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My current tire size is 195/50-15. I plan to get some tires in 205/50-15. I checked the miata.net tire size calculator and here is the deal:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/50-15 3.8in 11.3in 22.7in 71.2in 889 0.0%
205/50-15 4.0in 11.5in 23.1in 72.5in 874 1.7%

So my revs per mile will go from 889 to 874 (difference of 1.7%), how much will my speedo be off and will the car rack up miles slower? I wanna know if this is no big deal or maybe I should get the speedo recalibrated for the new tire size? And if so how is that done? Also is there any performance benefit for going to a 205 from 195 or is it miniscule? I am running a set of Kosei K1 wheels (15x7,+38 offset) Thanks for the help.
 

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I'm trying to remember since its been so long since I've used 195's but I think it's very minimal. Maybe 2mph. It doesn't make enough of a difference to think about it.. UNLESS you're a mph freak

But I would think you would get better mileage because a wider tire obviously has more grip. Maybe the calculator is compensating for a heavier weight tire which would have an effect.
 

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No I'm thinking more grip it would help you move forward more efficiently but I can see that side of it too.
 

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You'll get worse gas mileage with increased grip... unless all you did was burnout before because you had no grip

The performance difference will be minimal, especially if you keep the same wheels.

The speedometer is no big deal unless you always drive exactly the speed limit.
 

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My understanding is that your speedo will be off by 1.7% i.e. the mph difference will be more pronounced at faster speeds
 

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I've basically done exactly what you're asking.

Went from 195/50/15 to 205/50/15 (even on same width and offset wheel)

You've worked out the difference, being 1.7%

The speedo difference will be the same.

ie: mine at an indicated 110kph used to sit at 107
107 * 1.017 = 108.82

For me it corrected the existing speedo error (car was going slower than the speedo said)

No idea how the US speedo is calibrated.
 

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^ A good point. I've found that speedos are usually calibrated with a small degree of error; that is, the manufacturer wants you to go slightly slower than how fast you think you're going.
 

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A 195/50 tire has a 97.5mm sidewall, a 205/50 tire has a 102.5mm sidewall. Your overall rolling diameter is going to change by 10mm. If you're worried about how a 10mm difference is going to affect your speedo, you should probably save the money you would have used buying the tires and get some OCD meds instead.
 

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Depends on how much the police and speed cameras allow in the area.

Where I am we are given 2kph tolerance (1.25mph)

Do 3kph over, demerit points and about $150 fine.


For that reason, I want to know EXACTLY how fast I'm going.
 
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