Speedometer accuracy... can it really be this bad?

I got this 1972 D200 with a 360 and 727 last November as a non-running project which I've gotten back on the road in the last few weeks. I knew when I put 33" tires on it whatever speedo gear was already in it would be incorrect and I was right; speedo read about 50 when I would be going a tad over 60 (checked with GPS phone app). So going off the chart here Mopar Speedometer Pinion Numbers I got a 28-tooth gear since the truck has a 3.55 rear axle ratio, but now the speedo reads quite a bit over, like 60 when I'm actually going 50. Just to make sure I once again crawled under the truck and counted the turns of the driveshaft vs. tire and it is most definitely 3.5x:1. Tire diameter is 32.7" from the manufacturer they are 285/75-17 size.

What gives? Only thing I can think of is the speedometer itself is just way off, is that possible? This is a pain, speedo drive gears aren't exactly cheap and easy to come by to just buy 15 and try them all out until I find the right one.

EDIT: As a comparison my Duster has an AutoMeter 160-mph speedometer and with 2.94 gears and 28" tires the speedo reading was pretty close to actual speed with a 28-tooth drive gear as shown in the chart (29-tooth actually turned out to be perfect)