Use this to figure that out:
http://www.miata.net/garage/tirecalc.html
Kevin
George Graves wrote:
Can't speak to your situation, Will, but in my case I think that the
lower profile than stock tire (for any given wheel size, the lower the
profile, the smaller the diameter of the tire) is making my car go
slower per wheel revolution than would a larger diameter tire.
Therefore when the high-gear revs of the rear tires are of some
certain number, the speedometer "thinks" that the car is traveling a
certain speed. Each revolution equals a certain number of linear feet
traveled which equals the tire's circumference. A smaller tire has a
smaller circumference, therefore fewer feet are traveled with each
revolution. This will mean that it will take more revs per mile to
cover the same distance as a larger profile tire, this equates to a
lower speed than that indicated. Increasing the tire profile from a 50
to a 55 or a 60 ( Pirelli P6 15 X 60s are apparently what the car came
with) should raise the actual speed so with the proper (that is to say
stock) diameter tire, an indicated 80 mph should equal 80 mph (or at
least be closer to 80). Of course, the car will suffer a bit on
acceleration because the ratio between tire and road will be greater.
George Graves
'86 GTV-6 3.0 'S'
On Nov 10, 2009, at 9:44 PM, Will Owen wrote:
George Graves wrote:
The GPS SHOULD be much more accurate than any other kind of
speedometer. After all, if it can pinpoint where you are within a
few yards, it should be able to figure out how fast you're going
within a very small margin of error.
George Graves
'86 GTV-6 3.0 'S'
You'd think. My point was that my GPS is telling me that my
MPH/thousand is basically what your factory figures were telling you,
as opposed to what your GPS is telling you. So: is you GPS out of
whack? Or is *MY* GPS out of whack? Or has your car somehow been
endowed with some kind of mud-wrestling final drive?
FWIW, I've been driving the 164S over the last few days and its
gearing seems to be about the same. Have not hooked the GPS into it
because its cig lighter socket spits the GPS plug out every few
miles, which is a major recurring pain in the ass.
Will
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