Upgrading Sizes

13" tires are difficult to find nowadays and, even if they were available, performance tires wider than 185 mm on OEM 13" rims would not be ideal.

A simple solution is to replace the 13" wheel with a 14" or 15" wheel as these are readily available in OEM sizes with the correct bolt pattern and wheel back-spacing. Since the trend for tire sizes is generally going towards larger diameter wheels, smaller replacement tires (like 13") are increasing becoming very hard to find and it looks like 14" sizes are going this way too. The exception, for the time-being, seems to be specialty tires like the BF Goodrich Radial T/A, which is marketed towards the muscle car market. We could easily use a P205/70R14 (25.3") in place of our 6.50-13 (2.93:1 axle & 17 tooth speedo pinion) which would record 0.9% less miles that actual.

Going back to the early (1962-1965) Chrysler A-body as an example, its A904 transmission had 7 teeth on the output shaft. Because larger tires are easier to find that smaller, we can replace our orange 17-tooth speedometer pinion with a 16-tooth brown gear. Without changing the 2.93:1 axle gear ratio, our new corrected required tire diameter is 26.64" and would allow us to use a P225/70R14 tire (26.4" calculated static diameter). This would make our odometer record 0.9% more miles than actually driven.  This is a very large tire for an early A-body and, depending up your vehicle and brake set-up , it may not fit under the wheel wells.

The early A-body axles used 4" bolt circle (BC) wheels, which had an OEM rim width of 4.5" for 13" and 14" rims.  The minimum approved rim width of a 185 mm tire is typically 4.5" while it's typically 5.0" for a 195 mm tire.  Check your tire's specifications to be sure and ask your tire installer for advice.

While it's still possible to find tires now to fit the OEM 14" x 4½" rim, there are a few aftermarket rims with the early A-body 4" bolt circle that are wider than 4.5".  Cragar still makes the 14"x6" S/S with a 4" BC but it has a smaller offset (3-3/8") than a Mopar 14x6" rally wheel (4" offset).  If you're going to spend the money on new 14" rims, you should consider 15" rims instead because of the better tire selection.

Modern tires do not come in every conceivable size and are commonly available as replacement tire sizes. So if a particular vehicle sold very well, there would be a good selection of replacement tires for it. If no vehicles were ever sold with a size that happens to work well in your diameter calculations, it will be very difficult (and likely expensive) to find that size.

Before making significant changes to your tire's diameter and/or width, you need to measure and verify that your wheelhouses have enough room for your tire & rim combination. Small changes should present no problems and it is always safest to try to keep the replacement tire's overall dimensions as close as possible to the largest tires offered by the factory.