Here the deal. I have a 351m in a 78 and I have a few questions about fuel economy. Its actually in a Bronco and it gets only 10 mpg. I would like to get it up to 15 or higher. The 1st thing I am going to do is get rid of the fulltime 4x4 for lockouts and hoping that will help tremendously. Looking for a np205 transfer, I think. I have been reading up on the performance parts on the M block and was wondering if I did any mild performance upgrades (mainly Aussie heads) and left the carb to a 2bl if it would dramatically impact fuel economy or would it be a waste of money better suited eslewhere. Also what options are out there for AODs? Next option I have a 351w with trans from a 90 f150 would this be a better choice in achieving 15 mpg or better. I would like to stick the the 351m if possible. Your suggestions are appreciated.

 

Years ago I had a 78 F150 with a 351M 4x4 and a 4 spd, a guy showed me how to bore out the power jets in the carb (stock carb). I had the motor rebuilt, bored .30 over and an rv cam, the timing was CRITICAL for fuel economy! If I advanced the timing 2 degrees the mileage went down but the power went up (good for towing). I had two marks scribed on the distributor, I averaged 14mpg all around with it. I never realy drove it far.
I don't put a ton of stock in timing by a light, I mostly time them for what suits me, no ping, ease of starting, smooth power, etc.
The thing about boring out the power jets is to double their size, thereby letting in 4 times the amount of fuel, the stock carbs lack in that area which is where most of your use is.

 

I'm not sure you CAN get 15 in these trucks. Lockout hubs are a great start.

Clean the carb; Run her a little lean
Change air filter
Check/replace all vacuum lines and plugs
New ignition components...plugs, wires, rotor and cap
Check tires for full inflation
Possibly a radiator flush
Maybe others here can relate their experiences about a fuel additive?

Wax the finish and add a racing stripe....it makes it go faster!

 

Dont fool with it. Get a daily driver
I just dont believe in even picking up a wrench to work on these trucks if fuel economy is whats on your mind. They were made to excel in many areas, and do it well, but not this area. The only thing I would do to one of these, and may do it to mine at some point, is put a Cummins 4BT and overdrive unit in it. But hey, why not just keep them original and be content with what it is.

 

A few things to consider...best tank I've ever gotten out of any of my 351M/400's is 14mpg's...and I've owned well over 30 of these engines, and they all have run tip top shape. And 14mpg's was all highway driving.

Actually going to a 4bbl carb would be a better idea, if you can keep your foot out of the secondaries. Couple that with an intake and headers so the darn thing can breathe.

Beyond that, most other things are power adders and will deplete you mpg's.

Now if you swapped to a fuel injection set up from the likes of Holley or the other kits like it, I've heard guys getting upwards of 20mpgs with those.

A trans swap or OD, can be pricey.


So in thegrand scheme of things, there's really not much you can do to get better than low -teens mpg's. And also keep in mind all you would have to spend in new parts and swaps...how much gas could you buy with that money.

 

I'm not looking for a daily driver. I got a 78 f150 w a 300 that's cherry its my DD and it gets 22 mpg. I just can't get newhere on 10 mpg. I was looking to be able to drive more than 150 miles and not spend $60 everytime. It is my toy. I was just hoping to get 4-5 more mpg bc that another 60 miles. I am mainly wanting to swap out the fulltime 4x4 and was wondering what kind of gain I would get.