Doug and All,

 I believe, correct me if I'm wrong, that the speedo
is indeed affected by "bigger shoes." If the speedo
measures the revolution of the front wheel, not the
actual speed of travel, then it would display a SLOWER
SPEED on the needle with bigger tires.

Lets see if this is right; Normal tires would travel,
lets say, 10 feet in one revolution. The vehicle would
have traveled ten feet in, lets say, 60 seconds. (I
know that would be very slow but this it's
hypothetical).

The speedo is made to show revolutions per minute of
the wheel, I think. And it is calibrated for a certain
sized tire. So if the wheel travels 10 feet in one
minute the speedo would show a speed 10ft/minute with
normal, stock tires. Larger tires would travel, lets
say, 12 feet in one revolution. So a larger tire van
would travel FARTHER in one minute and the speedo
would display 12ft/minute.

If your actual speed was the same in both cases, The
speedo would show 10ft/minute with tires that are ten
feet around, and would show aprox. 8ft/minute.

They way I see it in my head makes sense, though I'm
sure you math people could describe it more succinctly
with an equation or two.

What do you all think?

Doran, '90 Weekender Carat

--- "Fitz-Randolph, Douglas" <dfrandolph@TALKAM.NET>
wrote:
> John Wrote: <<I've always kept it at 65-70, but
> would like to
> cruise around 75 MPH. >>
>
> My philosophy is that lower is better, but as long
> as I am in
> the "solid green" zone on the tach things'll be OK.
> I shared
> John's feeling, though - that I'd like to be able to
> cruise @
> 75 AND be in the "safe zone", which was my primary
> motivation
> for going to 15" wheels with larger diameter tires.
> Now the
> same engine RPM that produced 70 mph with my
> 185R14's produces
> 77.7 mph with my 215/75R15's - this is calculated,
> not
> measured. And of course, the speedo still reads 70
> mph because
> it has no idea it's wearing bigger shoes.
>
> Doug Fitz-Randolph
> Yarmouth, ME
> '90 Syncro
>

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