Tire size vs. speedo/odmeter error
06-15-2002, 08:11 PM
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Tire size vs. speedo/odmeter error
ok, i want to clear this all up...i already know that the STOCK tire/wheel combo has a pretty high error rate. with stock tires (215-50-17) the speedo and odometer read about 4-5% fast....so now that im starting to research for tires i know that i want 235 width (max for stock rim) and i want a lower profile....45 or 40...my question is with a 40 or 45 profile am i looking at a WORSE margin of error or does it get closer to being correct??
06-15-2002, 08:27 PM
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On my CL-S, the speedo is a lot closer than 5%, it's off by 1 MPH at highway speeds. (reads 80 at a true 79, as measured by GPS). Our TL reads high (reads 80 at a true 75). Here's a tire size calculator that can give you percentage change for whatever size you like:
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html
06-15-2002, 08:31 PM
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You have to figure out the overall diamter of the tire/rim combo that you want....
Stock tires are 25.5" diameter
A 235/40/17 tire is 24.4" diameter
The new tires size will make your speedometer 4.2% too fast.
If you want to make your speedometer read slower, you must get a tire with a larger diameter than 25.5"
Stock tires are 25.5" diameter
A 235/40/17 tire is 24.4" diameter
The new tires size will make your speedometer 4.2% too fast.
If you want to make your speedometer read slower, you must get a tire with a larger diameter than 25.5"
06-15-2002, 08:42 PM
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Originally posted by aciurczak
On my CL-S, the speedo is a lot closer than 5%, it's off by 1 MPH at highway speeds. (reads 80 at a true 79, as measured by GPS). Our TL reads high (reads 80 at a true 75). Here's a tire size calculator that can give you percentage change for whatever size you like:
http://www.miata.net/garage/tirecalc.html
On my CL-S, the speedo is a lot closer than 5%, it's off by 1 MPH at highway speeds. (reads 80 at a true 79, as measured by GPS). Our TL reads high (reads 80 at a true 75). Here's a tire size calculator that can give you percentage change for whatever size you like:
http://www.miata.net/garage/tirecalc.html
06-16-2002, 07:24 AM
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Checked my CLS speedometer reading against a police speed display at 30, 40 and 50 mph with original tires and the speedometer readings were the same as the police speed display.
06-16-2002, 10:10 AM
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The 225/50s I have are now dead-balls on. The 'watch your speed' sign entering into O'Hare would show 40mph and I had the cruise set at 40 for testing purposes...
With that, I'd have to say my speedo. was originally reading about 2% too fast based on comparing the overall diameter of 215/50s and 225/50s.
With that, I'd have to say my speedo. was originally reading about 2% too fast based on comparing the overall diameter of 215/50s and 225/50s.
06-16-2002, 01:45 PM
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ok, i want to clear this all up...i already know that the STOCK tire/wheel combo has a pretty high error rate. with stock tires (215-50-17) the speedo and odometer read about 4-5% fast
My Toyos I nave now are also dead on untill high speed. I'm about 1 mile off at 80mph. Below that there's really no difference. I haven't tested above 80 though, so I would guess the error will get worse past 80mph. However it will still be within 2mph.
235-45-17
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