Driving can give you a spare tyre and leave you more vulnerable to heart attacks and strokes, say researchers.
Motorists who spend an hour a day at the wheel were on average 5lb (2.3 kg) heavier than those who drove for 15 minutes or less.
The extra time in the car also adds around half an inch to the waistline - the additional fat making you more likely to have heart attacks, strokes or diabetes, the researchers said.
Men are more likely than women to put on weight due to the amount of time they spend in their car.
Professor Takemi Sugiyama, of the Australian Catholic University, said: "Prolonged time spent sitting in cars - in particular over one hour per day - was associated with higher total and central adiposity [fat] and a more adverse cardio-metabolic risk profile." This is a group of factors that identify individuals more likely to suffer heart attacks, strokes or diabetes.
Two-thirds of Britons use a car as their main form of transport, the experts found.
The research, published in the journal Preventive Medicine, assessed the driving habits of 2,800 adults from the Australian Diabetes, Obesity and Lifestyle study against health measures including body mass index and waist circumference.
The scientists wrote: "Relative to participants who spent 15 minutes a day or less in cars, those who spent more than one hour a day were likely to have a 0.8 greater BMI (equivalent to 2.3kg or 5lb for a person of an average height of 5ft 7in)."