Hey Guys. I think I've posted regarding this before, but bear with me. I'm more than a bit perplexed regarding mpg. The last time I filled the tank and did a X country test was about two years ago. I got 16 mpg average on a 400 mile trip that was 80% freeway.

I have since gone from a autolite 4100 to an Eddy 500 carb. I've also changed to the bbc lifters. Other than that all is the same. However, the other day I got 10 gallons of gas, and it lasted 77 miles, during 80% local street driving. Since I did not fill the tank, the mileage is not exact. I relied on the gas gauge to show me when I had burned the 10 gallons.

I was more than a bit surprised. I didn't think it possible for a 300 to get that mileage!! I've checked the carb, the plugs, timing, gas leaks, sniffed the oil, and it all seems good.

I'd like to hear what everyone thinks. Surely I couldn't get 16 mpg freeway and 7.7 mpg in the city. Someone tell me where the magic mpg switch is located....

 

First thing to do is run an accurate test, using the gas gauge doesn't qualify.

 

Looking back, I had been doing a bit of hauling with that tank of gas: 1500 lbs of sand, and my tools. But that was for a short distance.

I do understand that using the gas gauge does not give a precise measurement, but it does, or should, come close. Nobody runs their tank empty each time they calculate. I also know that using the gauge system got me close enough to be alarmed.

I believe I will try to route a cool air intake to the air cleaner. And yep, $4.20 gas may even kill my bbc roller rockers! Shot 'em dead.

 

Eight mpg in town in a big truck isn't great but not horrible.

A C6 isn't the thriftiest gearbox, so there's that.

Have you tried switching the pump shot on the carb to the top hole? I ended up switching to a smaller pump nozzle on mine. Don't know how much it would matter, but would be most evident around town.

 

Thanks for the tip. I'll give it a try. I will read up on the Edelbrock, may even call over and speak to one of techs there.

I think it an unforeseen byproduct of installing a header is that it raises the temp in the e. compartment considerably. And I've been reading how cool air into the carb helps mpg. That must have something to do with huge diff. between freeway mpg and city.

 

What ctubutis said. An accurate odometer and knowing how much one puts in the tank does wonders for mpg readings.

 

Of course, Harte3, I agree. Having said that, I noted the position of the gas gauge indicator, pumped ten gallons, and it lasted 77.8 miles before the needle returned to the aforementioned position. I know that is not exact, but nonetheless it does give a pretty good idea of mpg. And 8 mpg for mixed driving is horrible. That is near to the mpg of the 390 I had in the truck before.

I was not looking for you guys to quibble over the accuracy of my measurement, but rather hoping you would offer info. or instruction on how to improve what I have determined to be unacceptable.

Thank you.

 

I remember being perplexed with this way back when I had my 1bbl carb. My mileage around town was 7mpg, whereas my freeway mileage was around 20mpg. That's a huge difference and I could never figure out why.

 

Not necessarily. In fact, it can be totally bogus.

A 1975 model has a sending unit that utilizes a brass pad (or wiper) that passes up &
down across a coil of VERY VERY VERY fine wire wrapped around a piece of material.

It's basically a mechanical rheostat.

In any event, that wire eventually wears through. I wish I had taken pictures but, on one
of mine, that wire had unraveled and was floating around *****-nilly inside the triangular
container, making the gas gauge totally unreliable.

All I'm saying is that the gas gauge is good for *approximations only* and shouldn't be
relied on for measurements like this.

Best thing to do is fill it up and make note of the mileage. Drive it for a while, then fill it
up again and make note of the number of gallons needed. Determine the number of
miles driven since your last fill-up and do the math.

 

Thanks for explaining that. Some of the old stuff is a crack up when you look at how simple it was.

Of course filling and refilling, noting usage, would be a good system. But my tank is 38 gallons, and with gas over $4.10 per, I don't think I'll be filling up for a while.

 

Using the guage is, at best, just a WAG (military term Wild A** Guess).
I bet you can turn the key on 6 times, and get 6 different readings from the guage, without ever even starting the truck.
Ford built great trucks. They also built some of vaguest guages ever known to man at the same time......

 

I can see that posting this thread was a waste of time. Everyone is more than willing to tell me how inaccurate my mileage test is, or how bad the gauge is, but not one decent bit of help or info about how to improve mpg has been offered.

Point: Maybe my estimate was off. Maybe it was 5 mpg. Maybe it was 9 mpg. Either way, it still sucks. I guess I am trying to find a positive about the 300. "under-powered, expensive to work on, and the mileage is that of a bbf. But hey, don't they look cool under the hood?"

I'm so happy I spent $5k in an effort to get 50% better mileage than my 390. Only to find I get the same mpg. Hoorah!

 

It is attached to your leg, just below your ankle... I know another smart azz answer. Seriously, I have found that warming my Fords up, especially older technology, Granny shifting and only drive fast enough to "get 'er done" has worked well for me when I'm concerned about mpg. With that said, I tend to be quick on the draw while accelerating/shifting, never drive less than 80 mph on the highway and decell at the latest possible time. When my '93 was younger I remember 17-18 mpg all around. I think the last time I checked it was 15-16 mpg. it probable had a bit over 100,000 on it at that point. BTW I do run my tank stone empty and tickle it to the top for calculating mpg. I save the pump receipts for the exact gallons. I also do 2-3, maybe more, tanks and average them. A bigger sample should yield a more accurate average. With a 38 gal. tank 2 should be plenty. If I had a V8 and a four barrel with my driving habits I would probably be around 6 mpg. For a bit of comparison my '09 GMC monitors everything and calculates instant and average mpg. I am currently at 15.7 average after ~ 29,000 miles which is mostly butt hauling and occasionally carrying my camper and or dragging my travel trailer. Watching the instant economy is pretty entertaining. It will range from 3 mpg cold getting on the highway to 99 mpg coasting downhill.

Hold on to the bar... it's only going to get worse
Jim

 

F-250 restorer,

I can't offer much in the way of help as to specific tips to improve your mileage. I am in the process of swapping my 300 over from the stock intake and 1bbl with log exhaust to a Clifford intake with EFI exhaust and a 3" SS single exhaust system. I am using the factory pipes from the manifolds to the cat. and this includes a port for an oxygen sensor. I will be installing a standard narrow band O2 sensor and a meter. It won't give my fuel/air ratio, but will indicate if it is rich, correct, or lean. You might want to consider that as a tuning tool. The O2 sensor is about $30, plus another $35 for the meter. You will have to weld in a sensor bung if your pipes don't have one already.

With my stock setup, I typically get about 10 mpg mixed driving. That is with an F100 and 3.00 gears with a T18 manual. I am hoping for a big improvement. Once I get the new parts installed, I will see what the O2 sensor says and will track my mileage.

 

Do you still have the Autolite 4100 you could put back on?
I know I regularly get 11 - 14mpg around town with my Holley 390, which I've leaned out a bit. I'm wondering if it simply may be the carb.

I've heard Edelbrocks are preferred because they're a bolt-on-and-go, but I've never heard impressive mpg numbers from them.