My stock tires (new) have a notional circumference of 34.24"
Allowing for 0.2" of tread wear, worn tires would have a circumference of 32.98" - ie only 96% of their original diameter. That would make my speedo read 4% faster than real. (Assuming that it was calibrated for new tires).

Tire pressure would also play into this - since the average temperature has dropped somewhat in the past month, you can expect your inflation pressure to have dropped.

Maybe you need to go and do some non-empeg stuff to your car - flat bald tires sound like a great idea for drag racing, but not so good for the rain.