Ok, i'm not even sure if there is a problem, but it seems like my 85 pickup slows down way too fast when i take my foot off the gas pedal.
I have checked the obvious and non of the brakes are draging or sticking.
It kind of feels like it is slowing down due to it's engine compression, but my sl6 87 van dosen't do that.
It isn't as evident at low speeds, but anything over 40 mph it becames very pronounced.
I realize that every vehicle will somewhat slow down when the foot is taken off, but not as fast as my pickup.
The automatic tranny shifts really smooth through all 3 gears, so being stucked in 2nd gear is also ruled out.
The differential fluid has been changed recently, and it didn't help.
The only think that comes to mind is maybe it has higher than usual diferential ratio, but i can't find any stickers anywhere that would tell me what it is.
What were the ratios available from that year?
If i jack it-up then spin the tire, while counting the driveshaft revolutions, would that be the right and accurate way to determine the ratio?
I have this funny feeling that this "fast slowing down" problem has a lot to do with my mileage.
Anyone has any ideas what i should check for?

I think you're right about the axle ratio. If you have a 'tall' ratio, like 3.91 or 4.10, you will have a noticeable sudden deceleration as the axle tries to hold the engine at speed.

This would correspond to lower fuel mileage, as you are turning more RPM at speed.

I have a '77 Ramcharger with 4.10s, and brakes are mostly optional.

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Does it continue to slow unusually fast if you put it in Neutral? That'd show whether it was an engine drag effect or something on the chassis.

One other thought - lockup torque converters can sometimes make for more engine braking if they don't unlock on you. I'm not very familiar with when they are supposed to lock and unlock on your truck, but I would expect it to have one.

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When i put in neutral, the resistance is gone. I'm pretty sure there is a lock convertor in that tranny, i just don't know much about how it is supposed to work.
Could it bee that something might be blocking the air intake flow, and creating some un-wanted backpresure?

seems to me that means the resistance is in the rear end ratio, most likely your truck has the higher ratio rear gears to compensate for the weaker motor (as opposed to the V-8) I was giong to suggest you check the rear end oil to see if maybe it was losing gear oil but I don't think that would be the case if it freewheels nicely in neutral. Make sure when you jack up the back end and count the turns that both rear wheels are turning at the same rate if you have an open diff I think you only jack up one side and divide by two or something.

Lockup I don't know about.

Good luck.

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Middy.

jack up both rear wheels if they turn in the same direction turn one full revolution and count the #of turns the driveshaft makes.
if they turn in opposite directions let one side down and turn 2 revolutions counting the #of turns the driveshaft makes that'll be your ratio