My '85 f150 4x4 runs great...starts right up, and actually idles smoother than my '09 Hyundai Sonata. When I bought my truck in November of 2010 it had an NP435 and I was averaging 13.5 mpg highway.

Last summer I swapped over to a 4 speed O/D hoping for a little better economy. The best I got out of it was 14.5 highway.

Last month I swapped in a recently rebuilt C6 auto because the O/D started making noises...and I have a bad left knee. Truck shifts/drives great and I'm back to 13.5 highway as expected.

This truck only sees weekend highway use at speeds of 55-60 mph tops. What do I need to do to get at least 16 out of this thing? I have 3.55 gears in it now, but I have a set of 3.08's that are ready to go in. Will the gear swap make that much of a difference? All the tune up items are recent, using motorcraft plugs. Recently rebuilt carb too.

Truck is pretty much stock with Duraspark II, single exhaust with stock type muffler and no cats. The EGR has been blocked off by the P/O. Would that make a difference? What else do I need to check? Am I forgetting something?

whats the deal?!

 

DSII was not stock on the 85 300, TFI with a feed back carb were. Beings someone swapped out the TFI for a DSII did they also swap to a non-feedback carb? What size tires are you running? An overdrive or taller gears would help, like the 3.08's, but how much I don't know.

 

It has been swapped from TFI. It has the proper early carb, round coil, and single vacuum advance distributor. I'm currently running 30x9.50 tires.

 

The single biggest thing I did on mine was drop back to 235/75 street tires from 31X10.50 AT's. That along with checking/fixing any vacuum leaks, gapping the plugs to 50 instead of 45, and setting the timing with a vacuum gauge got mine from 14 to 18.5 mpg. But I'm running 3.55 gears which hurts it some.

 

I usually set timing with my vac gauge but ill see about the plug gap. I plan on swapping in the 3.08's next week. Hopefully it does a little better.

 

Going from 3.55 to 3.08 gears will reduce your RPMs by 13.2%.

At 55 MPH, my 1986 F150 with 4.9L [Duraspark conversion/1970 F350 Carter YF], SROD manual and 3.08 rear gears returns 22 MPG. With overdrive, the engine turns ~1570 RPMs at this speed. At the same speed, but in 3rd gear [1:1 direct], the RPMs rise to ~2200 RPMs. My tires are the original size: 215/75 X 15.

I feel that the "sweet spot" for my engine is 1600-1800 RPMs. This is about where the maximum torque is generated on a stock engine.

 

My 1982 4x4 F-150 used to get 10 to 11 mpg in the city & up to 20 mpg hwy! But, with the perverted gasoline nowadays my poor old truck only
gets 8 mpg & 15 HWY @ BEST!

 

Got the 3.08 put in last night. Went to the junkyard this morning and found the proper speedo gear. We'll see what happens with the mileage in a couple of days.

 

Let us know how they work out, been thinking about the same gears for mine.

 

I'm happy with the way it behaves in town and on the highway. I can cruise along at 55 at around 2000 RPM.

But...

I'm still at 13.5 mpg.

The only major player in the ignition system I haven't replaced is the coil. I'm using a standard round coil, with the "horse shoe" connector. I did the Duraspark swap keeping the TFI coil on a similar equipped truck I had in highschool (circa 2002) and got 18-19 mpg.

I've had two different rebuilt carbs (both non-feedback) on this truck with the same fuel economy.

I'm kind of stumped.

 

Man I don't know what to tell you. Even with the C6 and 4X4 it should be doing better than that. Given, mine isn't a 4X4 or C6 but it can do 18.5 all day long at 60-65. And with the 3.55's and shorter tires my final drive ratio is considerably lower than yours, but I do have an overdrive. Just a shot in the dark, is your exhaust in good shape? Just wondering if maybe the cat or muffler might be bad. I know the EGR should help mileage at cruse speed but don't think it's that much. Also the C6 will cost you a little mpg but don't know how much either.

 

I have a 94 FI e250 van with a C6 and 355 gears with 245/75/16 tires, no cat, running great and can't do better than 14mpg.
OD makes a big difference.

 

Just a side line question, does the DSII swap give you any hassle with smog checks up in Phoenix? I live in Casa Grande and we don't have them yet but been wondering if I did the swap would it be a problem latter down the line.

 

No problems with the DSII swap gong through smog. Been through many times. They just look for out of place stuff. Just as long as it looks stock they don't question it.

 

Thanks, that's good to know. My TFI setup is working fine for now but figure I'm going to have to do the swap in the future.