Mus, check out this link. It has some handy calculators:
http://www.corral.net/tech
the 3650 tranny has the same ratios as the T45. If you input 5800rpms, 17" 245/45 tires, 3rd gear of a T45, you get 101.89XXX.
And besides, I really don't believe the rev limiter is at 5800. I'm not sure what the exact rpm limiter is, but it appears to be past 5800, maybe 6000. Also, factory tachs can be off substantially. Some 200 or more as a precautionary measure by the manufacturer. You could possibly be shifting too soon on your runs? Try running every gear out to redline and crossing the traps in 3rd.
Regarding the fuel: this car has run excellent on exxon 87. I have noticed no pinging, even when the car was on the dyno with the hood open. It's kinda weird, but the 01GT we traded in for the bullitt pinged quite frequently on the same gas from the same exxon I always use. I have no explanation why since the car was bone-stock. Maybe a bad MAF calibration from the factory. Who knows?