It seems to be a much bigger difference when it is colder. Here is a quickly pasted together comparison to a nearby (1km) weather station, the last week of October. Higher temperature being the A3.

I looked up a warmer day in September.
16th September 02:00 Weather station reports +11.2c while the A3 reports +14.1c.
At 16:00 the same day Weather station reports +13.1c while the A3 reports +15.7c.

So putting in an offset will get it in the right direction, but to me it seems very difficult to get it to be correct all year around.