when I'm off island for college. The difference in price is pretty drastic, $3.89 opposed to the $3.29 I paid not an hour ago on my way to college.

 

In the 80's I had a truck that got a solid 12.5 mpg . Loaded , empty , up , down hill . It was solid as a rock ! My next truck verried from 14 to 23 mpg ! Calculating the same way ! My current F100 gets 15 to 16 mpg ( no freeway ) .

 

I agree with mike - the problem is that you can't tell exactly how much fuel you put in unless you refill it to a known level - i.e. full.

But otherwise as far as mileage goes, a lot of it comes with tuning. Vacuum connections/hoses are the first offenders. Ignition timing and points gap/dwell (if applicable) can affect it too, as well as carb settings. Then plugs, wires, etc etc. Vacuum would be the first step for me.

 

So going from full to empty, then filling back up to full again and using that as my gallons level isn't accurate?

 

You're doing great! My 300 six and four speed gets around 9 mpg. 300's are dirty running engines. They make gobs of low end torque, but fuel efficient they are not.

 

The 300 is a heck of a lot better on fuel than the larger V8's. A "dirty running engine" is improperly tuned/maintained.

 

Just wondering if you use the ferry or the bridge when you leave the island? I was on Whidbey Island around labor day. Was looking for your highboy but didnt see it. Just use whatever gallons it takes to fill the tank to figure out your mpg. Personally Id be happy with your mileage in a highboy even with a 300. Thats a lot of truck to move and keep moving. Plus everyone lies about what they actually get in fuel mileage ha ha

 

I think what the others are hitting on is the fact of the gauge not being accurate . Empty being the least accurate . So pick a half or quarter mark . Fill from there . Record the gallons . Next half or quarter ( Half may give better results ) fill and account for miles and gallons . Record over at least 4 fill ups to get an accurate record of MPG .

 

Well then Ford let A LOT of improperly tuned engines off the assembly line. I was doing emission tests on BRAND NEW fuel injected 300 six cylinders back in the late '80's and I had a problem getting the sixes to pass emissions with less than 5,000 miles on them.

 

Tires makes a difference too. And also age/exhaust...i have found that the nicer it sounds jumping off at a green light, the worse my gas mileage was.

 

When I say I run until empty, I literally mean empty, like the truck runs out of gas. I then record the mileage, switch to the aux tank, and drive to the gas station. And I do realize the gauge is inaccurate, because it sits on "E" for about 40 miles.

Mukilteo ferry. I'm on the south end, and I go to college in Everett. Labor day I was not driving the truck. I had it, but it was down for repairs. At the time I was driving a large, green chevy conversion van. Hard to miss if you saw me.

31x10.5's, 36" glasspack ending under the bed. If you mean age as in my age, know that in addition to college I'ma senior in high school. That should tell you what you need to know.

-Jameson

 

I'm referring to the older carb'd 300's, though what can you expect out of an early EFI, crude EFI system?

 

Well , If you are running it slapp out , marking miles and gallons ... Thats what you are gettin ! That my friend , is about as close as you can get .