If you dont feel like reading the gear ratio sticky.. You might get like, .5 less mpg, and higher gear ratios help make more rpms, therefore giving you better acceleration. Your rpms will go up about 200-400 depending on how high of a gear ratio you get.. but you WILL FEEL MUCH MORE POWER OUT OF YOUR CAR! Gear install can be from 100-300$. The actual ring and pinion (rear end gears) cost around like, 150-200. Kits are more.