The highest hourly rate for repair work was £240 at a garage in Surrey, according to research by Auto Express and Warrantywise.
The lowest rate was just £36 an hour at a garage in Birmingham.
The study found wide variations across the country, even given higher wage rates in London and the South East.
For example, the average cost of getting a car repaired in the Scottish borders was £91 an hour - more expensive than in North London, where the average cost was £81.
"Obviously, higher hourly rates in London are to be expected, but the huge difference in average costs across Great Britain shows motorists are not being treated fairly," said Graham Hope, the deputy editor of Auto Express.
"And a difference of £204 per hour between individual garages is completely unjustifiable."
Overall, the average cost of garage bills across Britain was £84.30 an hour.
London is generally the most expensive place, at an average hourly rate of £101.60. Scotland was by far the cheapest, at £71.42.
"Some of the differences Warrantywise has discovered are, frankly, offensive," said Lawrence Whittaker, the company's chief executive.
"Our advice is simple: shop around, negotiate and remember that as a paying customer, you are holding the ace cards."
To conduct the research, the company called 1,200 garages across England, Scotland and Wales.
