Congratulations for going in the right direction to solving your problem. If you are only achieving 3100 RPMs at full throttle you will not be travelling full speed. Something close to 4000 rpm is normal and that 900 rpm will make a big difference in your top speed and you are on the right track about checking your throttle cable. Your question about "which way to install the tires' is best answered by the owners manual but basically the smallest tire on the front ,then the biggest,then second biggest and second smallest on the rear. Look forward to hearing how you make out.