I just drove down to Missouri from NJ in a 1986 F250 4.9/C6. I am towing almost 2 tons otherwise I wouldn't have noticed. I was getting really bad power like the auto advance had just stopped working. I checked everything under the hood and the brakes and it all seemed ok. It was hard to get up to 60 MPH and on one hill (West Virginia) I had to downshift. I filled up near mile 125 of I-70 and the truck was transformed. Easy to get up to 75 MPH. Only slight slowdown on hills. Also felt smoother and I was not using so much gas pedal. This happened also a month ago on this trip and after filling up in Colby, KS I had the performance drop to 60 MPH max on a flat road with a similar load to what I've got today. Uhaul 6 X 12 trailer with 5 motorcycles. That's almost 2 ton. Has anyone noticed the big difference between brands of gas?

 

I only use Sunoco

 

 

I am saying most gas is inferior or wrongly mixed. Occasionally you get really good gas. If you never tow heavy loads you would not notice this. The difference in top speed and power on hills is striking. I don't suspect anything that will not spontaneously fix itself with a fillup. The troubleshooting is done over the last 8 months. You are stuck on the term "bad gas". That is off on the wrong track. The gas is good by your standards but it is inferior quality and there is far better. I think I have proved that. Most people don't run their tanks dry like I do (for this problem) so they may not get this dramatic effect.

 

 

One man's bad gas is another man's wrong ethanol mix. Anyway, same thing happened again today. I struggle to stay above 55 on the highway then I get to the intersection of I-70 and state route 40 and I refuel then head down 40 towards Eads, CO and I'm doing 65 - 70 with ease. What happened?

 

Might have gotten a hold of E85 instead of the E10 most stations sell. Poor fuel will do just what you describe. I used to haul fuel. It's not all the same mix everywhere. The base gasoline is, but the ratio of ethanol and the additive package (or lack thereof) is different. And occasionally a poorly trained fuel hauler will **** up and drop the wrong fuel in the wrong tank.

 

Next time it does it pull over and take a couple spark plugs out and see what they are lookn like, then post back.

 

I will do that. I have a new theory since my current theory is not much liked: My truck is loaded to the point where a very slight incline will cause the truck to slow down unless I am already doing 60 MPH in which case the momentum will prevent the slowdown and I get to a level road or slight downhill where I can keep my speed or maybe speed up a little.
If I am starting out from standstill and there is a slight uphill incline I will never get over 50 MPH until I hit a level patch or a slight downhill. That is why the highway works out well as long as I can get up to 60. Does anyone know what size load would cause an imperceptible incline to prevent a 4.9L reaching 60 with a twin axle trailer?
I suggest it is about 4000 lbs unless my 4.9 is particularly feeble.

 

4.9's aren't powerhouses. A freeway overpass will drag the speed down with any trailer. The transmission behind it makes a difference too, that and whatever gear ratio is in the rearend.

 

Here is what it says in the glove box book. My axle is 3.55. My GVWR is 6300lbs. But I can't work out the GCWR.

 

GCWR- Gross Combined Weight rating. That's tow vehicle and trailer combined. As far as being able to get it up to 60 mph, that will also depend on the frontal area of the trailer as well. Wind drag takes an additional amount of power once you get up to around 50 mph and the power needed to accelerate above that increases exponentially the faster you want to go. Add in a cross wind and all bets are off.