Rear end gearing calculations

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I bought my 91 ZR-1 #210 from a Dodge Dealer about 1 1/2 years ago and joined the forum a couple weeks ago. I have not been able to contact the previous owner for any details. I believe it is a stock motor. The first thing I wanted to do after all the normal maintenance was put 4:10 or 4:30 gears in it for that nice seat of the pants feel. So before I ran out to buy new gears, today my son and I drove it and took some rough readings. Our calcs based on the following shows I have 4:30's already installed. What might I be missing based on the following:

Tire (315x35x17) height 25.25" measured with straight edge from top of tire to garage floor using a tape measure
6th gear, 2100 rpm, 70 mph (car gauges only)
4th gear, 2300 rpm, 40 mph (GPS app and car gauges)
Per online gear calculator that figures out at about 4:30's

Haibeck gear chart for 4:10's
4th gear, 2000 rpm,37.35 mph
6th gear, 2000 rpm, 76.22 mph