Problems included tyres with illegal tread depths, under or over-inflated tyres, blown light bulbs and insufficient oil.
Peak Performance Management, which carried out the checks before taking drivers for on-road driver training, said the defects might be linked to increased service intervals on many modern cars, as they had less frequent checks at the garage.
In some cases, defects were so bad that trainers refused to take employees out on the road in their own vehicles.
It is PPM policy that all vehicles undergo safety checks before being used for driver training.
The checks for the study were carried out on a sample of 100 vehicles used for driver training. Some 34% needed attention before being allowed out on the road.
Tyre pressures were the most common problem, with 39 tyres being under or over inflated by more than 10% of the manufacturer's recommended pressure. A further three had less tread than allowed by their company's tyre policy.
Seven vehicles which covered a variety of makes had insufficient oil to make them roadworthy, there were three bulb failures, a further two with low levels of water in the radiator – and on one vehicle the bonnet could not be opened.
The spot checks covered a number of different makes of car and light commercial vehicles up to 3.5 tonnes.
PPM managing director James Sutherland said: 'We have now put in place a detailed six-month study to monitor the safety performance of all company vehicles that we deal with or come into contact with.'