Tire sizes are both standard and metric, however pretty easy to determine size.
...
A 295/40R20 would be 29.3”
A 295/45R20 would be 30.4”
Yes, a one inch difference, no need for a calculator.
The last example is also as simple, changing from a 265/45/20 to a 295/40/20 tire is not hard to do in your head(no calculator needed).
The
265mm to
295mm adds 1.5", and the
45 series to
40 series loses one inch. Together, that's a 1/2" change/gain in diameter, easy. It's close enough to accurate, no need for a calculator, or a link online to a tire calculator. The margin for error is within acceptable limits for a speedometer. If you match the sizes this way, the speedometer will be very very close.
Note OEM speedometers are not accurate when new, some are but most are not. Ford likes to error on the slow side, 1991-2001 Explorers read slow on all 15" tire models. 16" tires didn't become options until 1998, and those were rare to begin with. The speedometers didn't change when the tire size did, even though there is a 1" difference in diameters of 15 and 16" tires back then. The optimum size for the speedometer for all of those years was a 30" tire.
So if the speedometer accuracy concerns you, first check it yourself by a GPS device, or a roadside speed display etc. Then you can make a wise decision about the needed tire diameter, or what effect a tire size will have for your truck.