MD80 CREW DID NOT COMPLY WITH ATC ISSUED XING RESTR IN ZAU CLASS A AIRSPACE.

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MISSED XING RESTR. WE RECEIVED AND ACKNOWLEDGED A RESTR TO CROSS 90 MI S OF CHICAGO AT FL240. FL240 WAS ARMED AND AS THE PF I DECIDED TO STAY AT ALT AS LONG AS POSSIBLE IN THE INTEREST OF FUEL CONSERVATION. HOWEVER; I MADE THE MISTAKE OF USING THE DISTANCE TO GO TO DEST IN COMPUTING THE TOP OF DSCNT. THIS DISTANCE INCLUDED THE ILS RWY 27L APCH AND WAS CONSIDERABLY LONGER THAN THE STRAIGHT LINE DISTANCE TO ORD. NORMALLY I WOULD NEVER USE THE GPS DISTANCE FOR ANY XING CALCULATION AS IT IS NOT WHAT THE XING IS BASED ON. FOR SOME REASON; I FOCUSED ON IT AND DID NOT REVISIT THE TOP OF DSCNT CALCULATION UNTIL IT WAS TOO LATE TO MAKE THE XING. WE WERE APPROX FL265 AT 90 MI S OF ORD. SUPPLEMENTAL INFO FROM ACN 488051: ZAU GAVE US INSTRUCTIONS TO CROSS 90 NM S OF ORD AT FL240. WE WERE AT FL330 AND APPROX 130 NM OUT WHEN WE RECEIVED THE RESTR. WE CROSSED 90 MI S OF ORD AT APPROX FL290. APPARENTLY THE CAPT WAS USING THE MILEAGE OFF THE GPS. HE WAS IN THE MAP MODE AND I WAS RAW DATA. I DID NOT NOTICE THE PROB UNTIL ABOUT 97 NM OUT. ALTHOUGH I WAS PREPARING THE COCKPIT FOR DSCNT AND LOOKING OVER THE APCH; I REALIZE NOW THAT I WAS COUNTING ON THE CAPT TO MAKE THE ALT XING AND DID NOT SUPPLY THE NECESSARY BACKUP FOR HIM. NOT REAL SWIFT FOR SOMEONE WITH MY TIME IN THE ACFT. I WILL TAKE THIS AS A SERIOUS WAKE-UP CALL.