Under-inflated tyres add an extra £1.4 billion to annual vehicle fuel bills, according to a survey.
Vehicle parts replacement firm Kwik-Fit said as many as 46% of cars in the UK are driven with at least one under-inflated tyre.
About 2.2 million extra tonnes of carbon dioxide will enter the atmosphere this year due to under-inflation of tyres, the company estimated.
This year's figures compared unfavourably with last year's, when only 20% of cars were found to have an under-inflated tyre.
Kwik-Fit said this could be down to a decline in fully-working, publicly-available tyre gauges at service stations.
A recent report from Auto Express magazine found that 39% of service station tyre gauges were damaged, out of order or giving incorrect readings.
David White, customer services director at Kwik-Fit, said: "It's disappointing that, despite the extra financial pressure on motorists, this year's spot check found that the number of under-inflated tyres more than doubled in the past year.
"With fuel prices soaring, we hope that by showing how under-inflated tyres add to our fuel bills will provoke motorists into action. If not, then perhaps the impact on the environment will."
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