If you're racing, then you'll probably have more of a trial and error process. It all depends on how steep the hill is. You'll find that there's a fine line, where you either have too low of a gear ratio and it won't climb fast enough, or you have too high of a ratio and it maxes out the top speed and you still have extra power to go faster. I don't think any of us has the gut instinct to pick the best gear ratio the first time, based on tire size, hill slope, engine power, driver weight, etc. You just have to take a guess, try it, and go from there. The good news is that you'll be the expert after you figure it out.
If I were you I'd start out around 10 or 12 to 1, and check to see how it does. It will be simple enough to change to a different sprocket later to fine-tune the ratio. You'll need some extra chain, because if you put a bigger sprocket on you'll need a longer chain.
I'll give you a real-world example. I put a Harbor Freight 2.5 hp engine on a mini bike last year. It had an 11 inch tire and a 10 to 1 ratio. The engine was ungoverned, and it went up to around 5500 rpm, producing a 20 mph speed. I weigh 240 pounds, and this thing wouldn't accelerate up a gentle hill. On flat ground it was awesome.
You have twice the power, twice the tire, probably a steeper hill, and I don't know how much you weigh. Your vehicle also has 3 or 4 times as much ground friction than my bike (bigger tire-ground contact patch and 4 tires instead of 2), and it weighs more than my bike. That's a lot of data. Now you just have to make a good guess.
One thing I can say for sure is that you're using up most of your engine capacity with 6 to 1 and 22 inch tires, and that's on flat ground.
