 Newbie Speedo Question, Tires make THAT much difference? |
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| Grelber |
 Jul 17 2006, 08:50 AM
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#1
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Insert favorite Don Rickles joke here.  Group: Members Posts: 690 Joined: 30-May 06 From: McKinney, Texas Member No.: 6,107 Region Association: Southwest Region  |
My new 914 (yeeehaaaa) has a speedo problem, in that when I'm doing around 70mph, it's reading almost 90. The car has 195/50/15 tires. Will the smaller diameter tires make that much difference (25%)???
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| vertigo |
Jul 17 2006, 08:58 AM
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#2
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Member Group: Members Posts: 72 Joined: 12-June 05 From: Porterville, Ca Member No.: 4,262 |
yes tire size does make a difference. Tire rack has a calculator that will tell % of difference.
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| lapuwali |
Jul 17 2006, 09:17 AM
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#3
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Not another one! Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743  |
Here's an even better tire calculator: Ejelta
It's set up now to calculate sizes compared to a 165/80-15, which is about what a 165SR-15 (the stock tire) will come to. A 195/50 is nearly 11% off. OEM speedos are frequently off by 10% or more, even with the correct tire size. They're also often not off by the same amount throughout their range. For example, the speedo in my Type 3 is exactly correct at 70mph with its current tires (according to a handheld GPS), but it's way off at 30mph (30mph indicated is 25mph actual). My 914 speedo is about 5% off at high speeds, and 10% off at low speeds. I use 195/65 tires, which are within 2% of correct. Note also that even the calculated numbers are going to be approximate, as actual tire sizes are rarely exactly what the sidewall says they should be. |
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| Grelber |
Jul 17 2006, 09:57 AM
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#4
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Insert favorite Don Rickles joke here. Group: Members Posts: 690 Joined: 30-May 06 From: McKinney, Texas Member No.: 6,107 Region Association: Southwest Region |
Many thanks. I fugured it was a combination of speedo error & tire size, but a 25% error just seemed like a whole lot for just these two factors.
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| Joe Ricard |
Jul 17 2006, 10:01 AM
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#5
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CUMONIWANNARACEU Group: Members Posts: 6,811 Joined: 5-January 03 From: Gautier, MS Member No.: 92 |
195/50-15 tires are WAY short But also WAY funner. watch the speed bumps.
and you can probably pull 5600 RPM in 5th. |
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