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According to the National Highway Traffic Safety Administration about 30% of all vehicles had at least one tire that was 8 psi or more under-inflated. About 5% had all four tires seriously under-inflated. The same survey showed that only 4% of respondents checked tire pressure as part of their routine maintenance. That would seem to say that most cars had one or more tires that needed some air.
Under-inflated tires can cut fuel economy by 2% per pound. So if your tires should be at 32 psi and they're actually at 27 psi, you've reduced your mileage by 10%. In case you were wondering, you'll find the recommended level in your owner's manual or on a door jamb tag.