Accounting for variable grade

[GasSavers_maximilian]

Did some preliminary scouting of my coast down test strip and it's just long enough to be useful and is very straight. However, I also put a bullseye level on my dash and it varied a fair bit along the stretch. I didn't have a way to accurately measure it with me, but it was enough to noticeably effect coasting time in the different directions. Compared to all the other roads anywhere near here it's quite flat, just not flat enough. Since I don't want to drive hours every time I want to retest my vehicle, I'm trying to determine a way to account for its variations. If it were a constant grade, it'd be easy, but it's not. Since my current plan is to calibrate my Kiwi MPG's digital speedometer and then record that in a movie on my digital camera, I figure all I need to do is also record an inclinometer at the same time. I'd only need to include enough data from each stretch to account for the variance, so fewer points in constant grade sections, more during the variable parts. The math is straightforward, but the practical concerns with getting the data accurately is concerning. I'd need an inclinometer that is responsive, but doesn't jitter all over the place and is cheap.