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1989 5.8 liter: How to Improve Mileage?

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Old 11-09-2010, 07:02 PM   #1
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Default 1989 5.8 liter: How to Improve Mileage?

Specs: 1989 F150 Lariat with 5.8 liter (351) with three speed automatic (no overdrive) and EFI. 94,000 original miles. Runs fine, with a slight rough idle. Horrible mileage, maybe due in part to the three speed automatic. My question to the forum: what's the most efficient and economic way to improve the gas mileage? What's the best bang for the buck?
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Old 11-09-2010, 07:32 PM   #2
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Nothing. We all are suffering due to the garbage the feds are making gas companies put in their fuels, such as methanol which burns twice as much as gas, a couple things you can do, but it will cost you, put higher gears in but towing will be affected, full headers and duals, try new o2 sensors, but other than that there is no true way to get better mileage out of a 351\c6,
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Old 11-09-2010, 07:44 PM   #3
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10-12 MPG is close to the best City mileage you're going to get with a
89 F-150 5.8L / 351W engine automatic without over-drive.

The 4WD Regular Cab LWB F-150 weighs 5200 lbs. minimum and
it takes a lot of fuel to get it up to speed and maintain that speed.

Is your truck a 4x4?

What's your gear ratio?

Does your truck have an Air Conditioner?

Keep the engine tuned-up (Plugs, Wires, Air Filter, etc.)

Keep EGR System working as designed.

Keep engine RPM below 2000, let it get to 3rd gear as smoothly and as soon as it can.

Remove anything Heavy that you do not need from the truck.

Avoid jack-rabbit starts.

Do you have the stock tire size (P235/15) or larger than stock tires?

All the above can have an effect on the fuel mileage.

Rodney, you need to go to your truck profile and change 5.3L to 5.8L.

Oh, you are talking about modifications to the engine aren't you?

Last edited by 1987 F-150 XLT 4x4; 11-10-2010 at 05:45 AM.
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Old 11-10-2010, 08:39 AM   #4
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Thanks for the feedback guys. I knew it was a low percentage play but wanted to make sure I wasn't overlooking anything. Will update my truck profile as suggested.
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Old 11-10-2010, 07:15 PM   #5
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Honestly, stop driving it. You'll get 0 miles for every 0 gallons burned, it's infinity mpg!

In reality, you're boned. There's no winning. Even with OD in a 351 trans, it's still a 11city/15hwy beast
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Old 11-10-2010, 07:19 PM   #6
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highway cam, edelbrock performer intake, edelbrock carb, crane fireball ignition, duraspark distributor, more power better mileage.
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Old 11-21-2010, 02:10 PM   #7
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My truck is a 1990 with the 5.0 in it, but it should still be applicable. This is what I did to mine and I noticed a pretty good increase in mileage.

-Electric Fan Upgrade
-Accel Distributor Cap/Rotor
-Accel (or equiv) Spark Plug Wires
-Bosch Platinum Plus Spark Plugs (Platinum Electrode = Hotter Spark)
Note: If you run a "hotter" ignition coil, you can open up the gap on your spark plugs to allow a larger spark, which should give you a more complete burn.
-Accel or Jegs Ignition Coil Upgrade
-3G Alternator Upgrade
-Synthetic Gear Oil (75w-90 if you tow occasionally and 75w-140 if you are towing frequently)
-High Flow Air Filter
-Quality Motor Oil (5w-30 is optimal for mileage)
-Fresh Transmission Fluid
-Double Check Ignition Timing
-Obvious stuff like make sure your tire pressure is correct
-New O2 Sensor
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Old 11-21-2010, 09:12 PM   #8
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i have an easy way to improve gas mileage...
take a 3/4 inch dowel rod, measure it about 3 inches and cut it. place it on the underside of the gas pedal and secure it with a good ole wood screw skinny part against the gas pedal.
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Old 03-28-2014, 06:23 PM   #9
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Sorry folks have a 89 5.8/351 in my Elite motorhome and am lookin for ideas to improve the stock 3 speed transmission as well as mileage some good ideas from qdeezie as i have set aside 2 grand before the alberta spring rving/rodeo season.Thanx
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Old 03-28-2014, 06:37 PM   #10
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Whats the stock rearend ratio we are thinking of changing the rear ratio 300 bucks plus beer then a full tuneup then go from there
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