If the speedometer is reading faster then you are going, then I think you need to have a slightly larger gear (more teeth) on the driven side to slow it down. I think that would be the easy way to correct it. There should be a chart out on the net somewhere.
Here is a site that compares tire size: https://tiresize.com/chart/
I don't think they make a 225/75 -14
Maybe a 225/70 - 14
They are very similar in diameter.
The old tires were 80 series, that means that 8.00-14 had a diameter of 26.79 inches or 68.06 centimeters.
It's easy to calculate: 8 inches = 20.32 centimeters, 80 % of this means 16.25 centimeters (this is the height of the side of the tire)
2x 16.25 =32.5
32.5 + 35.56 (diameter 14 inch rim, 14 inches = 35.56 centimeters) = 68.06 centimeters or 26.79 inches
The closest thing to the diameter of an 8.00x14 would be a 245/60R15 which is 26.6
The speedometer on my Ranger is off and when I go past the electronic speed signs I see how it compares to my speed and I just compensate as I drive.