Do rough idle repair problems track registered cars and trucks per resident across states?
Across U.S. states, NHTSA owner complaints about rough idle repair run higher where registered cars and trucks per resident is higher, after adjusting for the makes sold in each state. This is an association across states, not proof that registered cars and trucks per resident causes this repair.
State reports of this repair against registered cars and trucks per resident, one dot per stateEach dot is a state: how much more (or less) than expected this repair is reported there, adjusted for the makes sold in the state, against its registered cars and trucks per resident
How to read it: Each dot is a state. Further right means more registered cars and trucks per resident; higher up means more complaints for this repair than expected (1.0 is average). If the dots climb to the right, reports of this repair rise with registered cars and trucks per resident.
What it shows: States with more registered cars and trucks per resident tend to sit higher on the chart across 28 states: a pattern in the data, not proof of cause.
Data: FHWA Highway Statistics 2023 Table MV-1 with Census 2023 population, joined to NHTSA owner complaints, downloaded 2026-10-06 · 28 states. Chart: CalcScrape · calcscrape.com
The 9 states with the most registered cars and trucks per resident (Arkansas, Alabama, Kentucky, Michigan, South Carolina, Oregon, Tennessee, Wisconsin, Ohio): reports of this repair run 17% above what the makes sold there would predict, and 8 of 9 are above expected. The 9 with the least (New York, New Jersey, Massachusetts, Texas, Connecticut, California, Maryland, Illinois, North Carolina): 4% below, and 3 of 9 are above expected.
Exceptions: Wisconsin is among the highest in registered cars and trucks per resident but reports below expected.
Split the states into thirds: the third with the least registered cars and trucks per resident runs 4% below what the makes sold there would predict, the middle third 14% above, and the third with the most 17% above. Most of the registered cars and trucks per resident effect sits between the lowest and the middle third.
For registered cars and trucks per resident, after allowing for the 1562 comparisons we ran, luck alone would produce a pattern this strong about 1 time in 129. Take out any single state and the registered cars and trucks per resident pattern still holds.
Weak spot: compare only states with similar auto repair shops per 100,000 registered vehicles and the pattern keeps about 37% of its strength, so part of it may come from auto repair shops per 100,000 registered vehicles instead of registered cars and trucks per resident.
For registered cars and trucks per resident, of the 4 exception states, 1 read the same way for the other parts of the system (so that is the system there, not this repair) and 3 are specific to this repair.
More registered vehicles per resident means more cars on the road and more older vehicles kept in service.