WOULD A SIZE 18 TIRE IN THE REAR PROVIDE QUUCKER ACCELERATION?

 

It would lower your gearing, and lower your center of gravity, but also your ground clearance! If you are riding a groomed track, that's great. If you are riding trails or open desert, it will probably be a disaster.

It is easy and way cheaper to get lower gearing by other methods, like changing chain sprockets.

 

How much would the top speed be affected? I have my eyes on a couple 18" for ice racing. I like the lower gearing aspect.
Thanks.

 

18" tires will make your machine rev much higher and you will lose a considerable amount of top end, by i would say at least 10 mph. but it will make your low and mid range like a rocket, if you like the lower gear idea i would definelty go with lowering your front sproket wich cost around 15 bucks and 15 mins of your time in the garage, you can pick one up at your local ATV shop.