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I have a 1969 Mark III and i would like to get larger rims & new tires for it. I would like to keep the diameter apx the same because i like the height of the car as it is right now but i want to lose the 15" rim and get an 18 or 20 depending on what the ride is like.

I have been googling today and found a "tire size calculator" that allows you to put in the size data and it calculates the diameter and circumference.

I have the original hubs & caps but the tires on them are so bad that i can't read a number on them. I like the size of the current tires so its not a big deal but if someone has the info for the original wheels please share.

The currently installed rims/tires are..

Existing Tire 235/70/R15

Diameter 27.95275590551181

Circumference 87.81617260034461


So after getting those numbers (and having no idea what i am doing) i assumed i would look for 18" & 20" tires that would have a similar diameter and circumference.

I found these sizes...


New Tire 235/50/R18

Diameter 27.25196850393701

Circumference 85.61458404782894

Or...

New Tire 235/35/R20

Diameter 26.476377952755907

Circumference 83.17799447004472

So... Am i on the right track? Or have i driven off a cliff? Can someone help me with the math?

Thanks.  

First, get rid of the extra digits after the decimal point.  The nearest 0.1" is close enough.  You don't need to worry about circumference as it is directly tied to diameter.  And you are looking for a diameter within 3% of the original.

Second, your problem is going to be the WIDTH - as that is the most likely dimension to interfere with body panels, frame, and suspension components - AND - the worst condition for this is when the steering wheel is fully turned and the spring is completely conpressed.  It is difficult to be sure you don't have a problem just by looking.  You need to have someone who has done these kinds of changes in the past and knows what to look out for.

My suggestion is to work with a retailer who does this all the time.  You have an unusual car and this is likely the only time they will work with such a car.  A little extra time and a little extra money at this stage will prevent HUGE problems later.