The response of late model year vehicles to front-end fuel volatility was measured in a consumer survey carried out in Houston, Texas in the summer of 1980. About one hundred ninety customers were given gasoline of varying front-end volatility on a weekly basis. Responses to these changes in volatility were measured by weekly telephone interviews with each customer. Results show that Volatility Class C of ASTM D 439 provides high temperature driveability protection at 96°F (35.6°C), its highest vapor lock protection temperature. Most of the time Class C is overprotective, however, because the protection temperature must drop below 85 °F (29.4 °C) before a more volatile fuel, Class D, is permitted. Problems observed by participants on highly volatile fuels were sluggish performance, occasional stalling, and poorer start-up when hot. There were no cases of “vapor lock” (disabling of the vehicle because of fuel pump starvation) during the survey.