Thanks Scott, as a side project I'm thinking about putting a absolute pressure sensor, temperature sensor and humidity sensor in a box and having a microcontroller look at the three of them and spit out a relative air density value.

One of the other guys that used to race TZ250's had a Kinsler RAD gauge and it was handy when making jetting changes throughout the day, so it'll be interesting to make up something similiar and see how things change. Even with the YZ250, the two places I ride are about 300m different in height and it's noticeable. I'm playing with the pressure sensor (red arrow) and will (very) slowly get around to adding the rest. Only 948hPa in my office at the moment...

3.812V = 947.6hPa