VIDEO: Hennessey Venom GT convertible clocks 427km/h

Hennessey Performance Engineering has set what it claims is a new record for the world’s fastest convertible by clocking a Venom GT roadster at a speed of 427.4km/h at a US Navy base in Fresno County, California.

The record verified by Racelogic, the performance testing equipment maker and Commander Darren Fouts from the Naval base, will not be considered an official world record because Guinness World Records was not present to officiate.

The top speed run was also only recorded one way, instead of the two-way average required for an official world record - but the 427km/h top speed easily beats the unofficial 408.8km/h record Bugatti set for its Veyron Grand Sport Vitesse back in 2013.

To beat Bugatti, Hennessey ran its updated Venom GT Spyder that’s powered by the same 7.0-litre twin-turbo V8 fitted to the previous Venom coupe, but with its boost wound up from 19psi to 26psi.

This change is said to have raised peak power to an incredible 1082kW.

Now capable of running E85 ethanol mix, the engine boasts a power increase amounting to 155kW.

Instead of a sequential ‘box, a six-speed manual gearbox made by Ricardo was used for the record attempt. The transmission was chosen because it was capable of dealing with the 949Nm of torque the 7.0-litre powerplant generates.

At SEMA 2015, Hennessey claimed the more powerful Venom GT Spyder had a theoretical top speed of an incredible 450km/h - but the new record falls some way short of that.

It’s not known whether the US car-maker had set out to beat the 2014 record of 435km/h for the coupe or whether it always intended to set a lower speed for the roadster.

Weighing in at just 1244kg, the Lotus Exige-based Spyder takes just 2.4 seconds to hit 97km/h and reaches 322km/h in a mighty 13.0 seconds.

Guinness demands a top speed average for its official world record because it neutralises any advantage gained by elevation and wind speed.

Speaking following the record run, Hennessey Performance’s founder, John Hennessey said: "I thought that this would be a special way to celebrate 25 years of making fast cars faster. I've wanted to test the top speed of our Venom GT Spyder, without the roof, ever since our coupe ran 270.4 mph on the Space Shuttle landing runway at NASA's Kennedy Space Center in 2014. This was a great way to validate the technical excellence of our car, which includes high-speed stability with an open roof."

Off the back of the record, Hennessey plans to offer a limited run of 3 “World Record Edition” Venom GT Spiders that will also commemorate the firm’s 25th anniversary.

Each car will cost $1.7 million plus local taxes.