Hey Technobear,
Here's a comparison using a very close match to our stock (205/55/16) N/A PT tire size, that I got from my favorite tiresize calculator website:
http://www.dakota-truck.net/TIRECALC/tirecalc.html
Tire A: 205/55 R 16 (stock)
Tire B: 225/50 R 16 (wider)
Stock (205/55/16) dimensions:
Width 8.07" (205.00mm)
Sidewall height 4.44" (112.75mm)
Overall height 24.88" (631.90mm)
Circumference 78.16" (1985.17mm)
(225/50/16) Dimensions:
Width 8.86" (225.00mm)
Sidewall height 4.43" (112.50mm)
Overall height 24.86" (631.40mm)
Circumference 78.09" (1983.60mm)
Speedometer Error
Actual Speed Indicated Speed with Tire B Speedometer Error
5 mph (8.04 kph) 5.00 mph (8.05 kph) +0.00 mph (+0.01 kph)
10 mph (16.09 kph) 10.01 mph (16.10 kph) +0.01 mph (+0.01 kph)
15 mph (24.13 kph) 15.01 mph (24.15 kph) +0.01 mph (+0.02 kph)
20 mph (32.18 kph) 20.02 mph (32.21 kph) +0.02 mph (+0.03 kph)
25 mph (40.23 kph) 25.02 mph (40.26 kph) +0.02 mph (+0.03 kph)
30 mph (48.27 kph) 30.02 mph (48.31 kph) +0.02 mph (+0.04 kph)
35 mph (56.31 kph) 35.03 mph (56.36 kph) +0.03 mph (+0.04 kph)
40 mph (64.36 kph) 40.03 mph (64.41 kph) +0.03 mph (+0.05 kph)
45 mph (72.41 kph) 45.04 mph (72.46 kph) +0.04 mph (+0.06 kph)
50 mph (80.45 kph) 50.04 mph (80.51 kph) +0.04 mph (+0.06 kph)
55 mph (88.50 kph) 55.04 mph (88.57 kph) +0.04 mph (+0.07 kph)
60 mph (96.54 kph) 60.05 mph (96.62 kph) +0.05 mph (+0.08 kph)
65 mph (104.58 kph) 65.05 mph (104.67 kph) +0.05 mph (+0.08 kph)
70 mph (112.63 kph) 70.06 mph (112.72 kph) +0.06 mph (+0.09 kph)
75 mph (120.67 kph) 75.06 mph (120.77 kph) +0.06 mph (+0.10 kph)
80 mph (128.72 kph) 80.06 mph (128.82 kph) +0.06 mph (+0.10 kph)
85 mph (136.76 kph) 85.07 mph (136.87 kph) +0.07 mph (+0.11 kph)
90 mph (144.81 kph) 90.07 mph (144.92 kph) +0.07 mph (+0.11 kph)
95 mph (152.85 kph) 95.08 mph (152.98 kph) +0.08 mph (+0.12 kph)
100 mph (160.90 kph) 100.08 mph (161.03 kph) +0.08 mph (+0.13 kph)
You can see that they are VERY close in overall diameter, so would be the first size I would check out in terms of keeping the speedo error difference to a minimum. Plug in a few values of your own and have some fun!
Rock on,
Brian
(remember, there ARE limits to what will mount onto a 6.5" wide rim [^] )