I have a 1978 F100 with a 302 and a C6 Transmission. Here's my question. I've only had it about a year and I was wondering if it should be coasting better than it does. Let me explain a little. Occasionally, I take it out on the highway at higher speeds, 65 or so. When I let off the gas she immediately slows way down and doesn't coast freely. I don't really have anything to compare this to as my last one was a 76 with 3 on the column. I do know that years ago I drove my uncles Olds 442 and once you got up to speed you could let off the gas and that thing would roll for miles at speed on the flat roads of my home town. Is this just the way these are or do I have a transmission or torque converter problem? I have already jacked it up to see if the wheels spin freely. The front do, the rear are a little harder, in neutral of course, than I remember my 76 being but not overly so. Ideas? Thanks in advance!

 

Our trucks can have a lot of roll resistance. You didn't mention what kind (or size) of tires you're running, but a soft compound or aggressive tread will have a lot more resistance than a hard compound or highway tread.

Bearings can also be dry or misadjusted. Brakes could have a drag to them

The C6 also draws a lot of power. Just shifting from Neutral to Drive can drop the engine 150-200 RPMs

Anyway, lots of things to check

 

As Papabearyuma said I think it's more roll resistance on car vs truck. I've had plenty of Gto's and Firebird's with TH400's in them and as far as rpm drop/drag it's the same to me (and a '70 Vista Cruiser and a '70 Toronado GT). The Vista and Toronado were pigs if you had to push them as there just heavy vehicles but not the weight of a truck. Again I think it's a truck vs car. Types of tires, vehicle weight and aerodynamic's and not to mention gear ratios etc....

Later, Shortbox4x4

 

Mine will engine brake if at anykind of rpm, then once dropped down it will free wheel. I have not driven a car with c6 much but I do believe they were different, so maybe they were coasting easier, or maybe just the taller gearing made it seem so, but the truck to car c6 was different, so maybe that was it. I like that feature because coming from the mountains with firewood, I just drop to 2 and engine brake all the way down. I guess maybe a spragless converter might eliminate this, good luck. Hard to say without seeing, but sounds ok tome.

 

If that is your only symptom, I would not assume a transmission or converter problem. As everyone said there are many factors which would determine this characteristic. I have a '76 F-250 with a 460 & C6. It acts similarly. When I examined the drag on the front disc brakes I discovered much drag and the need for a brake overhaul. (The rear drums didn't have the excessive drag.)