Force India's F-duct system is working better for Adrian Sutil, his teammate Vitantonio Liuzzi insists.
"It's not working as it should for me," the Italian is quoted by Auto Motor und Sport.
"In terms of top speed I'm generally slower than my teammate," he insisted.
Italian Liuzzi, 29, is regarded as having struggled against German Sutil in 2010, having scored 32 fewer points in the sister VJM03.
But reports recently indicated that Liuzzi is already under contract for 2011, despite speculation Force India would like to give its promising reserve driver Paul di Resta his F1 debut next year.
Di Resta, a Scot who won his second successive DTM race last weekend, will not be driving the Force India on Friday morning in Singapore, as he has at several race weekends this year.
Lewis Hamilton believes that if a Force India driver is ousted at the end of the year, it will not be his friend and former F3 teammate Sutil.
"Adrian is doing a pretty good job and it is very difficult to see anyone dislodging him from that seat," the 2008 world champion told the Times of India.
German reports suggest Sutil, 27, would be the leading candidate to drive for Mercedes next year if Michael Schumacher decides to return to retirement.