how many tooth gear for 3:55 gear speedo?

All the formulas I've found online say my Dart with 518OD, 3.91 gears and 26" tires need a 40-tooth gear.
Still, measured against GPS, the speedo is exactly 10% off, both in speed and distance. So I need to get me a 44-tooth gear-unit, which don't seem too commonly found.

Go with the GPS. That's good that the speedo and the odo are off the same amount as that tells you the speedo magnetic gismo which is triggered by the odo is spot on. As you know just need to get the correct speedo pinion gear in the trans. If that (44 tooth) gear ain't commonly available then might need to source a speedo gear adapter which has metal gears of a different ratio in a little box that goes inline in the speedo cable to alter the ratio so you're final ratio will come in correct

I recalculated the error-difference and I should've said the speedo is 13% off. (I was measuring the wrongway around)
When the speedo reads 60mph, I'm actually doing 54mph on the GPS.
At 70 speedo-miles, it shows 63 on the GPS.

I made an 11 mile drive yesterday (on the speedo), which was 9.7 miles according to the GPS triptracker-app in my phone.
And I just rechecked the same route with Google Maps and come out at 9.57 miles.

Rear tire diameter is a measured (horizontally) 25.8" (235/60/15" BF Goodrich).
(Maybe I should've measured the tire height vertically to take tire compression into account.)

Step 1 would be to get the ODO reading right with the correct pinion gear in the trans. I thought GPS's were dead nuts accurate but Kunkel knows more about this than I do. According to the GPS the speedo is reading 11% fast (70 divided by 63) so you would want a speedo gear with ~11% more teeth. I would double check by measuring rollout by putting a rear tire valve stem at 6 0'clock & making a mark on the pavement at that point then rolling the car forward 1 revolution till the stem is back at 6 0'clock & with your tape measure that distance (rollout) then divide Pi (3.1416) into rollout to get actual diameter then going to the MP chart in the archives along with the rear gears/dia so see if their recommended pinion tooth # matches. When you get the ODO confirmed accurate then see if the speedo is accurate either by GPS or a roadside digital MPH sign. If the odo is accurate but the speedo ain't then the speedo head needs to come out & go to a speedo specialty shop to be recalibrated but one thing at a time.