Thank you. I'm here for the knowledge and share where I can.

Edit: As posted before- I have a 94 f150 302 rclb 4r70w with 3.08's. I just bought the truck in October. Its not my daily. I'm married to this fuel mileage for now but I don't love it,lol.I'm in SE Michigan where its as flat and as straight as it gets.Set your big toe and go,lol. This truck replaces a 95 f150 302 3.55 5 spd ext cab long bed. That I felt did fairly well with power and mpg. I'm looking to see what type of mpg I get towing a smaller pop up.

 

Not so sure about this. I've had the understanding that the only time, for emissions reasons, that a factory ecm allowed open loop was either above 4000 RPMs or WOT.

 

My 96 will sit in open loop if I drive 80+mph with a head wind, in overdrive, no where near full throttle. It'll do this across all of South Dakota, and get about 9mpg. Drop to 70, closed loop except up hills, and mileage jumps up to 13-15. I can watch which protocol the PCM uses, and open loop happens regularly anytime engine load is high.

On edit: Force of drag increases with the square of velocity. Meaning there's a big difference from 55 to 70 mph in wind resistance. It takes fuel to go fast in these old bricks, but a lighter (RCSB 2X4), lower brick, with the i6, on stock tires should do fairly well at 70mph.

 

No that is not the case.

 

Then what is the case? Lets hear the conditions needed to open the loop.

 

open loop comes into play when throttle angle, map reading tell the pcm the engine load has reached a certain tipping point.Now exactly where that is programmed I'm not 100 percent sure,it also factors in engine temp, o2 temp,transmission gear etc. The reason they can do this is emission standards are mostly at idle and cruise speeds measured in ppm, grams per mile etc. Emissions are not tested under heavy throttle loads pretty much the same as epa mpg ratings weren't real world.

 

Also in the bad old days of carbs and cats it was found out that prolonged heavy loads melted the catalyst and caused car fires. The solution they came up with and still use today is altho it sounds counterintuitive is add more fuel to cool the converter. The reason is the same as when your mixture is to rich an engine wont run it takes a certain amount of oxygen to burn the fuel, addin g extra displaces the oxygen so it cant ignite and it cools the catalyst. Sounds crazy don't it but its true. It works the same as the air pump injecting oxygen causing the catalyst to light off but in reverse.

 

Exact conditions for open loop is going to vary with each PCM family but the bottom line is it is based on a dynamic factor called Engine Load and that parameter takes into account engine rpm, throttle angle, manifold vacuum or MAF flow rate, vehicle speed and a few other things. The PCM will transition into open loop as the vehicle travels along when Engine Load increases beyond a set point that is somewhere around 50-60%.. I can't remember exactly right now. It doesn't take much to do this, accelerating at anything more than a very moderate level will do it, simply driving down the read at a constant speed and encountering a hill will do it, this is a big part of the reason that automatic transmissions are programmed to downshift so easily, the whole idea is to try to keep the engine in closed loop as much as possible for best fuel milage. I had a tuner on my '90 that would allow realtime data logging and this was something that could be monitored, it was painfully obvious that when cruising along at 55mpg on a rolling highway that it was far better to keep the trans in Drive vs OD, when encountering hills in OD the engine would bog, Engine Load increased and forced open loop and the instantaneous fuel milage that the software calculated would drop from 20+ to 8-12mpg.

 

I see. I've only tuned aftermarket EFI systems,so I don't know much about factory PCM tuning. Just wondered if those comments were guesses or if someone actually knew the conditions for open loop.

 

And being that he is trying to push a barn door down the road at the speed he wants to drive, going to a gear with LESS mechanical advantage he will load the engine more. Thus causing vacuum to be lower and be more likely to drop out of closed loop.

His best bet to test whether or not a gear swap will help is to try to find a friend with the same bolt pattern and taller tires. And then go for a test drive.

 

What shape are your tires in? You could just get taller tires and forget the gear swap. 33" tires will bring you pretty close to a 3.08.

 

They are fairly new 235/75R15's.

I was crunching the #'s for 3.31 and 3.27 gears as well to maybe find a happy medium.

 

Ditto on the torque peak being about 2000 rpm. I have a '96 M5R2 5-speed F-150 4x4 with 3.08 gears. First gear is low enough that I have no problem pulling away from a stop. I never tow, but if I do, I can always use 2wd-low range if I have a lot of weight. I don't have a tachometer, though, so I have no idea what I'm spinning at. Replacing the intake manifold gasket now, hoping for a couple mpg's increase when done.

 

For some closure on this topic, the $$$ I would have spent on gears, gaskets and gear oil went to putting new heads on my Ranger and having it back to being the main daily driver getting close to 20 MPG at 70 MPH. The F150 is the boat hauler and Home Depot trip truck. Towing the boat, I am fine with bad mileage and it actually does pretty good in the city cruising at 40 mostly.