I have been told that the fuel economy of 351 EFI is about the same as a 460 EFI. Can anyone give me feedback as to what kind of milage I can expect from these two motors?

 

Fryguy, I back that theory, old truck 1988 f-350 3:55 38" tires 5.8 litre, 5 spd, present truck 1997 f-350, 4:10 37" tires 7.5 litre, 5 spd . You would think the 88 got better fuel economy "not" .
Same camper, same trip, I get better fuel economy and more power with the new truck (and it's a xtra cab compared to 88 regular cab).
As for actual fuel economy with the 7.5 empty with a tail wind 13/14 mpg.
Fully loaded passing the competition up hills like there are standind still (honestly I dont know ) I guess 6/7 mpg but it's woth it, I come back from a camping trip it takes a week to get that grin of my face.
Regards, Domenico......

 

My experiance involves my E350 15 passenger van. When I got it, it had 122k miles, 351W, C6, stock exhaust & everything. It is a 1 ton van, with 3.55 gears & LT235 85R16 tires. Running empty, with all the seats, I was pulling 12.3 mpg, but had TPS trouble & didnt know it, plus would drop a cylinder when I was trying to push it. I didnt think that was bad, considering how it was running.

It got 9-10 mpg pulling a dolly with a S10. I think it got a little worse, as my TPS went nuts, causing it to idle way to fast (uh, 40 mph!), and causing the EEC to think my foot was about 3/4 of the way down, no matter what. I will also add that my #5 injector was way lean, as evidenced by the white plug tips, as compared to the others that were dark.

I have all that fixed, with true dual exhaust, using 2 aftermarket cats & straight piped. Idle is fine. New O2 sensor. I havent driven it enough since the repairs to give you an actual MPG, but I know it has improved. The engine has quite a bit of blowby, and uses oil, so it is in need of a rebuild. I feel I can hit mid teens when I get everything running right & do some head work.

Hope this story helps.

 

Take it from a long time 460 owner, the difference in mileage is small, but the driveability of the 460 is superior. I don't tow anything heavy with mine, I just like the throttle response and overall power. On the resale end, most people with a choice seem to want the 460 in the trucks they were offered in.

 

Well, here are the disappointing numbers for my beast. Making the trip from Southern Illinois to the Texas Mexico border, towing an S10 with a tow-bar. It pulled a consistant 10.5 - 11 mpg. I found the O2 sensor lead had come off, and reconnected that. I drove another 600 miles with everyting running right, hitting a strong headwind, no trailer, it topped 12mpg. Not very impressive. The engine is still stock, and I havent played with the timing or anything.

I cant understand why a carburated Dodge van, 15 passenger, 3/4 ton, same height tires, well worn 360, 3sp auto, gets 15 plus - using stone age technology, yet my fuel injected van gets only 12!

Just my thoughts ...

 

this is a most interesting discussion. I originally wanted a F250 with 351W or some small block like that.

I ended up with the big block 7.5. It of course gets predictable 9 in mixed and 11-2/3 on highway driving.

No experience with 351W but from what I hear, it gets about 13mpg on the average in an identical heavy duty truck.

Really, disappointing. But not that surprising. I think that Chevy 350 is also gets pretty poor mileage in a 1 ton truck.

 

My 1/2 ton with 351/C6 got around 10 with the Holley, and a best of 13.3 since I changed to the Q-jet, but still far from great. In my experience, the 350 powered Chevies I've driven did better than that, but I don't know what the gearing was on them. The Ford has 3.00 gears, so I wouldn't think they'd be much higher than that, although the Ford has better low-end grunt. As far as old-tech goes, I'm still trying to figure out whey my Dad's old carbed 300 I-6 got 21-23 Mpgs with a 2.73 and three on the tree, while neither of our injected sixes(both with overdrive) will get over 16-17. I have seen larger engines get the same of better mileage than a smaller one, simply because they aren't being pushed as hard to do the same job. With my lousy mileage, I'm getting tempted to star looking for a 460. If I'm going to burn that much fuel, I might as well have some fun with it.