I know you can adjust tire by increasing the first number and decreasing the aspect ratio, and vice versa. I have a 1990 Oldsmobile Cutlass International with 215/60-16 tires. I‘ve heard that 205/65-16 works, is there a way of telling how it will work properly, so long as it fits? Also, I was told that the first number is how wide your tire is and the second is the percentage of width as height. Does that mean the width as how far the tire goes into your vehicle, or if you were to take a tape and go across from one tread across the wheel/rim to the other tread? Is height from the tread to where the tire touches the wheel/rim? I don‘t really understand how you can adust one number higher and lower the other to keep the same speedometer/odometer readings and stuff. How would a shorter tire register the same speed just because it is fatter? I apologize if I‘m speaking ignorantly, I just cannot understand how this works. :)
The 1st number is the tire tread width, not the width of the rim. The 2nd number is the side wall height as a %age of the tread width as measured from the ground to the bottom edge of the tire rim with the tire off the ground. Most speedometer errors are less than 3% error when changing tires sizes with the same rim diameter Going from 215/60-16 tires to a 205/65-16 will cause your speedometer to read 1.3% slower than your actual speed. If your speedometer says 60 mph, you are actually traveling at 60.8 mph. Speedometer accuracy is really tied to the circumference of the tire, not the fatness of tire. When you adjust tire sizes, the closer to the circumference you are to the stock tires from the car manufacturer, the accuracy of your speedometer will be maintained. Otherwise, you have to change the speedometer gear in the tailpiece of the transmission. The speedometer is calibrated to the circumference of the tire and thus the revolutions per mile. When your tire is is shorter (not as tall) than stock tires, the circumference is less so the tires have to make more revolutions per mile, thus the speedometer reads faster than your actual speed.
