Porsche has accused Nissan of cheating in the GT-R's record bid at the Nurburgring racetrack.

Porsche has just run its own back-to-back tests with the Japanese company's GT-R supercar and says it could not get within 25 seconds of Nissan's claimed record time of seven minutes 29 seconds in April.

It also found its 911 Turbo and GT2 were both quicker than the GT-R.

"This wonder car with 7:29 could not have been a regular series production car," says August Achleitner, the 911 product chief for Porsche, speaking to the CARSguide at the Australian press preview of the latest 911 Cabrio.

"For us, it's not clear how this time is possible. What we can imagine with this Nissan is they used other tyres."

He believes the time achieved by Nissan with ex-Formula One driver Toshio Suzuki would only be possible with a semi-slick race-style tyre.

Achleitner says Porsche took a standard GT-R, running on regular road tyres, and ran it around the Nurburgring within two hours of its own cars, on the same day with exactly the same weather conditions.

He says there was no tweaking of any kind and the GT2 and Turbo both ran on regular Porsche road tyres, the Michelin Sport Cup.

"We bought the car in the US. We drove a GT-R with new tyres," he says.

Achleitner was initially protective of the exact lap times, which were run during a program when Porsche also compared its upcoming four-door Panamera with a range of potential rivals.

But he eventually revealed his team clocked the GT-R at 7 minutes 54 seconds, with the 911 Turbo managing 7:38 and the GT2 getting down to 7:34.

The laps were not run by Porsche's usual hot-lap specialist, former world rally champion and race winner Walter Rohrl, but one of the company's chassis development engineers who is an expert on the Nurburgring.

Achleitner says the back-to-back comparison was run because Porsche was concerned by Nissan's claims for the GT-R, which is heavier than the 911 with similar power.

"The Nissan is a good car. I don't want to make anything bad with my words," he says.

"It's a very consistent car. But this car is about 20 kilos heavier than the Turbo . . ."

In the end, Porsche believes its testing has achieved the right lap times for the Skyline GT-R and benchmarked it against its own 911 heroes in the right context.

"For us it has been clearly the result. This technical puzzle now fits together. With the other numbers we had problems to understand it," he says.