Hi all, just wanted to know if this is normal: I've got a 76 F-150 4x4, 4spd granny low, 32's on factory suspension. 390 Big Block with a 700cfm carb, mild performance intake manifold, headwork, mild cam, headers, true dual exhaust. No AC but I've got the ramrod assist power steering. I've yet to be able to drive due to a blown rear 9 inch. The prior owner tells me when he daily drove it to work a few years back, it got around 8 -10 mpg. THAT IS HORRIBLE! Is this possible, even with his mild performance mods that he did to the truck? The rear end was a Ford Factory posi, soon to be replaced by a Trac Loc posi, and its either 3.89 or 4.11. I want to mount 35's if its got 4.11's, and that granny low is low enough to handle those tires with those gears I believe. That motor has plenty of power, and runs great. Motor and tranny both rebuilt three years ago, and it was only driven about 9k since then and parked for the remainder of the time. I just want to know if that mileage quote is legit, and if so, I need some help massaging some more out of it! I know to put a wooden block under the pedal, drive civilized and keep it about 60 on the freeway, but Im in college and I dont have a tow vehicle, so this thing's gotta be able to get me to my favorite wheeling spots without taking too big a bite out of my wallet. I know it is purpose built vehicle, and its mods make it worse on the fuel, as well as its inherently heavy and durable design, but come on now, why not 12 or 13mpg? I'd love to be able to get an overdrive unit for that nice semi truck like gear splitting ability and overdrive, but that is very expensive and will require a large amount of time to pay off with the mileage increase. Any input here is greatly appreciated.

 

8-10 MPG is common for these bricks. Some have reported getting higher mileage, as good as 15-17MPG. But, I suspect those were 2wd, 1/2tons running 3:1 gears etc etc.

Aside from the obvious tune up stuff. You need to make sure your cam, compression ratio and other engine components are well matched. Higher compression will generally improve mileage, but too much compression will require higher octane. Do you know what cam/pistons you have?

I think you can get 10+MPG. I got 10MPG in my stroked 428 driving near 70MPG.

Above all else- SLOW DOWN!

 

Morning Greg....

Well with my mild 395" er in my 70 F250 2WD with a C6 trans, when I had 3.73's in the rear end and running 33" tires on the back, I got 10mpg no matter what..

I dont know what I get now with the 4:10s in the back ..but i'am sure its worse, and it going to get worse yet when the new motor goes in and 28" tires in back and 4:56's in the rear...But unlike yours its not a daily driver... Sometimes ya just have to pay the cost to be the boss...

Ya might try and lightening the truck up some...We have a member on the site Scouder and he has a 4WD that weigh's in at about 4500lbs right now and he is still dropping the weight... JMO!! Good Luck..

Russ

 

That would be me. And it's 3.50 gears.

Lean out your jetting, check the timing. Advance the timing until it pings, then back it off about 3 degrees.

 

Geese, sounds like I'd better get a tow vehicle! No, I do not know offhand what was done to the truck, but I can find out, as the prior owner tells me he has receipts for over 4k worth of work done to the motor, so I assume it is good running order and the cam/compression matching is done with some skill, as the work was done at a reputable shop. Once I get a look at the receipts I will get that information posted. I will play with the timing once I get the new third member installed and I get the truck home so I can go through it, but when I start it up where it sits and let it warm up, it runs great and is VERY responsive. I assume the only major mileage mod I can do at this point is EFI conversion, which would definitely be a plus, but it probably will cost more than the 1k that I paid for the truck. However I do plan on having the truck a long time. Maybe E-fan(s)? I dont think trucks had actual clutched fans in the 70's, but I could be wrong on that one. I definitely want 35's, anybody out there with a similar setup with my engine/tranny/gears/tire combo that can give me a mileage figure? Thank-you all very much for your opinions, they are appreciated greatly

 

take a look here to get started:

http://www.angelfire.com/fl/procrastination/rear.html

 

all trucks and cars by ford got clutch fans starting in the early to mid 60's. edsel in 59. for 1k its a deal.

 

Too bad you haven't driven it yet. No matter where in the RPM range you like to drive, these motors have plenty of torque. The heavy iron cranshafts store energy (inerta) that will get you through stuff you normally think twice about with a smaller engine.

The build specs gtex mentioned determine where in the RPM range torque comes in, and peaks. Find out which transmission, transfer case, and axle ratios you have for sure. Once we determine the gear reductions, we can determine the RPM range you'll be cruising on and off road with different size tires.

