Past studies on tire pressure monitoring have revealed that about 28 percent of light vehicles on our Nation's roadways run with at least one underinflated tire. Only a few psi difference from vehicle manufacturer's recommended tire inflation pressure can affect a vehicle's handling and stopping distance. Poor tire maintenance can increase incidences of blowouts and tread separations. Similarly, underinflation negatively affects fuel economy. In 2005, NHTSA's FMVSS 138 required automobile manufacturers to install tire pressure monitoring systems (TPMS) on light passenger vehicles with phase-in period from 2006 to 2008. Prior to the regulation, NHTSA’s National Center for Statistics and Analysis conducted several surveys and studies to estimate and compare the benefits of Direct and Indirect TPMS. The results of the most recently conducted survey, Tire Pressure Monitoring System Study, are presented in this report. Data collection in this survey ceased prior to its completion. This study outlines a Bayesian approach to compute the case weights so that the estimates could be representative of the universe considered for the survey. Subsequently, effectiveness of TPMS is studied by comparing estimates of percentages of underinflated and overinflated vehicles with and without TPMS, as well as the average underinflation and overinflation over vehicles in the two groups: vehicles with and without TPMS. Testing some relevant hypotheses provides statistical support to claims made in favour of TPMS based on the above comparisons. The analysis also covers comparison of direct and indirect versions of TPMS, concluding that direct type of TPMS is more effective as compared with the indirect. Since this data collection, improvements have been made to both kinds of TPMS, so different results are to be expected if the study were to be conducted presently. (Author/publisher)
Abstract