Front wheels are the call. From USAC:
http://www.usacracing.com/news/item/...y-photo-finish
"A glance at the timing showed Leary on top of the leaderboard as Cottle celebrated his triumph in victory lane. However, the transponder on the rear bumper of Leary’s car crossed the timing loop prior to the transponder on the tail of Cottle’s car, positioning Leary in front of Cottle on timing.
However, per USAC rule Part VI 9.4.C, a car will be officially credited with a lap only when its front wheels cross the start/finish line after completing one entire circuit of the course. The USAC chief scorer and race director unanimously agreed that Cottle’s front wheels crossed the start/finish line before Leary’s."
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I think this is what WinglessLovers was referring to earlier in the thread.