The Motor Insurers Bureau (MIB) saved more than $205,000 (£145,871) by successfully taking a personal injury claim to the Court of Appeal.
Last week the court found the wrong multiplier had been used when originally setting the amount needed for the claimant's long-term care.
The case involved a US serviceman who was hit in 1994 by a car driven by an uninsured driver in the UK.
The serviceman suffered severe head injuries, including a skull fracture and frontal lobotomy that caused personality and behavioural changes.
Liability was agreed at 75% to the motorist and 25% to the injured pedestrian.
MIB technical director Roger Snook said the serviceman was originally awarded $1,513,159 (£1,076,659), using a multiplier of 25 years.
However, Snook said the MIB argued that the correct multiplier was 17 years.
The MIB said the claimant's evidence did not tally with the care regime allegedly required.
It also argued the original amount set did not reflect that the care regime was designed to lessen over time, so the claimant could become more independent.
The Court of Appeal found in favour of the MIB, reducing the award to $1,307,846 (£930,572).
Snook said there were differences between the US and UK compensation cultures.
"Damages awarded in the US are usually somewhat higher than in the UK.
"As he was advised by a US lawyer as well as one from the UK, we think his lawyers may have been more optimistic than usual," he said.