Ack... I didn't realize that I was using a number from another tire
size. This 5.60 wide tire may not have the same overall height as the
6.40 tire... so my measurement may be wrong.
Basically you will be cool if you keep your step-ups uniform. For
example
145/90/14 (O.D. = 24.27")
155/85/14 (O.D. = 24.37")
.
.
.
195/65/14 (O.D. = 23.98")
205/60/14 (O.D. = 23.69")
.... ect
This is why going up a size in wheel diameter is so nice... More choices
in tires are available. It really depends on what the roadster can fit
inside the wheel wells (I don't know... I don't own one, yet). Racers
don't really care. They are mainly concerned with getting nice and wide
sticky rubber on the car to keep them planted on the track. But since
you are trying to keep the O.D. of your new tires exact to the
original... you may have a hard time trying to find something like that.
>From experience in selling these things, if you were keep the O.D. to
within .25"-.5" of the original... you should be just fine with your
speedo gearing. Adding more height, will make your speedo read slower
than you are actually going going. Don't be afraid to even go so far as
to add an inch to the O.D. It won't take you that far off with your
reading. But then again... 80% of cars on the road are accurate to only
65MPH anyways. Since that is the top speed limit for most states, there
is not need for mfgs. to correctly calibrate speedos for anything more.
European cars on the other hand.... are another story.
Ken
San Ramon, CA
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