Originally Posted by
HudsonHawk
205 is the tire width in milimeters (mm). 45 is the sidewall height as a PRECENTAGE of the tread width. So the sidewall height of that tire would be 92.25mm (9.225cm or 3.64in.). This give you an overall tire height of 24.28 inches.
The 40 would result in a smaller diameter tire that would slightly lower your gearing (higher numerically) and technically would improve acceleration. Your speedometer would also read fast compared to what it is now. To campare to the above the 40 would give you a sidewall height of 82mm (3.2in.). This will give you an overall tire height of 23.46 inches.
Tire one has a circumference of: 76.27 inches
Tire two has a circumference of: 73.74 inches
This is a difference of about 3.3%
This means your speedometer would be about 3% fast and your gearing would be about 3% lower then it is now. At 50MPH that would be 1.5MPH difference.