SpeedWeek reservations are usually allowed around the beginning of February. The date will be announced in advance - or someone here will ask the casinos and then post. Many of the lodging facilities will be booked up solid (for racers, at least) within a few days of when they're available. I'd try calling around to the motels (not the casinos) and asking when they'll take reservations, and then ask to be put on the list of folks that want rooms. And then call on the day or two before the date you've been given and see if they'll give you a reservation then. And then call on the date they told you they'd begin. Yes, it really is that bad.
And when you make a reservation -- don't be too shocked when they ask for a big deposit which will probably be non-refundable, and the room rates will by WAY higher than the regular room rates, and they're likely to require a minimum stay of - say, 5 nights or so whether you asked for that many or not.
The easiest way to reserve a room for SpeedWeek is to ask to have a room next year -- when you check out this year. Some motels will do that -- have customers that have been staying in the same motel for years, or maybe just this year - - and since you've already made it to the list -- you're able to reserve for next year. There have been times when even that didn't work - like when a motel gets sold during the intervening year, but give it a try if you can figure out how to do that.
Does this sound like gloom and doom for SpeedWeek lodging? Sorry -- but by and large, you'll run into some or all of what I've mentioned here. Good luck, and feel free to ask more questions. Maybe one of us will have some ideas that'll help you.