Mid-cap tool firm Snap-On ( SNA) is having a banner year in 2012, driven by a vehicle fleet in this country that's older now on average than it's ever been before. Shares of Snap-On are up more than 24% since the start of 2012 as sales continue to outpace analysts' expectations. Snap-On sells its tools and diagnostic equipment to a diverse mix of customers, most of whom are professionals who work on cars, trucks, planes, or other machines. Independent car techs are the biggest buyer of Snap-On products, served by a fleet of 3,200 vans that sell and deliver tools directly to potential client shops. That control of the product from manufacture to delivery gives SNA precision control over its costs, a fact that's helped the firm react to economic headwinds in the last few years. An eye to emerging markets could provide significant growth opportunities for Snap-On, as the firm looks at courting the tool market in China. Even a modest success in the People's Republic could yield significant performance improvements for SNA's shareholders -- we're watching this Rocket Stock this week. To see all of this week's Rocket Stocks in action, check out the Rocket Stocks portfolio at Stockpickr. -- Written by Jonas Elmerraji in Baltimore.