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I put on shorter tires because they were available and free but that throws my speedo off. I got to looking for an answer and here is the formula I used tell me if it makes sense.
stock short
225/60/16 225/50/16
rev per mile 781.3 837.0
vss pulse 23430 25100
per mile
variable 29.98
does that make logical sense or not? it does to me, I just wanna get the speedo as close to right as possible
225 multiplied by 0.50 (aspect ratio = 50) times 2 (2 side walls) divided by 25.4 (to convert mm into inches) plus 16 (wheel diameter) equals 24.85826 inches.
I just went through the same thing. From RussK....
"The formula for a different dia tire is; new advertised tire dia/old advertised tire dia* the stock current tire dia value. Then enter this new value into the new tire size field. Also enter the stock gear ratio.
Russ Kemp"
If you want to be exact and take into account the tire squat and tread wear, you can take a bit of yellow (or something bright) and paint a dot on the edge of one of the tread blocks. Then roll the car forward or backwards until this paint hits the floor of the shop or road twice. Then measure the distance between the two little paint spots that are left on the floor. There's your new "accurate" rolling circumference of the tire.
circumference = pi* diameter
so... take that number divided by pi (3.141593) to get your diameter to enter into HPT.
Question....the factory tire size setting for a 99 Bonneville in HPT 25.95 inches.
(OEM was 225/60/16).
Using the formula above...
225 multiplied by 0.60 (aspect ratio = 60) times 2 (2 side walls) divided by 25.4 (to convert mm into inches) plus 16 (wheel diameter) equals 26.629921 inches.
the factory tire size setting for a 2003 Bonneville in HPT 26.33 inches.
(OEM was 235/55/17).
Using the formula above...
235 multiplied by 0.55 (aspect ratio = 55) times 2 (2 side walls) divided by 25.4 (to convert mm into inches) plus 17 (wheel diameter) equals 27.1777165 inches.
Obviously, someone is wrong. It's either the OEM or HPT. Who is it?:shrug: