I easily grind at Indy oval using the race modded ZR1 Corvette, just tune it including full downforce and you never have to get off the gas (like the A.I. does); except avoiding the slow A.I. cars
I prefer the LeMans cars for it, like the 908 hdi, the 787b, the R8 race cars or the R10.
You can also do it with a Furai and if you're just grinding the X2010 is king, because it will win you those 86000 credits in less than 2:50
The premium Zonda is great. Comes with a good bit of downforce...with the right tune, no need to even use the brakes at Indy.
It is basically a street version lmp.
I originally did daytona with the modded veyron (made all the ground up on oval sections), For indy ZR1 RM but you run full front downforce and less than 50% rear downforce, this gets the car turning in much better, don't forget to set the suspension up as its bad after you mod it. Stiffen the springs up (heavier at front by 1 kg), dampers 8 all round, stabalisers 6 all round, neg camber F 2.0 R 0.8, hope this helps Oh BTW I run TC on 1.