I have an 89 F350 XLT Lariat dually crew cab with a 514 ci stroker motor and a marine cam, tri wye headers and 3" exhaust out the back. It has the German 5 spd manual tranny and a Ford 4:10 ratio rear end. The motor is newer because the starter motor housing is identical to the starter housing that was on my 92 E350 box van. At this point I am trying to get ideas about what to do about mileage. I have every intention of having the diff re-geared to a 3:54 as I don't intend to tow anything heavy like the very loaded 12' U-Haul I did when I moved up here to NorCal Coastal from San Diego. I met a seemingly knowledgeable mechanic that suggests, I go from 20-50 motor oil to 10w-40 as my pressure gauge reads 70+ psi when warmed up. Then, at some point sooner or later, have the injector system cleaned thoroughly and he described the process and ok I see that....after that then convert over to platinum plugs. Finally, later, change the gears as soon as money builds back up. This motor has not lost power but mileage. I used to get almost 200 miles to 18 gallons. On the trip up here, coming up the San Joaquin Valley I was getting 125 miles to the 19 gallon tank. It has low compression pistons to run on regular as it towed the drag boats the the original owners built. It is usually really cool, cold and or rainy up here near the Oregon border of Ca, so I was concerned that the O2 sensors and computer would read the cold dense air and call for more fuel to be added to the injectors...any comments would be welcome...we are currently paying $4.39 a gallon for regular up here so..speak up!

 

a 514 is a big motor, its gonna suck alot of gas, thats the cold hard truth, i would personally rip out that motor and sell it (im sure you can find a buyer) and put in something smaller

 

And the dually is going to get less mpgs than a single rear wheel axle any way you slice it. More rolling resistance, unsprung weight (in your wheels & tires that you have to get rolling, and load-rated tires are HEAVY) and additional wind drag due to the fender extensions. It sounds like you've only got 2WD so you've got that advantage, mileage-wise (less weight, less drag, vehicle sits closer to the ground).

I'm with the others, there's not much you can really do. You could go to a smaller motor such as the 351 (forget about a 302 in that size vehicle) but you won't be happy with the power after having the big block, and you may not get that much better economy because the smaller engine has to work harder - note, you have to watch your rear end ratio with the smaller motors, as they often want to spin a little faster than the big blocks which have maximum torque just north of 2K rpm.

I just bought a 460-powered 1990 F350 crew cab single-rear-wheel with 3.55 gears and the 5-speed manual, and I'll be really pleased if I can get into the double digits for fuel mileage.

The cat's meow for top fuel mileage is the diesel - your truck would probably get in the mid to upper teens with the 6.9 or 7.3 IDI diesel on the highway, and might even break into the low 20s with the Powerstroke 7.3.

 

Few things I see..

1. You have a stroker motor under the stock 460 EFI system, that isn't going to run optimally without some tuning.

2. In 1989 the stock transmission in this truck is likely a C6(what is the trans code on the drivers door decal). If that is the case the single best thing you can do for fuel milage will be to find an overdrive transmission for it, you would have to go all the way down to 3.00 gearing in the rear axle to get the same effect.. which is another option I guess.

3. You have a 500 cubic inch motor in a 7000lb truck, consider yourself lucky if you can pass any gas stations.

 

As I stated in my post, I have the German 5sp manual trans. which is a very rare tranny. Thanks gentlemen for your responses...when I bought it 3 years ago, I was getting 12mpg. I agree as to your statements about the truck but am not going to sell it yet. My sources tell me that I should be able to cruise 65 mph 1800 rpm instead of 55mph @ 2000rpm after changing the ring and pinion gears. I will do the necessary tune and maintenance first. I am going to buy a friend's 2000 Ford Taurus with the 24 valve V6, (of which I have a lot of driving time in this particular car), next year, as she is going to sell it. It only has 70k original miles and I could go from San Diego to Vegas @ 80mph avg without having to fill up, 13 gallons of regular, and that tranny was just rebuilt in Dec while I was still living down there.

Thanks again for your advice gentlemen...much obliged,
Don

 

Sorry I missed the part about the transmission, the ZF does have an overdrive ratio.. it's not very deep (0.8:1) but it's better than no overdrive and 3.55 axle gearing should improve things quite a bit.

 

Hello, I have a couple sets of 3.54 gear sets and a good running 92 7.3 NA, 750 bucks owns them both. Add 300 for crate and shipping, if it does you any good I'm @ 414 303 7079.