All right report time.
So dropping a single tooth does indeed have a big effect. It looks like I am pulling to 142ish kph by EOS versus 140ish.
It makes a big difference. I went into face-off and sure enough, I was pulling on em as the straight progressed.
What changes is the rhythm. I am having to relearn all the turns slightly. S1 has gotten slower but in time I have managed to get it within 2/10 of my best (15.41). That should be good enough for sub 45.
Changing from 25t-24t on the axle sprocket is a 4% change so would result in a 4% increase in EOS speed or 145.6 kph. at the same revs.
It would seem that the 1t change is too much.
What James’ charts do ( amongst other things) is to show the 4 combinations of engine /axle sprockets between 16-25 and 16-24. so we have in order 16-25. 18-28. 15-23. 17-26. 19-29. 16-24.
If all these sprockets are available you could spend a few minutes!!! working through these ratios to further improve your laptime ??
I can change the front and rear so currently I am at 16/24 down from 16/25.
So the steps you describe are in order? Beginning with 16/25…ending 16/24? So, 17/26 would be “between” 16/25 and 16/24?
Thanks
You have it! And the best of luck.
Thank you. 
Sorry again… am I correct in understanding the following:
If 16/24 means I am EOS at 142kph but ideally it should be 145. Since I’m not getting to 145, I need to bring it down so that my theoretical max speed at EOS matches what the gearing max is?
It may be that the engine just has not got enough horsepower to send that kart along at 145.6 kph.on that track.
All you can do imo. is to try the various ratios to see which gives the best result.(laptime)
It could be that with more practice the 16-24 may come out on top but to me that looks unlikely atm.
I’ll try the 17/26 combo tonight. The 16/24 resulted in gains in s2 and s3 but lost time in s1. It does come out ahead relative to 16/25 but you are correct, it feels a bit like trying to make a square peg fit a round hole.
Thanks again.
@KartingIsLife would you happen to have a spreadsheet that could help me figure out the steps? I started at 11/73 for X30? (109kph top). I am looking to increase top speed. 10/71 (112 kph top) is in ballpark but want another KPH.
Is there a similar relationship as the one john described?
This shows the gearing steps between 16/25 and 16/24 for Modena KZ
10/71 is 7.1 ratio
11/73 is 6.6 ratio
So quite a different ratio. If you want more top end, just drop a tooth. When you start splitting ratios, it’s such a small difference it almost doesn’t matter.
If you wanted to change drive gear to split the ratio, a 10/66 would be a half step down from 11/73.
As you have around 73t on the axle sprocket on the X30 cf around 25 on the KZ you can get a much finer adjustment on the X30 by just altering the axle sprocket teeth.
One tooth change is equivalent to 13000 /73 or 180 rpm at 13k for example.
I think this should be as fine an adjustment as you could possibly need, but equivalent chart would go: 11-73,12-80,13-87,11-74.
No doubt James is busier than me!