Shaun James, 32, was run down in Cinderford in 2002 shortly after leaving his local pub.
A cold case review has led Gloucestershire Police to now believe Mr James was kidnapped before he was murdered.
The money offered from Crimestoppers is on top of a £5,000 police reward.
At the time, officers thought Mr James had died in a hit-and-run accident.
But CCTV footage has been one of the factors prompting police to examine the case again.
Tony Butler, from Crimestoppers, said: "This is an awful crime where a young man lost his life in horrendous circumstances.
"The person or people responsible need to be brought to justice and we would urge anyone who knows what happened to contact us anonymously."
Mr James' case recently featured on Crimewatch, which prompted more than 50 calls, several of which were described as "interesting".
A second reward of £5,000 has been offered for information about the death of a builder from the Forest of Dean 12 years ago.