Hi all, Ive got a 1997 F250HD with a 5.8 but the thing is I'm only getting 10-12 MPG Highway. From all I've read this is decently low.

So far I've changed my oil, Cap/Rotor, Fuel Filter, Air Filter, 3in Straight pipe exhaust after CAT, and the biggest thing is I don't slam on it. I try to stay below 2,000 RPM.

Now my truck has SD not MAF, 4x4, 3.55 gears, Ext Cab, Long Bed, Auto Tranny, 113,000 miles, 33in tires but I didn't see a difference going from stock size to these.

Any help would be great, only thing I haven't tried doing is Spark Plugs, and Spark Plug wires.

Thanks, Mike

 

Did you recalibrate your speedometer when you changed tire sizes?

Otherwise, the mileage isn't really abnormal. How fast are you driving to get "highway" mileage?

It's an engine built with 25 year old technology, pushing a large, unaerodynamic truck that weighs a bunch.

Jason

 

It seems low, but not super low.

Since you're changing out most of your ignition components, I would recommend doing this:

ignition upgrade and timing bump (no 56K) - FSB Forums

When you replace the plugs, make sure you get copper Autolites or Motorcraft plugs. Increase the gap to about .055 or so on the plugs. After that, you can advance the timing a couple of degrees. The timing advance will give you more power and an increase in fuel economy.

I did this on my MAF 351 and did notice a difference in power and an increase in MPG, but not to the extent that some of the people over on FSB are claiming.

 

Besides the good info already mentioned make sure your truck doesn't have the fuel transfer problem if it's a dual tank truck, pull codes and make repairs to clean anything that is flagged, and even if there are no codes if the O2 is old change it anyway as they do wander off spec over time.

 

Double check your tire pressures also! It really is amazing what a few pounds of pressure will do when you're on the low side of the pressure range. The higher pressures help to decrease the rolling resistance.

 

I'd say 80% of the time I'm driving 55-65MPH. Speedometer is fine after the tires, have gone through many Radars and all say I'm doing same speed as speedometer says I'm doing.
Checked tire pressure only one was down considerably a bit all other tires were down only 3PSI.
Check engine light isn't on so I think my O2 sensor is still good, However, I think im going to run out today and get new SparkPlugs and Wires. Any certain brand I should get or just the Motorcraft?
Thanks

 

10-12 MPG out of a F250 SC 4x4 isn't all that bad, really. My 351 Bronco only got around 15, and it was quite a bit lighter than your truck, and typically saw speeds of less than 60 MPH. Even my '89 F150 300/4M only got around 16 in typical driving, and many guys I know are only getting in the 11 MPG range in F250s with the 351/auto/4x4 combination. Even the diesels in that truck only get in the 17 MPG range typically, if that, so not sure how much you can do to get noticeably better fuel economy.

 

That's a bad assumption, you can still get codes without a check engine light and even of there are no codes if that sensor is original it's waaaaaaaaaaay past it's due date.

Also make sure the front hubs are unlocked when commuting if this is a 4wd.

 

I have a 92 F150 2WD with the 351, and recently performed the "six liter tune-up", and am pleased with the result. The 9mm wires, wider plug gap, and MSD cap and rotor allowed me to push timing to 18* BTDC with no pinging.

Also, I had a plugged up cat, and recently replaced the entire system. You might want to confirm that you don't have a partially plugged cat, even though you have already done a cat-back system. And yesterday, I changed my O2 sensor, which appeard to be original. Haven't driven it much yet since, but expect it to help.

I'm now considering a cam change after looking at the specifications on the factory camshaft. There should be a little bit of mileage to grab with a more efficient "towing" cam.

You're truck is quite a bit heavier than mine, but I think the same changes will help you out.

Cheers,

JD

 

do you have a high-rise canopy back there ? carrying a few hundred pounds of junk ? passengers ? if yes to any of these, then your mileage is probably about normal.

also, did you arrive at your mpg figure after 10 tanks of gas or just 1 ? it's amazing how many people use the latter method, which is not accurate .

 

The 10-12MPG has been constant for a year now. Used to get 6MPG but did what I mentioned above and got it up to 12MPG. Just went out today about bought the MSD ignition coil, Autolite copper plugs and Autolite wires. Put the MSD ignition coil in today and plan to get the plugs and wires done tomorrow.

The ignition coil seemed original and back when i first did the distributor and rotor that seemed original so the O2 might be as well. Ill probably run to see if any codes come up tomorrow too.

No extra weight in truck. Only pull my boat in the summer and that's it and hubs are never locked unless i need them, then I immediately unlock them when I'm done.

I'm shooting for 14-16MPG. Think that's do able after I do this Tune Up and possibly if I have a bad O2 and replace it?

 

Check out your CAT. If you don't need to pass emissions inspection where you live, you might consider gutting your CAT for a little easier exhaust flow.

 

Don't feel bad, I'm getting 10.9 on the highway with my '94 F150 4.9L

 

I did the spark plug wires today and ended up breaking my M.A.P Vacuum line... So I had to do a temporary fix to that. Anyway, drove it around a bit today and got 67 miles on a quarter-tank (usually only got 45-50). Cant wait to get the original Spark plugs out and see what happens after that!

 

Hi JD - I wonder if you may be able to answer this - I am getting like 8 MPG out of a 90 F250 351 with 160K - 4x4, Fresh plugs, rotor, cap, wires 0- and I am considering pulling the Cat completely off - FOR NOW. I will eventually pick up a Cat-back system but in the interim I can't quite afford it. Do you (or anyone else) happen to know of any ramifications of doing so to the Mixture, Running of the truck, Etc?