Gear ratio question

I took my car too the track and just couldn't believe the RPM's I was spinning by the traps, seems way crazy for what was supposed to be a 3.91 gear set in my Duster. 340 V8, built up mediumish, 27560R15 tires(28"), A-833 4 speed which is obviously 1-1 final drive. So at 3500 rpm I'm doing a true 64mph,( I know my speedo isn't right but I've watched enough police radar signs to know it's off 4mph.) Plugging this info into a formula I come up with it being a 4.56 rear gearset, just looking for someone else to confirm this for me. Thanks.