I had an '86 f150 300/3 spd w OD I bought new, as I recall I avged 18-19 mpg on the hwy but that was with ac wide open and running 70-75 mph, no cruise control. Wish I had kept that truck ;(

 

I did an experiment last night. Me and my friend adjusted the idle to the correct idle speed, and me and him filled two 1 gallon gas cans and put them in the back of the truck. We then ran the truck out of gas on the interstate, coasted to a mile marker, put in the 1 gallon of fuel, and ran till I ran out.

Just by getting the idle speed correct, my mpg jumped to 15.6 (at 65 mph) So i'm on the right track.

Does anyone know where i can get a diagram (or if anyone already knows) of where the vacuum lines coming off the carb hook up to? I'd like to play with that a little after I rebuild my carb this weekend and see if I can maybe get better mileage by getting that EGR back up and running.

Thanks for all the suggestions so far!

 

If you are actually traveling faster then the miles read on the speedometer then the would make your odometer off in the same direction right? If you do 60 on the speedo and 65 actually, that would mean you odometer only clock 60 miles when you travelled 65? Only asking to make sure i havent lost my marbles.
And I do get your argument on the 300 vs 400, but hear me out on this also. My 4.6L gets worse gas mileage then about 70% of 5.4L because a bit stronger engine moving the same weight works more efficiently. Hence why Ford stopped putting the 4.6L in regular manufactured F-150s.

Boomer

 

If your speedo says 65 and your really running 60, in an hours time you will show 65 miles on the odo but you really only covered 60 miles. So yes, basically what you said lol

 

Reread my post, not 65-60 backwards.

Boomer

 

A speedo reading "slow" will also result in the odometer not recording the true miles covered, in most cases. This should result in MPG numbers that are lower than actual MPG, since the number of miles recorded would be lower than the number actually travelled.
Going by my odometer, my truck barely gets 12 MPG. But, the speedo reads 55 when I am actually doing closer to 65 (about 63, IIRC) due to oversize tires. back when I first got it, I was bit underwhelmed when it calculated out to just shy of 15 MPG, until I realized the speedo/odo were a bit slow. At that time, it read 55 at about 59 actual MPH.
Stock tires were 215/75-15, when I bought it, it had 235/75-15, and now it has 31 x 10.50-15
Using GPS/ mile marker signs, I can usually average close to 16/17 MPG empty, and between 12-15 towing. Of course, I also have a "right lane" mentality when driving the old truck, loaded or not. It's a I6, so passing people is doable, but it's gonna cost you a couple MPG at a minimum.......

 

Thats what i thought I understood, that even with bigger tires the reading im getting for mileage should not be more then the true mileage of the vehicle same with my 04' F150 going from 28in tire aprox to 33 in tire aprox. Thanks,

Boomer

 

Anybody know how to get that data for trucks older than 78? I tried changing the link to: http://www.fueleconomy.gov/feg/epadata/74guide.txt and a few other years, but it didn't work.

 

It's easy to check both your speedometer and odometer on the freeway if there are milepost markers you can see. Using a stop watch run your truck up to 60 mph on your speedometer then time it from milepost to milepost a couple of times. One mile per minute = 60 mph. If it's showing that then it is correct,if not then speed up or slow down until you are doing one mile per minute.That will show how much it is off if any. Change the gear on the cable if it's off to get the speedometer to show correct readings. You can check the odometer with the milepost markers at any speed if you have a stretch of freeway to do it on. I like a stretch about 10 miles straight if possible. A lot of people fool themselves in to thinking they are getting good mpg when there odometers are way of. I wish i could get more than 10.62 mpg on my f-100 /460/c6 and 2:75 gears ,speedometer corrected. But then i do dream a lot. L.O.L.

 

A couple of my buddies with lifted Jeep's go by their GPS when calculating mileage for this exact reason.

 

13 seems to be on the low side... I'd suspect that it's just the entire combo netting you the bad MPG.

 

Depending on how much their GPS cost, I wouldn't trust any GPS system.

If it's one of the cheap ones, you could easily be 100 meters off target based upon your surroundings and the accuracy of the unit.

The company I work for does testing for GPS systems and we often find some radical things happening even in an everyday traffic setting. For some reason, dealership lots (FORD, CHEBY, ACURA, etc) make a GPS signal bounce. The signal bounce isn't from a build that's nearby, but it's from a bunch of cars that are all shiny, metallic and have some ability to make a GPS signal inacurate.

Another weird thing I've seen is near a semi that has a large piece of equipment on it or a semi that's hauling a cattle trailer. Both are large metallic and shiny (for the most part), so they make the signal bounce like crazy based upon the surface it hits (curved, flat).

I highly doubt many GPS systems have this issue, but it's not uncommon to see. Drive by a sky scrapper with nothing but tinted glass for walls... your GPS will tell you you're in the next state/province...

This is exactly why I wouldn't trust GPS over actually timing yourself at speed. Manual still works better than whatever technology has to offer...