 Speedometer with different size tires |
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| RonnieJ |
 Dec 26 2009, 02:58 PM
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#1
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RonnieJ  Group: Members Posts: 230 Joined: 16-June 03 From: Sterling Heights, Michigan Member No.: 829 Region Association: Upper MidWest  |
How far off is my speedometer going to be since I changed to 205-55-15 tires.
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Replies(1 - 5)
| GeorgeRud |
Dec 26 2009, 03:11 PM
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#2
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Advanced Member Group: Members Posts: 3,725 Joined: 27-July 05 From: Chicagoland Member No.: 4,482 Region Association: Upper MidWest |
Unfortunately, there's no perfect formula as there is quite a difference in rolling radius from tire manufacturer to manufacturer. If you know your old tire's circumference and the new tire's circumference, you could easily apply a correction factor, but the speedos were never calibrated that accurately in my opinion. They often tend to be optomistic!
You might want to find a buddy with the iPhone app that gives you a speed while you're driving (obviously using GPS), and compare that with your indicated speed. Otherwise, find an interstate and time yourself while driving 60mph for a measured mile (using their mile markers). Either way should give you a pretty good estimation. I believe that if you need to make corrections, North Hollywood Speedometer can probably do the calibration work. I've been happy with their services in the past. |
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| Kirmizi |
Dec 26 2009, 03:13 PM
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#3
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Senior Member Group: Members Posts: 836 Joined: 12-February 06 From: Wyoming, US Member No.: 5,568 Region Association: None |
Cllick on the "914 Info" tab at the top right of the page.
There's a tire size calculator that will give you a pretty close estimate. Mike |
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| TravisNeff |
Dec 26 2009, 04:10 PM
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#4
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914 Guru Group: Members Posts: 5,082 Joined: 20-March 03 From: Mesa, AZ Member No.: 447 Region Association: Southwest Region 
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I got a GPS for xmas and just went out for a spin a few minutes ago. With my 205x55x15 tires the speedometer reads about 4-5mph fast (meaning speedo says 40, gps says 35)
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| Jeffs9146 |
Dec 26 2009, 05:14 PM
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#5
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Ski Bum Group: Members Posts: 4,062 Joined: 10-January 03 From: Discovery Bay, Ca Member No.: 128 |
QUOTE North Hollywood Speedometer can probably do the calibration work I have used them to do two of my speedos over 25 years and they do a great job but not cheep either! |
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| RonnieJ |
Dec 26 2009, 05:14 PM
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#6
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RonnieJ Group: Members Posts: 230 Joined: 16-June 03 From: Sterling Heights, Michigan Member No.: 829 Region Association: Upper MidWest |
Thanks all for the responses.
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