275/60/15 rear tires that original poster has are pretty large diameter that cuts the effective gear ratio down.
Your overall ratio is about 50 rpm more than a bone stock 71 340 Duster with 3.23 gears and F-70-14 tires. They sold 1,000 of Dusters and Demons like that.
These are not new cars. They drove originally down the road at 3100 at 70 mph.
A F70/14 that originally came with 340 dusters is 26" diameter.
275/60/15 are 28" diameter.
This is a very handy quick visual tool to figure out rear end ratio changes:
http://vexer.com/automotive-tools/speed-rpm-calculatorWhat is the exhaust system on the car? Full to the back, turn downs, Flowmasters even?