Another consideration is the type of wheelin you do; mud, sand, trails. Driving strategies that work for one are certian disaster for another, I'm a go-real-slow kinda guy myself.

I'm going to buck convention and say mount the biggest tires you can. FEs have enough torque to get em turning and keep em turning-with less throttle.

FWIW, my '74 came with a 390, 600 CFM 4v, 3:54/3:55 axles, 31" tires-10 to 12 MPG work or play. Could have been better but I have big feet, heavy boots.

 

8-10 mpg is not unheard of with these truck/engine combos. Since the engine was rebuilt and the work was probably done when gas was relatively cheap, I'm guessing the rebuild was aimed at maximizing power over economy. These two are not mutually exclusive, but generally, if you increase power you decrease mileage. I'd start tuning the dickens out of it. Proper advance, lean out the carb a little, proper tire inflation, synthetic oil in engine, trans, diffs.

Keep in mind that at $1000 for a good running truck that leaves about $600 a month for gas in car payments, insurance and licensing fees you don't have to pay. To put it another way, the same truck today that will do the same job will cost you $35,000 +. That's a lot of gas money!

 

Bigger tires can hurt gas milage in 2 ways, one being higher resistance to turn them, and the other is by causing the truck to sit higher in the airstream.

Larger tires generaly have a larger contact patch with the road. great for handling, but it means more resistance, and it takes energy to overcome resistance. In other words, more gas.

General rule of thumb is from a 20MPG base, you lose 1 MPG for every inch higher your truck sits over stock at highway speeds.


Also, what Transmission does it have? If it's an automatic, does it have high stall converter?


Things you can do to help would be Removing the tailgate and adding an aero net, rejetting to run leaner, making sure you are using a carb with vacuum secondaries, running your tire pressure above 30 on the road

 

EH?

My '74 highboy w/360 had a "solid" fan - no clutch... and my brother didn't change it out, either.

I remember seeing my friend's '65 t-bird had a solid fan... some other '70's Fords too...

 

If you are planning on 35" tires, then for mileage reasons you will want to stick with a 3.73 gear ratio. Plenty of pull, but will help on the highway. The 4.10's are GREAT, but you would want to install a tranny with an over-drive gear like the NV4500 or as you mentioned an over-drive add-on.

First, I think your carb is to big for your 390 (this is said not knowing the specific's of the build), but you should be running a 600cfm and max of a 625cfm. Of course re-jet (primary) for a lean mixture at idle and re-jet secondary at WOT for fuel mileage.

Second, you will here several different "pros" and "cons" on this next one, but upgrading your stock DuraSpark ignition with a high-out-put system. This will allow you to widen the gap from .044 to maybe around .055 for a more complete burn.

Third, again...a lot of opinions on this suggestion too. But I would suggest either installing a clutch style fan or going electric. The electric fan will reduce drag on the engine and free up more HP. Resulting in a better efficient engine.

Good luck with your quest...with my '76 F150 4x4 390FE 32" tires 4" lift 3.50 gearing: I get consistantly 13-15 mpg on the highway...at 70mph or less. Wtih the modifications that I mentioned above.

biz

 

My '74 highboy w/360 had a "solid" fan - no clutch... and my brother didn't change it out, either.

I remember seeing my friend's '65 t-bird had a solid fan... some other '70's Fords too...


really? i have never ever seen a truck or car from that era with a non clutch fan. maybe it was a west cost thing that had them.

 

Could be, those CA people would do anything to reduce smog

But yeah, lots of FE's without clutch fans, that I've seen.

Back when I was really redoing my highboy (15+ years ago), I'd look at a clutch fan like "Hey, that's a nice new-fangled thing" ...

By the way, with 33" tires and 4.10 gearing, my '74 highboy with an 11:1 390 and big lopey cam got 15 MPG on the highway ... long trip to Rochester from Long Island... so, lots of running at 3500RPM. That's not to say that it wasn't running way lean ... like the secondaries were open and overall not enough vacuum to pull the correct amount of fuel.

 

Quote:
By the way, with 33" tires and 4.10 gearing, my '74 highboy with an 11:1 390 and big lopey cam got 15 MPG on the highway


How 'come I only get 10 MPG with my 3.25 gears, 390 with lopey cam, 1/2 ton p/u, 9.7cr, on the highway?