Glass reinforced poly(ethylene terephthalate) polyester molding resins are currently used in various applications for automobiles. These include assemblies such as windshield wiper plenums. Expanding its use into new applications requires that the molded polyester part be able to withstand additional automotive fabrication steps and be tough and dimensionally stable in use. While PET based material is currently used for applications that are affixed to the car body after oven treatment, customers have specifically expressed the need to simplify the automobile assembly sequence by using materials that can withstand the 200°C ovens used to cure car body coatings. Existing commercial grades do not meet all of the additional specific dimensional requirements, although the heat deflection temperature at 1.8 MPa of these types of materials can be well above 200 °C. A laboratory sag test using tensile bars also indicated dimensional changes occurred at 200°C but failed to reveal the causes for this instability. A three point bending creep recovery test was developed to investigate this issue. It was found that cold crystallization of the polymer matrix took place during the heating to 200°C within the first 5 minutes and corresponding dimensional movements occurred. Use of this technique also predicted that some specific additives would improve the resistance to deformation in the end use. A new Impet® 1grade, EKX-163, is developed that has higher deformation resistance. The preliminary results from a customer trial confirmed the superior dimensional stability of EKX-163 in the end use, and the validity of the test.