Tyre size/ speedo deviation calculator...
Submitted: Saturday, Apr 22, 2006 at 23:58
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Member - Blue (VIC)
Some time ago one of you lot posted a calculator which advised how far your actual speed would deviate from that displayed on your speedo as you increased/dereased tyre size from stock... I always sucked at math so I'm not even going to bother trying to figure it all out myself. Don't suppose a link is available...???
Reply By: Member - Norm C (QLD) - Sunday, Apr 23, 2006 at 15:32
Sunday, Apr 23, 2006 at 15:32
These web sites are great for calculating the speedo change in theory. I recently put on bigger tyres and the miata (I think that's the one I used), gave me a speedo variation of 7%. I made myself up a spreadsheet to calculate the difference across the full range, printed and laminated it for the 4B. By the calculation, when the speedo showed 100, I'd be doing 107; a tad fast.
Then after fitting the tyres we went for a drive. I used the GPS to
check speed. What had actually happened was that the bigger tyres corrected the speedo error that was built in to the vehicle. Our speedo is now just about spot on and only get 1 or 2 KPH out when we get over 100KPH. We have checked this on three separate trips, and I reckon we have it right (GPS error notwithstanding).
So, I'd suggest you do a practical
test as
well to
check how it compares with the theory.
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Follow Up By: Member - Pezza (QLD) - Sunday, Apr 23, 2006 at 23:21
Sunday, Apr 23, 2006 at 23:21
G'day Norm,
I thought that only happened on the patrol, a few of us have found the same thing, when I went to the larger 265/75 from the standard 265/70, a 4% increase, the speedo now reads 100% accurate, on a recent trip the GPS and the trip metre were an exact match to the metre, and the speeds shown are the same aswell, yet with the standard tyres on the speedo would read 100kph and the GPS would read 96kph.
I wonder if the manufacturers are doing this on purpose.
Avagoodn
Perry
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