[Fot] [Mgs] Tires

[Jack Wheeler]

 Thanks Rick.  Your equations are definitely going into my "Technical Information" file.
Jack

    On Thursday, January 30, 2020, 2:58:44 AM EST, Richard Lindsay via Mgs <mgs at autox.team.net> wrote:  
 
 Mike, I recently did the same thing for my Land Rover. We like to think we're smart but we still make mistakes. And speaking of mistakes, here's what I would do...
Look up the original tire size for your car (or see if TireRack has that info). Then calculate the circumference of that tire. For example if the OEM tire is a 165r14 then the diameter is:
D = (165×.8×2)/25.4 + 14
The radius is:
R = D/2 
The circumference is:
C = 2xRx3.14159
   This is the tire circumfrence, in inches, for which your speedometer is calibrated. If you want to keep that calibration, just reverse the equations above and start poking in numbers from available sizes. I do this in an Excel spreadsheet. WAY easier.   Just remember to include the new tire's aspect ratio. Old, unlabeled radials have an aspect ratio of about 0.8, as used above.
Rick
On Thu, Jan 30, 2020, 12:11 AM Michael MacLean via Mgs <mgs at autox.team.net> wrote:

Well, I screwed up.  Reading the MG Experience forums I bought Douglas tires at a great price.  Taking off the old tires tonight I found they were Dunlop SP-20s made in 1999.  Wow!  The size on the tire was 165R15.  Without looking and going by the recommendation of the MG Experience web site I bought 14 inch tires.  Crap.  Now I have to try to take them back.  They have not been mounted so they are still just like I got them.  My question is, what the heck size tire should I get for a 15 inch wire wheel rim?  The MG Experience forum is saying 185/70 R15 is closest, (1/4 inch smaller) but not readily available for a decent price.  A 185/65 R15 is the next most popular tire, but smaller in diameter by an inch than the Dunlops I am taking off.  165R15 tires are 25.65 inches in diameter.  My head is about explode.
Mike MacLean
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