In Holland we have about 5 or 6 beach races. And 18 nov i'm planning to ride the Redbull Knock Out in Scheveningen. Like old fames Veronica beach race 80's and 90's but neven heavyer. Last year
I ran 15-43, at the long section its great (180km/u+), than back thrue the deep sand even a K-5 has it hard.
So I was thinking if i put a longer 5th gear in it I would stil have awsom top speed, and can go back a lot easyer.
You are not going to have any gearbox options for what you are trying to accomplish. 4th and 5th gears on a stock KX500 are both overdriven ratios: 4th gear = .954 and 5th = .791
In theory you would need to gear your KX500 16/40 to pull 180 Km's (and that's "IN THEORY". I seriously doubt you can get a KX500 to pull anywhere near that top speed off road.....
I have a neat Excel based gearing caculator if you want it PM me your email and I will send it to you. It calculates theoretical rpm's and speed in all gears, and can modify the values from different final drive variables that are entered.
Rich