Converting tire sizes

[auburnseeker]

Does anyone know of a website that will convert the old tire size like 6.50x16 to a modern tire size?

I'm not looking to change to a modern tire but I found a great site for computing tire changes. The only problem is it's only designed for new tire sizes.

[Guest bkazmer]

I don't think aspect ratio is in the old sizes. 6.50 x 16 is I think just 6.5" wide tread (nominal) and 16" rim. New sizes have 3 bits of info. 225-60 x 16 means 225 mm tread, side wall height is 60% of tread width, 16" rim. So you can convert assuming a high aspect ratio as was typical back then (like 75)

[nzcarnerd]

I think you will find that the aspect ratio of most old tyres was 100%. A Ford T tyre size is 30x3 which is a 3" wide tyre on a 24" rim. In 1929 the smaller Buicks used 30x5.50 tyres and the big ones used 32x6.50 tyres - they are actually both 19" rims. Somewhere around 1930 they went to using the rim size - usually expressed with the rim size first - 6.50x16 - but there was a time when both systems were in use which has caused confusion ever since. Oddly enough I see that modern bicycle tyres use that old system - at least for the kids sizes. A 24x2 tyre is for a 20" rim.

[ply33]

I think you will find that the aspect ratio of most old tyres was 100%. A Ford T tyre size is 30x3 which is a 3" wide tyre on a 24" rim. In 1929 the smaller Buicks used 30x5.50 tyres and the big ones used 32x6.50 tyres - they are actually both 19" rims. Somewhere around 1930 they went to using the rim size - usually expressed with the rim size first - 6.50x16 - but there was a time when both systems were in use which has caused confusion ever since. Oddly enough I see that modern bicycle tyres use that old system - at least for the kids sizes. A 24x2 tyre is for a 20" rim.

It does not surprise me that early cars used the same measuring scheme as bicycles: A fairly large number of early cars used bicycle derived parts and/or were designed and built by people who got started by making bicycles.

For what it is worth, it seems that the 5.25x17 tires available for my car have an aspect ratio of 1.01:1. Very close to your 100%. This is based on looking at the stated size and the "mounted tire diameter" for those brands and models of tires that publish the information.

[Guest palosfv3]

Coker Tire has a chart on their site. Wheel and tire diameter can be used to compare aspect ratios to new style tires.

[Jim Rohn]

here is a good one I found:

Tire Tech - Tire Size Conversion Chart

[Guest bkazmer]

what does wheel diameter have to do with aspect ratio? Aspect ratio is the tire sidewall/tread width.

[nzcarnerd]

Aspect ratio has only had a bearing since WW2 when those ratios began to change. Pre-WW2 - as I said in my earlier post - the sizes were expressed differently and it mattered that the ratio was around 100%. Even with modern tyres different makers' tyres vary in dimension for a quoted size. To have all tyres on a vehicle the same they need to be from the same maker.