Some keyword: race, racing, drive, fast, car If you could give some info about the game, maybe what keywords you already have in mind, might be able to get some more focused advice.
racing,race,driving,fast,speed,car and more Check this out - https://adwords.google.com/o/Targeting/Explorer?__c=1000000000&__u=1000000000&ideaRequestType=KEYWORD_IDEAS
Well If you looking for some good key words, then you have to use Google key word tool, From there you can got all competitive key words.
Check http://appcod.es. It is a SaaS application that helps you choose the best keywords for your iOS game. (I'm not associated with AppCod.es)