<div class="htmlview paragraph">One very effective way to improve fuel economy of motor vehicles is to reduce the viscosity of the automatic transmission fluid (ATF). However, viscometric constraints often exist for a particular ATF application. The proper choice of viscosity index (VI) improver allows viscosity reduction at lower temperatures, while maintaining viscosity at higher temperatures. This study shows that molecular weight of a VI improver is a key factor to achieve high thickening efficiency, improved shear stability and limited metal fatigue failure simultaneously. Based on this study, an optimum VI improver has been found and a new fuel economy ATF was developed.</div>