Figuring speedometer change - Saab 9000 Bulletin Board - Saabnet.com
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Figuring speedometer change
Posted by Robert Davis (more from Robert Davis) on Wed, 21 Mar 2001 00:41:38
In Reply to: Tire sizes vs speedometer error etc...., saabhartha, Tue, 20 Mar 2001 15:46:54
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Whether you will have a speedometer error after changing tire sizes depends on how much error you had to start with. You can figure speedometer *change* from one size to another by comparing the rolling diameter of the tires. First, a few definitions:
The first number is the width of the tire (TW) in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4.
The second number is the aspect ratio (AR). This is a ratio of sidewall height to width.
The last number is the diameter of the wheel (WD) in inches.
Thus, the formula and your two examples:
TW
(---- x 2 x AR) + WD = Rolling diameter
25.4
((205mm/25.4) x 2 x .60) + 15 = 24.68"
((195mm/25.4) x 2 x .65) + 15 = 24.98"
24.98 / 24/68 = 1.01
This would mean your speedometer would read about 1% slower than before. That might increase or decrease your speedo error depending on where it was originally.
Posts in this Thread:
- Tire sizes vs speedometer error etc...., saabhartha, Tue, 20 Mar 2001 15:46:54
- All tires are not the same, Brian C, Wed, 21 Mar 2001 22:08:13
- 205/60/15 is 1.1820330969267128% faster than 195/65/15 n/m, jack b, Wed, 21 Mar 2001 09:08:31
- Figuring speedometer change, Robert Davis, Wed, 21 Mar 2001 00:41:38 <-- Viewing This Message
- Re: Tire sizes vs speedometer error etc...., jack b, Tue, 20 Mar 2001 16:26:46
- 0.2 mpg, jack b, Tue, 20 Mar 2001 16:19:55
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