Put skinny draglites on front, now ABS light comes on. Why?

I just put on 15x3.5's on the front. I had to use 1/2 inch spacers to clear the brakes too. But now after about 3 minutes of driving the ABS light comes on. Anybody know what might be causing this? Any help would be appreciated. Thanks.
 
It happened to my car too, its not the skinny's but the different tire sizes, i have a 28 inch radial on the back, and the skinny's on the front tire height is closer to 24.5 if i rememeber correctly, the computers is picking up the different wheel speeds between the front and back wheels, and thinks abs is causing it, nothing to be worried about.
 
That's probably it then because I got 28's on the back too. Thanks alot. That eases my mind. This dumba!@ at NTB while I was getting it aligned told me I need different calipers for some reason because that's what was causing it. Thanks again man!
 
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Yeah. I would have actually wanted the 4.5's because it puts the back tires perfectly flush and makes the wheel dished another inch which looks better IMO. The front tires are nexen 165/80/15 and the rear are MT ET streets 275/60/15. The 60's in back really fill out the wheel well nicely I think. But yeah go with the 4.5's. Thanks for the compliment!