Coast down test methods and determining cD?

[puddleglum]

Does air density also change with humidity like it does with temp. or is it not an issue. E.G. would air be substaintially denser in fog than a dry sunny day?
Is there any rule of thumb for the change in rolling resistance relating to temp. or is it dependant on the vehicle?

[Ernie Rogers]

Originally Posted by puddleglum

Does air density also change with humidity like it does with temp. or is it not an issue. E.G. would air be substaintially denser in fog than a dry sunny day?
Is there any rule of thumb for the change in rolling resistance relating to temp. or is it dependant on the vehicle?

Hello, Puddlegum,

I am checking in a handbook for you (Handbook of Chemistry and Physics, CRC Press). At 95 deg F, here are the air densities at sea level--

Dry air at 95 F.........1.146 kg /cu.m.
100% humid air........1.122 kg /cu.m.

That's a density decrease of two percent. Suppose you are driving at a speed where 70% of the energy is used to oppose air drag. Then, a 50 mpg in dry air becomes---

50 x (0.7/0.98 + 0.3) = 50.7 mpg in 100% humidity and 95 F

At 70 deg F, the moisture content in humid air has less effect--

Dry air at 70 F.........1.201 kg /cu.m.
100% humid air........1.190 kg /cu.m. This is 1% less

Effect of greater density of cool air--

50 x 1.146 /1.201 = 47.7 mpg.

Effect of raising humidity from zero to 100%--

47.7 x (0.7/0.99 + .3) = 48.0 at 100% humidity and 70 F

If you lower the temperature another 25 degrees to 45 F, the effect of humidity drops to about 1/2%. (Air density rises to 1.226, a bigger effect.)

The effect of fog is to lower fuel economy, partially cancelling the effect of high humidity. The fog effect is smaller, so forget it.

I hope that puts your mind at ease.

Ernie Rogers