A quick ?- "What is the formula-or how do I tell my actual speed?"
Any info would be appreciated!!
-Mel

If you throw some numbers at me ( before & after ), I'll check it out & give you some answers.

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2007 C 230 Sport.

Maybe I didn't ask the right question.
I just went from 14" to 15" wheels (I forget tire sizes for each wheel size)
I'm guessing that with the 15" wheels I'm gaining more ground per revolution and wondering how this affects (effects?) speedometer. For example reads 30 mph...actual is_________, reads 70 mph...actual is______ .
Is there some way to figure difference?
-M

I have to know the old & new tire sizes.
Then I can tell you the exact difference.
Going from 14 to 15" wheels does not necessarily mean anything, if you compensate with tire sizes, i.e. 195/65/14 could be the same as 205/55/15. See what I mean ?

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2007 C 230 Sport.

I'm up to speed now. Sorry-just thinking rim size!
I'll take a look at old ones this week and get back to you if you can suffer my stupidity once more.
Thanks for the assist!
-Mel

Try this

http://www.miata.net/garage/tirecalc.html

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R