Some garages charge "extortionate" labour fees of up to £170 an hour, according to a Warranty Direct survey.
Labour charges vary across the UK
The highest hourly rates were in London, the survey found, but in 80% of UK regions surveyed labour charges peaked at more than £100 an hour.
The survey of 3,000 garages revealed big differences in rates charged by franchise dealers and independents.
However, a motor industry body said the survey gave a "misleading" view that motorists were being overcharged.
The differences between franchised dealer and independent garage labour rates were at their widest in the West Midlands, the survey said.
In the region, franchise dealers charged on average of £66.35 an hour and peaked at £117.45. The cheapest independent garages charged just £23.50 an hour for labour.
Generally, the highest fees were to be found in the south east of England and London.
In the capital, the average labour charge was £116.53 an hour, with the most expensive garages charging £170 an hour.
The least expensive regions were mainly in the south west of England and Scotland.
Duncan McClure Fisher, Warranty Direct spokesman, said some garages were charging excessive fees.
"Garage rates are still extortionate, but motorists can help themselves by asking how much they charge an hour. Unfortunately, once you get the bill, it's too late to yell," Mr McClure Fisher said.
"After a couple of years of excessive rises, mechanic labour rates have started to stabilise but it's obvious that there is still a major gulf between regions, and independent and franchised garages."
Matthew Carrington, chief executive of Retail Motor Industry Federation, said that there were good reasons for the price differences.
"Franchised dealer labour rates are generally higher overall because of the extra costs that these businesses incur in order to meet the standards expected by both consumers and car manufacturers," Mr Carrington said.
"Costs are naturally highest for businesses in metropolitan areas, especially central London. Those in the regions will be able to charge much less per hour because their overheads are lower," he added.