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Detroit,where my home was
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On my "A" Ascona [1900] with a stock 13" rim would be 165/80 x 13 the next size to have almost the same speed reading is 185/70 x 13, om my brothers "C" Record stock was 175/80 x 13 and the next size to have almost the same speed reading is 195/70 x 13
A simple way to remember add 20 mm tire width and reduce tire height with 10%.
 

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Opeler
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So, I was running some gear calculations and needed to look up the standard tire size again. I found a website that has the tire size of 165 HR 13 converted into the modern tire size of 165 / 82 R13

Here are a few pictures of the original equipment tire on my '73 GT, a Dunlop165R-13. It was a radial tire although that "R" designation isn't shown on the tire. The aspect ratio isn't shown on the tire either, but most of the skinny tires of that time were about an 80 or .8 ratio. So I think that your guess is certainly close. I always use a .8 ratio on my speed calculations.

edit: After reflection, I'm not entirely sure that the Dunlop was a radial. I didn't like the the Dunlops and replaced them with a set of Michelin XAS Radials within a short time.

7C08B0CC-661C-416E-80EB-C150CA20A08E.jpeg 21195AE0-2C39-43E3-A313-00FF7A2E8C32.jpeg 55C6EDF3-545E-45F7-8B88-D8B0DB73CA5D.jpeg B22531BA-BBD8-45DD-B6C6-287C27DB886C.jpeg 5BCBD807-568F-4028-B650-19CBABE9D23F.jpeg
 

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Here are a few pictures of the original equipment tire on my '73 GT, a Dunlop165R-13. It was a radial tire although that "R" designation isn't shown on the tire. The aspect ratio isn't shown on the tire either, but most of the skinny tires of that time were about an 80 or .8 ratio. So I think that your guess is certainly close. I always use a .8 ratio on my speed calculations.

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Interesting!
 

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Interesting!
When I purchased my new wheels back in 1980/1981 or there abouts I, without thinking, just went with the standard wheel size, what ever that is. Then somewhere around 1999-2000 when I was getting the GT back on the road, thinking I was nearing the end of my restoration, and needed new tires, and by the way, I am still running those tires at 3000 miles accumulated in the last 20 years, no dry rot - and run great, Good Year Tires. Anyway I asked the dealer for the widest tire he could put on the wheel which turned out to be a 185/70 13 or so he said, but that is what I got. As I recall the original tires that came on the car were not very wide, skinny actually and these, tires shown below, have a good look regarding the width and general overall appearance. Probabely not available anymore but I have looked around and there does seem to be a pretty good selection out there of 13 inch tires. I still like the conventional tire as compared to the really large wheel small tire combination that just makes me think if you ever hit a big rock or curb you are so screwed - wheel damaged beyond repair and it only gets worse from there. Just my humble opinion - to each his own.
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One of the interesting things about mag wheels, or custom wheels as compared to hub caps, have you noticed that even grandmother's Mini Van has special wheels/mag wheels - hub caps seem to have gone the way of the dinosaur. So now they are not as special or eye catching as they use to be, everybody has them. In 1970 I had a silver mustang, a 67 model, and I added wide tires with white letters, and keystone mags that were a dark grey with a silver rim, what a match, I mean what a combination, looked great - never saw another silver mustang with that combination and again there just weren't that many cars on the road with custom wheels at that time. I personally think I got lucky in the early 80's when I purchased the current wheels on my GT. I had seen them before and knew that is what I wanted on my GT. Of course wheels then for the cost for 4 = the cost of 1 wheel was about the same or in some cases 8 to 1 would be a better comparison and at that price I like what I have. But you never know...................
 
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