Thermoforming involves stretching of a heat-softened polymer sheet to large strains. This was simulated by uniaxial tensile stretching (UTS) experiments at different temperatures and strain rates. These experiments were used to identify an optimum window of thermoforming temperature and strain rate and to compare the thermoformabilities of a blend of polyphenylene ether (PPE) and polypropylene (PP) with thermoformable acrylonitrile butadiene styrene (ABS) resin. It was shown that the PPE/PP blend generally has a wider thermoforming window than ABS. Using Considere construction, it was shown that the PPE/PP blend underwent more homogeneous deformation than ABS. The higher homogeneity in deformation of the PPE/PP blend resulted in a higher uniformity in the distribution of draw ratios, which implies more uniform thickness distribution in thermoformed parts. Draw ratio and crystallization studies ruled out the possibility of postforming shrinkage in the PPE/PP blend under ambient temperature conditions. The PPE/PP blend was also found to have a higher shrinkage onset temperature than that of ABS. An attempt was also made to relate the small deformation viscoelastic properties with the large deformation UTS properties through the use of Deborah numbers. These results bring out the importance of studying both the small deformation and large deformation properties in conjunction, while comparing the thermoformabilities of different polymers.