I am looking at buying some racing tires for my revo and LST, and come across a problem:

I don't know what the difference is between the 40 SERIES TIRES(not 23mm) and the Standard Size Monster Truck Tires is.

so what exactly is the difference?

40 series rims are bigger and come in 23mm and standard (example: proline velocity 6) ... the 40 series tires are not as fat (kinda like 1/1 scale 40 series tires), therefor giving you more stability & better handling but will be less forgiving when landing the big jumps

here ya' go... the left one is a Proline Velocity 6 40 series rim (standard hex) w/Crimefighter tire and the right is obviously a LST / Aftershock rim & tire .... now - I know the racing tires for the standard size rims are smaller than the claws in the pic, but this is mainly to show the difference in the rim size. Overall, I think the standard size racing tires are the same height as the 40 series (or really close), just, more rim - less tire