Here's a thought. Just get a piece of clear vinyl 1/4 inch tubing, about 12 feet long. You can fill it about 1/2 full of dark colored water. Then use a coupler to join the two end's together. Then you just run it along the inside door pillar, back to the back of the passenger compartment, down to the floor, back to the front, up to the front door pillar.

The distance between the front of the tube and the back gives it a larger differential in terms of the height of the liquid in the tube and it would give a pretty fine degree of sensitivity.

Get a digital Level used in wood working.

I think Lug Nut has the right idea. Why not just go with either a big dial vaccum gauge or digital vaccum gauge? With the SG I just leave it on MAP and it's great for showing the slightest imputs to maintain speed with slope. Using it and your eye's you can use it for just what your talking about.

Its a V6 MT longbed 4X4. Basically I drive it in the same manner as I do my Civic (of course it doesn't coast worth a darn, so I can only use of few of the downhill coasts I can use with my Civic). I only do ICE off coasts if there is no traffic around and the road is fairly straight. The only mods I have done to it so far is to air the tires up to 50psi and to install an almost full grill block. 23 mpg isn't the best I have been able to get from it. Last summer I got 26mpg in local (mostly under 45mph) driving. I plan to do some underbody panellng on it this spring and hopefully get it up into the upper 20's on a regular basis this summer.

I don't think any of the plumb bob, bubble level, 4x4 inclinometers, or other gravity based sensing systems will work since they can't distinguish between the gravitational pull of the earth and the "gravitational" pull from changes in the car's velocity. Even changing to a thicker liquid is not going to have any effect in trying to avoid detecting the continuous gradual decceleration of coasting on a flat road.

Silveredwings' gyroscopic pitch indicator would work, but is not inexpensive, the cheapest device of this type I could find, the E-GYRO, was $500.

Lug Nut's string "torque-o-meter" or zpiloto's vacuum gauge would be more likely to work since they could detect the engine's "null point", where it is not either driving forward or holding back on the wheels. I'll investigate my car's engine compartment to see how I can implement one or the other of these two suggestions.