Hey everyone. I posted a couple weeks ago that I picked up a new truck. It's an 86 with a 7.3 swapped in. Still has the 4 spd with 35" tires. Just finished my first tank of fuel through her and only got about 11mpg! Every other idi I've had got 17 or better. What gives? This is my first with the 4 spd. All others were either zf's or automatics. This one has a sidewinder turbo on it as well. Doesn't smoke. Seems to run well, if anything a little under powered for having the turbo.

Most driving at about 55-60mph. Is this really because of the 4 spd?

 

most likely is the 4sp and 35's. how many miles on the IP and injectors? positive you have no fuel leaks.

 

About 60k on the injectors and pump. I can see the 4spd, but technically the 35s should up the gear ratio a little to help the revs. No tach, but I'd guess I'm at about 2500ish cruising.maybe a little higher. My 6.9 and 5spd cruises just below that.

 

large tires will hurt fuel economy more than they help in gearing.
they are big and wide,thus more rolling resistance.they are also heavy thus more power/energy required to make them turn.
to top this off,since they are taller,your introducing yet more air flow under the truck,where everything is setup to function,rather than for aerodynamics.
no OD,is of course a large economy hit too.

how does this one sound to ya compared to the other idi's you've had?
does she seem to have a lot less rattle? her timing could be retarded too.
old trucks and wheel bearings.....when do you suppose the po owner repacked the bearings? might want to get started on that.
gear lube...same thing.check it out.consider synthetic gear lube,and grease.
how did the big tires seem to balance out? you don't want her wasting energy going up and down,that could be used for forward motion instead.this one was a major player with a DRW issue i had.hopping is devastating to fuel economy.

lastly,
(and should be first on your check list actually)
large tires alter your odometer.make sure it's set correctly,or your calculations are a lost cause.

 

if your ear tachometer is calibrated right, then that is your problem/reason for low mpg. my truck is turning ~1900rpm at 55mph.

 

what rear gear ratio do u have. i have a banks turbo a 6.9 and a 4 speed and get 17 to 20. avg doing 55 to 60 and i have 37s to boot. if it is feeling under powered for haveing a turbo i would look in to replaceing the ip and injectors.

 

you must of found some 50+ cetane juice out there in cali you lucky sob Joe.
ship me some of your good stuff,would ya.lucky to get 42 around here and in all actuality,id bet anything most of it tests out 38 lmao (and i wish i was joking.)

 

Thanks for the good reply. Been very busy. Drove the truck solid for about 2 1/2 weeks. After calculating another 2 fillups, looks like i'm getting closer to 15mpg. I'm a lot happier with that but think better is possible. There is a fuel leak at one of the injector caps. That will be replaced when I get home again mid-December. Didn't get a chance to shake down the front end this time home. I should have a lot more time in December and I'll give it a real thorough look.

Come spring i will be swapping the 4sp with a zf from another truck. This one will see a lot more highway time and I would rather have the zf here instead. I expect a couple mpg there.

Thanks again for the input just wanted to "finish" the thread!

 

I have 1986 F350 crew cab 4x4 with 6.9 non-turbo (yet... one waiting for installation). It has 4:10 gears and the 4 sp and a Gear Vendor Over/Underdrive. With the overdrive working I got 17 mpg, when the speedometer cable broke and disabled the overdrive I got 15 mpg. The Overdrive is .78 ratio change, the ZF in 5th is .76 so basically the same. If you're getting 15 now you will probably get 17 with the ZF

 

That's about what i'm shooting for. The truck that currently has the ZF pulls that or better all day long (unless i'm towing). So this truck with turbo on it should about as you say with the 35's on. My guesses anyways!

 

I would bet you have 410 gears + 4 speed=death for mpg. you need a tach.

 

I have an 88 F350 3 speed auto with 4.11 gears and when I get 12 out of it I am doing good. Depending on how I drive it, foot in it too much or just easy cruising it easily varies between 9 and 12.

 

AGREED! When the ZF goes in, a tach will definitely be added. Funny, this owner took the time to do a very nice guage setup in it but neglected a tach.... still think that's strange. BUT it WILL get one when I do the swap

 

hey trust me I don't get 15+ I have a 92 F350 ATS turbo...I get 13 mpg and 10-11 towing 9,000-11,000 lbs. Try this though I measured my fuel mileage from Chevron and Maverick. Repeatedly Chevron gave me 11 mpg at most...Maverick gave me 13 mpg consistently...and I was really conservative with the Chevron fuel. Don't know how to explain this one...beater trucks take beater fuel I guess lol. My question to you is, does your truck cold start now w/out plugging it in?

 

Forget the ZF5, too much work.

My old '86 6.9L with the T-19 and 3.55s got 18 @ 65 all day long. No turbo, no overdrive, nothing. And that's not just a poorly calibrated odometer, because I drove a known 90 miles to the house in Spokane and measured between fill ups, so the distance was exact.

Just put 3.55s in it. I haven't checked yet, but I suspect that my new truck has 4.10s. Those gotta go.

-Matt