Trying to understand why the mileage on my truck is so low. It’s a 1978 f150 4x4. It has the np435/np205 combo behind a 302. Also has 3.50 gears and is sitting on some 31x10.5r15s. I’ve driven this truck roughly 1000 miles since I swapped the 302 in and I’m averaging about 5mpg. Does that sound close to what you guys are getting? When I researched the swap most people I’ve seen are at least getting 10. Any thoughts or suggestions? Below is the truck in question

 

My 360 gets 10 if i coast downhill. Otherwise lucky to get 8-9 range around town. Maayybbbeee 10-11 on a flat hwy

 

That sounds low for non 4wd use. Check for a dragging brake shoe/caliper. Our 300 gets about 10. Do you use the granny gear in the 435?

 

Your t-case isn't in low range is it? Is the engine modified or rebuilt at all? Is the emissions crap still on it? Is it running smooth?

 

I rarely use 1st gear. Generally always start in second so I don’t have to instantly shift off the line

 

engine was rebuilt about 1000 miles ago by me. .040 over, gt40 heads, performer 289 intake, brawler 500cfm carb, DUI HEI distributor, shorty headers, and 2.5” exhaust. Absolutely no emissions on it whatsoever. Runs great as far as I can tell but I’m new to the engine building world so I could be wrong. And it’s always in 2hi

 

You should be doing better than that.

Do a good check on your timing curve, and make sure your vacuum advance is pulling in spec as well. detonation or retarded timing can both kill fuel economy, and your engine as well.

 

But first, verify that your speedometer and odometer are reading correctly.
Even though 31 inch tires are not that far off of original, maybe it originally had a different transmission also? There are possibilities.
But like we said, check the timing and tune. Check, spark quality and carburetor jetting.
You didn’t mention a cam I don’t think. What did you put in it?

And how exactly are you testing your fuel economy?
Are you filling the tank all the way, then driving it, then filling it again? Then comparing that with the miles driven?
If so, verify that the miles are correct.

 

according to the VIN it still has the original transmission. The cam I installed was a 2122 kit from edelbrock. And yeah I have been doing that but I know it’s accurate as I drove 45 miles to the next town over a week ago and it took half a tank.

 

Look for vacuum leaks too. As stated, check the timing with a timing light. Then tune carb with a vacuum guage, try to get above 18 inches of vacuum if your cam will allow it. If all that is set as good as you can get it, then you have something causing drag on your engine. Are there any burning or hot smells anywhere around the truck after a drive? Like hot brakes, hot clutch, burning metals? Any weird noises like metal on metal contact or anything?

 

Is gas sloshing/leaking out of the tank? i say sloshing because maybe it’s only doing it while you drive and gas is moving around.

Thsts really low mpg. I would expect 10 easy from your truck especially on a fresh streetable rebuild.

 

5 or so mpg is absolutely aweful. My FIL once had a '70s F100, 302, auto. It was running rough and using excess gas, some black smoke too. I found it had a bad accelerator pump diaphragm and the power valve was leaking. Carb kit / cleaning fixed it.

My 351M, auto, 3.5 axles, 31x10.5x15s, '77 long bed, has a few mild mods, 4bbl carb & intake, Crane fireball hydraulic cam, and I will occasionally check timing. To time it, I disconnect, plug ported vacuum and set timing at a point above distributor centrifugal advance is al in, like up over 2,xxx or 2,5xx rpm as it has a quick timing kit in it. Then afterwards, I unplug and reconnect ported vacuum. I know from a long history, it's a solid 11 mpg truck. I've seen it do 13-14, but 11 is what I can count on.

I live 15 miles from a town, nearer a "village" (but they are building a Dollar General), off an old 4 lane US highway. Most of my driving is on that 55 mph highway, or back roads. I can hear interstate traffic over the hill, but I avoid interstates as much as I can practically do. I worked them my whole career, I know what travels them. When interstates were only 65 mph tops and 1/10 th the traffic, I used them more, and I found that my truck is by far happiest at 60-63 mph. Otherwise, only occasionally do I see stop lights or anything above 50-55 mph on my roads.

With my GPS, and before that using my radar, the 31x10.5-15s put the speedometer about "dead nuts accurate" (it's an industry term).

 

Is your truck lifted? Thats a mileage take away, Mudflaps, take away. wider tires are a take away. The bigger mirrors are a take away. Nothing against the American flag but if that's mounted in the box it's a take away. Ford did studies back in the day of the cafe days and people would complain they didn't get the stated mileage. Ford published the findings for dealers to help them check for deviations from stock on mileage complaints. Nothing wrong with 302's, I have one in a half ton 2wd but if you are driving it in the 80mph Dakota's it really runs out of steam. Small blocks are a little short on torque. When the 80's came around and the long runner F I intake really helped pump up the torque. 5 mpg is pretty bad, as others said you may need the distributor checked. My 2wd F150 gets around 15 at 55 mph speeds,

 

My high boy with a 428 CJ and 33's got about 9 before the Gearvendor. now around 11. I had a brand new '79 SWB 4x4 with a 400 and it got around 12. I can't imagine the OP shouldn't get over 10 from what he's saying.

 

I doubt that the cam from Edelbrock is helping, but it still should be over 5 miles per gallon.
I’m not sure I understand your verification method though. You can never, and I mean never use a fuel gauge as your guide. Especially one of our Old Ford gauges!

But it sounds like you were using the “fill the tank, and then refill it and check the number of miles driven” method. Correct?
You’re not using the fuel gauge as your check, but only as proof that your numbers are accurate. Also, correct?