The total value of all options bought by drivers in both countries last year was more than €7 billion (£4.68bn).
JATO Dynamic’s study looked at 10 specific options, including audio and navigation systems, additional airbags, lights and wheels, which were ordered with new cars in Germany and the UK.
The total list value of orders for these 10 options in 2003 was €5.5bn (£3.67bn). In 2005, this dropped 13% to €4.7bn (£3.14bn).
JATO said: "The fall in the total value appears to be due to two main factors. Several options, particularly air conditioning, were more widely available as standard equipment in 2005. And, certainly in the UK, the list prices of the options we looked at fell by an average of 10% between 2003 and 2005."
Of the 10 options included in the study, satellite navigation systems were the leading option by value in the UK last year. UK buyers spent £107m on factory-fitted sat nav systems in 2005.
In the UK, the value of air conditioning options sold fell by more than 60% to £45m. JATO said this was due to increased standard availability, plus a 25% fall in the average cost of having it fitted as an option.
UK buyers benefited from a 10% fall in the average price of audio options compared with 2003. UK drivers were more likely to steer towards in-car technology such as 'smart' keys (up 270%), traffic information systems (up 260%) and sat nav (up 85% compared with 2003).
The top 10 options by total value of revenue in 2005