Britain should set an early date for converting its road signs from miles to kilometres, a report by the UK Metric Association says.
Campaigners hope metric signs will be in place for the 2012 Olympics
The imperial signs are a "confusing" exception as most of the UK officially operates in metric measures, it claims.
It says conversion would make it easier to calculate fuel consumption and enable more finely tuned speed limits.
However, the government insists it has no plans to make the switch, which would be a "waste of taxpayers' money".
The UKMA says conversion of road signs was originally intended as part of metrification when it started in 1965 and should have been completed by 1973.
But it was put on hold in 1970 and never restarted.
UKMA chairman Robin Paice said: "The Irish have shown how easily, safely, and economically it can be done. The British government should just get on with it."
Ex-Labour leader Lord Kinnock, who is backing the report, says the UK's imperial signs contradict its modern image.
In an introduction to the report, he says: "Our imperial road signs are perhaps the most obvious example of the muddle of measurement units in the United Kingdom.
"They contradict the image - and the reality - of our country as a modern, multicultural, dynamic place where the past is valued and respected and the future is approached with creativity and confidence."
He said the conversion could be completed in time for the 2012 Olympics if the report's recommendations were followed.
But the AA Motoring Trust said a "key flaw" in any planned conversion lay in the fact that car speedometers still mainly measured miles per hour.
Paul Watters, head of roads and transport policy at the trust, said: "A move to make UK road signs metric will take far longer than five years.
"Any precipitous changeover will create confusion, danger and anger, particularly where misunderstanding leads to prosecution for road traffic offences, such as speeding."
The trust said a solution could be to have signs showing both measurements during a transition period of a number of years.
A spokeswoman for the Department of Transport said it had "absolutely no plans" to change the signs - a move which she said would cost several million pounds.