You can do this:
(You have a 42t spur right??)
http://scriptasylum.com/rc_speed/_gear_calc.html
Using the bottom calc with 42/16 gearing it says 31mm shaft spacing. Without changing your motor mount, you can gear 48/10 with 31mm spacing, leaving your motor mount the same, bringing you to a easier 50mph... (probably closer to 45 when all factors are considered)
Here is a 10t pinion and some 48t spurs, not sure which config you need:
http://www3.towerhobbies.com/cgi-bin...?&I=LXUPN2&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXLMT7&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXVHB7&P=7