Hmmmm.
indoors and bad lighting, getting the shutter speed up is a factor in your blurry photos.. You should still be able to get some nice panning shots if the car isnt moving toward you or too erraticly.
With my camera indoors at 1600iso I would usually still struggle to take a photo without moving the camera at all, You really need to be moving with the car(s) to keep them from blurring.
This is the sort of thing I mean, ive somehow managed to move in time with that first car (and not the 2nd). lighting in there was really bad.
You can try using the flash also.
Im no flash expert and im still learning to use it.. I'm used to seeing flash shots where the car might be over exposed and the background is totally black. Which looks rubbish ofcourse.
Probably the best way to use the flash is in fully Manual mode, where you can control both the Shutter speed AND Aperture at the same time.
If you can set the shutter speed to something like 1/250th and the Aperture to the biggest it will go (F2.8 ?? or lowest number).
The big aperture controls the background / ambient light, bigger aperture will make it brighter.
You need to be fairly close to use the flash, especially the built in one on your camera as they dont have the power to light things further away.
Using the flash is a bit more complicated than I ever thought, its a balance between the flash power, the available light and your shutter / aperture settings.