I would run the smallest pinion that fits, which is going to be around 15T. FYI, I've seen someone else run into the following problem, which is when that massive tire balloons it will increase your overall tire diameter which will increase your top speed which will cause you to run hot. Also the weight of that tire will cause more heat. That combo will put you at 51 mph no-load gearing, but if the tires balloon 1" that takes it to 66 mph, if they balloon 2" that takes it to 81 mph. Just keep that in mind. Ideally, you would want to belt the tires with fishing line braid to eliminate ballooning. Another option is to just choose a lower kv motor.