Guys,
DOH, I forgot to give you a comparison of a 155/80-13 versus a 175/70, so,
here we go:-
155 x .80 = 124mm x 2 = 245mm / 25.4 = 9.76" + 13" = 22.76".
From below, you can see that a 175/70 has a lesser Overall Diameter at 22.65".
So moving from a 155/80 to a 175/70 Tyre would result in a reduction in
Overall Diameter of just about .05%;
i.e., what was 60 MPH on the Speedo, (and if THAT was 60 MPH on the road!)
would now be 59.7 MPH on the road. Pretty close, I should think.
Ron, in Ireland
'59 AN5 "Oggibib"
'64 Riley 1.5 NoNameYet
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Guys,
Here is how to calculate the Overall Diameter of Radial Tyres/Tires:-
Standard Sprite size:= 145SR-13. Where no "/xx" is given, assume "/80".
So,where the tyre is145/80-13, you multiply the Thread Width of 145mm by
.80 = 116mm,
which is the height of the tyre from the rim to the thread.
Multiply this by 2 to allow for a Top and a Bottom = 232mm.
Convert this to Inches; divide by 25.4. = 9.13"
Add this to the Rim Diameter of 13"
So, Stock Overall Diameter is (9.13" + 13") = 22.13"
Now, to compare with a 175/70XR-13 tyre (the "X" is in place of a Letter
which denotes the Speed Rating)
Multiply the Thread Width of 175mm by .70 = 122.5.
Multiply by 2 to allow for Top and Bottom of Tyre = 245mm
Convert to Inches; divide by 25.4. = 9.65
Add this to Rim Diameter of 13".
New Overall Diameter is now 22.65"
This is an increase in Overall Diameter of just over 2.3%. Say 60 MPH on an
(accurate !!) Speedo would translate to just about an actual 61.4MPH on the
road if nothing else is changed. Be warned!
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