XRAY - Model racing cars

I am needed a little help to get me going in the right direction. I am the only X-ray that runs in our on road club, I got it used from a guy that ran it in the mod class with great success. I am running in the 19t class. The car is a T2 I have went over the car and upgraded the bearings to ceramics. The problem I am having seems to be getting a descent top end speed. I have been to 2 races and seem to get left behind on the top end.
I have worked on the gearing and if I go up on pinion it over heats?
Here is what I have we run outdoors, asphalt, tracks about 130ft x 80 ft with 1 long straight the rest sorta choppy
T2
Foam tires for main the Dia was 2.534
84 Spur
23 pinion
I am using Checkpoint 19t money motor
ESC is a GT7
4200 EP promatch 6 cell

I noticed it the setup sheet that came with the car says to use a 87spur/18 pinion
Am I missing the boat here?

Any advice is welcome


Thanks Sherman