Jason Lynch, aged 30, from Whitfield near Brackley, died on September 2 last year after the car he was driving crashed just north of Silverstone on the A413 in Northamptonshire.
John Underwood, from Northants police’s serious collision investigation unit, said there was a possibility the speedometer had not been calibrated to the replaced gearbox and may have been giving false readings.
The inquest heard from another driver who was passed by what is believed to be Jason’s car around 30 minutes before the crash.
He estimated the speed of the Metro to be around 100mph.
Assistant coroner for Northamptonshire, Hassan Shah, said: “Jason had been ejected from the vehicle and suffered catastrophic injuries.
“The only thing I can say is the extent of the damage and that Jason was thrown from the vehicle all point towards a speed that was probably in excess of 60mph.
“For the purpose of the record I conclude that Jason Lynch died as a result of a road traffic collision.”