I had a 78 F-150 4x4 with a stock 400m. I averaged about 10 mpg.
About a month ago I replaced the ignition with a Jacobs system, replaced the stock exaust with 2.5 inch duals and Heedman headers.
I wanted to improve my milage along with power but after all these mods my mpg went to 7. I've tried different plugs, timing, and have not got better than 7. I also went from a 31 inch A/T tire to a 32 inch M/T tire.

Any ideas?

Thanks.

 

Couple of things to check and/or consider, the tire size changed, this will lower the true MPG based on the fact that larger tires turn slower, giving a lower speedo reading as well as a "loss" of MPG. With the Jacob's, you have to play with the gap of the plugs, I found with mine(79-400)that the best is with the gap around .067, to much gap and it misses, to little and it just doesn't get full burn. Make sure you only have copper plugs in there-it needs them to read the resistance thru the plug to optimize the burn. Also, give it a few miles to adjust to the new plug setting before changing again. Make sure when you gap the plugs that the "gap" is straight . I've found it takes a little fiddlin' to find out what the engine likes in reguards to the ignition and timing, I do best with 1-2 degrees of advance over stock, less does not hurt, but to much will and you won't know it because it does such a good job of "masking" spark knock, please don't ask how I know! Lets just say the loss of performance is noticed goin' up hills the most. And of course, check the obvious, vacuum leaks ect, ect. Chuck P.

 

Chuck P., what are the specs on your engine? What mpg do you get? I've heard people getting as much as 16 and as little as 5. I just need to get my engine dialed in.
Dreaming of 10 mpg ........

 

Well, the truck is a SWB stepside with a 351M converted to a 400 with eldebrock performer intake and 600 cfm manual choke 4 bbl. It has a Reed cam (mild) with roller rockers. Rolls on 35" tires with 4.10's in the pigs, tru-tracs front and rear, C-6 and NP 205 case. Large trans cooler and a small power steering cooler, Oberg oil filter in-line with standard filter(has by-pass in case it gets clogged) a K&N 5" x 14" open air filter. I only get around 11-12 mpg in town, puddle jumpin' and about 15-16 mpg on highway runs, the a/c doesn't really effect the mileage much at all. Off-road I get 8-9 mpg which is better than I thought it could. The speedo has been corrected so these are true readings. Don't carry much except 44 gal. of gas and tool/spare parts boxes with full size spare and sometimes two spares. These mpg are summer numbers, this time of year I get about 1 mpg less, down with you, back and forth to work ect, still hold the highway miles though. Perma-balancers have also helped economy a little. Truck weighs in at just over 5000#. I can do much worse than these mpg #'s, if I'm not smooth on throttle movement and jump on it a little these will drop 2-3 mpg. Power comes with a price if you want to use it!! Oh, have Hedman fenderwell exit headers with 2 1/2" pipes, crossover and trapps on the ends with front of rear wheel exit. Depending on mods, your plug gap may have to be just a little less, the more mods the larger the gap seems to be the rule of thumb. Chuck P.
P.S.-sorry I didn't respond right away, been doin' the holiday visiting thing! And I beleive your on the right track with gettin' it dialed in, just don't expect too much, a 10% increase will only be 1 mpg!

 

What kind of heads are you running? Any idea what hp?
I don't want to start a fuel injection vrs carb debate but 16 mpg seems real good for a carb V8.
I have friends that get 13 on the hwy with newer fuel injected motors with 35' tires and 4.10's.

 

Just stock that have been massaged, port matched and cleaned up/smoothed out, new everything, valve guides, SS valves(stock size), springs, locks and roller rockers, it took about 30 hrs to do the porting/matching but was worth the time and effort invested. Also run synth fluids in everything, every little bit helps! Sorry, no HP numbers, much stronger than it was! Tranny has a trans-pak installed with an aftermarket convertor, RV type, a little more efficient at speed than the stock one. Vacuum gauge mounted in the cab has been one of the best "tools" for gaining MPG, that smooooooth throttle thing! Let me know if you make any headway in your pusuit of MPG or need anything else. Chuck P.

 

Do you have to push the go pedal harder to get the same power?.After your improvements your engine may be excessively lean requiring your foot to push harder.Take your truck for a drive and get it up to operating temp .Hold it at a steady speed pop it in neutral and cut the engine. pull a plug out and look at the firing end .If its white its running lean which means you have to open the carb more which opens the power valve and kills your mileage.