XPost: alt.home.repair   
   From: none@none.net   
      
   On 4/4/2020 6:14 AM, micky wrote:   
   ...   
      
   ...   
      
   > It's 215/50R-17 instead of 215/55R-17, so iiuc it's 50/55ths of the   
   > diameter, 10/11ths, or 1/11th smaller, so it will have to rotate   
   > 11/10ths more times. So 25,000 miles will seem like 27,500 miles to the   
   > bearings and the tire tread.   But these are the rear wheels in a FWD   
   > car, and I don't even drive much, so does it really matter?   And it's   
   > half the price.   
      
   Aspect ratio gives profile and is ratio of sidewall section height to   
   section width...but the overall diameter is   
      
   Doverall = Drim + 2*Hsection   
      
   So the ratio of the two tires is   
      
   (Drim + 2*Hsection)/(Drim + 2*Hsection*55/50)   
      
   Well, this is simple w/ MATLAB which is open in other window...   
      
    >> fnHSect=@(SW,AR) SW*AR/25.4/100;     % Section Ht inches   
    >> fnHSect(225,55)   
   ans =   
              4.87   
    >> fnHSect(225,50)   
   ans =   
              4.43   
    >> fnD=@(Drim,SW,AR) Drim+2*fnHSect(SW,AR)  % Overall diameter (in)   
    >> fnD(17,225,55)   
   ans =   
             26.74   
    >> fnD(17,225,50)   
   ans =   
             25.86   
    >> fnDrim(17,225,55)/fnDrim(17,225,50)   
   ans =   
        1.0343   
    >>   
      
   So there's only roughly a 3% difference in overall diameter instead of   
   10% because the rim diameter is the bulk of the overall dimension, not   
   just the section height.   
      
   All in all, it doesn't matter enough that you'll notice it other than,   
   perhaps, the lower profile will drive and ride a little more stiffly.   
   Whether you'll actually notice unless you drive the vehicle very hard is   
   doubtful.   
      
   Why there's so much difference in price I've not looked at at all...more   
   marketing than anything else one would suspect.   
      
   Nice drawing/explanation at   
      
      
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