ZX5 RS help please

[Speeed Freak:)]

Hi guys, I have a ZX5 running a Losi 6.5, currently a 78 spur and a 21 pinion, i'm more than happy with the way it runs, in fact its one of the fastest on the carpet track we run! Motor/esc runs cool too!

Well I got hit and damaged the spur so need a replacement, I have a 76T spur, if i run that with a 18/19 pinion what difference would i expect to see against the 78 with the 21?
Thanks for any help!
Andy