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IndyCar teams are desperately seeking ways to stem tyre degradation with the new aerokits, says 2004 champion and '13 Indianapolis 500 winner Tony Kanaan.

The veteran Brazilian, who has moved to AJ Foyt Racing for 2018 after four years at Ganassi, said Monday's Sonoma test highlighted how much impact the aerodynamic changes were having.

"2000lbs of downforce removed, and you feel it missing everywhere - from braking, to turn in, to power down. There's less," Kanaan told Autosport.

"This track has always been really hard on tyres, but with less downforce your problems are coming sooner.

"The tyres were sliding less when we had a lot of downforce with the aerokits we had the past three years.

"Now, you've got to really manage the tyres, and that's because of the downforce being reduced.

"I don't want anyone to think it's because Firestones aren't good - the degradation occurs because you don't have this aero grip pushing you into the track. You're sliding across the surface more, and right away. So the tyres start degrading sooner.

"It's hard to go flat through the chicane on worn tyres, and it gets harder and harder each time through.

"But the track is quite green, we only have half the usual number of cars here. It will improve."

Reigning champion Josef Newgarden, who led a Penske 1-2-3 in Monday's unofficial test, argued degradation rates had not changed significantly.

"The tyres feel similar to what we were getting before," he said.

"They degraded a lot last year, from laps 15-20 you could really feel it, and now I'd say it's the same but it's offset by having less grip overall."

Newgarden also played down the differences between the 2017 and '18 cars.

"Obviously you notice a lot less grip on this car, a significant drop," he said. "It's moving around a lot more, we were way more stuck.

"But I'd say the traits, the overall balance and feel, are not that different.

"The set-up [from the old car] has maintained some significance here.

"There are other tracks where you are starting from a clean sheet - Sebring seems very different, and I think street courses will take a bigger step.

"But it seems like here you can start from a decent baseline [from the old car] and work on changes from there."

Kanaan emphasised that he was not complaining about the new cars.

"This is how we wanted IndyCar to be," he said. "Now it's up to us to find the right grip level.

"We wanted to get more like the balance of CART Indycars back when I came into the series, and for sure these remind me of the CART cars.

"Back then we had a little more downforce than this but also more power, but this car is a step in that direction."

Firestone has been working with teams during testing of the 2018 aerokit and has made tyre construction changes accordingly.

It will also introduce a new wet tyre for the coming season.