It's hard to find real rules of thumb, a big part of adjusting a setup to different weather settings is just trial and error, but that's what the warmup session is good for.

If temperatures are higher than 70°F, you have to take off some tape and maybe increase diff ratio. If temperatures are lower or if you're running with cloudy weather, you have to reduce diff ratio, that's clear.

Rules of thumb for tape / gearing adjustments:
- 5% tape per 10°F
- 1 click of diff ratio per 20°F
- cloudy instead of clear: reduce diff ratio by 2 and tape by 1 click (on most tracks; if two possible diff ratio values are very close to each other you might have to reduce it by 3 clicks)

After finding good settings for tape and gearing, the final step is finding a good balance. That's the trial & error part about it as weather changes influence the handling in a different way on the different kinds of tracks.

Colder or cloudy weather means that the atmospheric pressure is higher which means that more air hits the car. The results are more downforce (mainly at the back of the car), more drag and more engine power as more fuel can be burned. Higher temperatures have the opposite effects, of course.

More downforce at the back makes the car tight overall and more power loosens it in the middle and at the exit. The additional power is usually more important, cool or cloudy weather will make the car tight at the entry and loose when you're back on the throttle. And as cars will spin most likely at the exit, you'll have to tighten the car and make the best of the tightness entering the turns if the weather is cold (in most cases). Use front bias or track bar (keep the bar flat though, that means make both sides (nearly) equal) to adjust the car's balance, you will rarely need more than 2 clicks.

If the weather is very cold (in the 40s or even below) and / or cloudy, you might get another problem on many tracks: The car will bottom out too badly due to increased downforce. In that case stiffen the front springs by 50 lbs each and increase front bias by an additional click (yes, stiffer front springs make the car loose).


There's an exception for Michigan:
If it turns cloudy, reducing the diff ratio by just one click is enough because the gap between those diff ratio values is really big.