Re: Help! Can the speedometer be adjusted for different... - Saab 9-5 Bulletin Board - Saabnet.com
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Re: Help! Can the speedometer be adjusted for different...
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Posted by John Davies (more from John Davies) on Sat, 2 Feb 2002 11:53:44 Share Post by Email
In Reply to: Help! Can the speedometer be adjusted for different..., davec, Fri, 1 Feb 2002 19:52:28
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Keep in mind that most tires vary in actual measured diameter, even though they may have the same size markings on the sidewall. You were correct to measure them - that is the only way to verify this. You can compute the percentage reduction in "rolling diameter" - the answer will be the percentage increase in indicated speed the smaller tires will give you. Rolling diameter is not the actual diameter from tread to tread, but twice the distance from the wheel centerline to the ground. This takes into account the squish of the sidewall, which effectively gives you a shorter tire. Be sure the tires are inflated correctly.

However, you will probably see negligble error in your speedo, _if_ the dealer installed the correct 235/45x17 tires. I suggest you take the car out on the freeway to a "speedometer test section" and check your indicated speed. Or borrow a friend's handheld GPS and check it anywhere.

My 2001 Aero Wagon speedo indicates 2 mph slow at 70 mph, as verified by my Magellan GPS. Smaller diameter tires would "speed up" this reading and actually make it more accurate.

Whatever you do, don't have the computer altered - that is fixing the symptom, not the problem. And you would have to have it done twice a year, which would be a royal pain in the @ss.

posted by 64.38....

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