I have a Scan Gauge II and use it to monitor the transmission temps on my Jimmy's transmission when towing my 13' Trillium
. I am surprised how much difference the ambient temperature makes. Last week I drove the I95 between New Brunswick and Portland ME. In the afternoon it was 82* and I had a high of 213* after hills but leveled off at 204*. At night the ambient temperature dropped to 41* and my transmission temp stayed steady at 176*; the same road and driving conditions except temperature. I also noticed that if I allow the transmission to shift itself into 'passing' gear rather than shifting myself into 3rd, the transmission temperature rose more quickly and higher.
The ScanGaugeII has a LOD gauge which shows the engine load. I use this to let me know the point at which the transmission will shift into passing gear and shift down manually just before.
Of course, it is also great for reporting fuel consumption.