Many older vehicle owners eventually have to decide whether to continue using hard-to-find or increasingly expensive bias-ply tires in their collector cars. The old bias-ply tire sizes were typically much taller (skinnier) than modern low-profile tires, which makes them practically impossible to directly upgrade. While it is possible to try to find a similarly sized radial tire, the upgraded radial tire size isn't always exact and is often undersized, which in turn affects the accuracy of the speedometer and odometer.

Rather than looking up a radial equivalent of a old bias-ply size, it is more accurate to determine the tire size that will keep your odometer (and likely speedometer) accurate. The odometer is simply a counter that relates tire rotations to distance traveled. The speedometer is simply a spring-loaded pointer that relates the speedometer cable's rotational speed to road speed. Therefore, to determine the correct tire size, we need to work backwards from the odometer. Generally, 1000 revolutions of the speedometer cable record exactly 1 mile and, obviously, 1/10th of mile requires 100 speedometer cable revolutions.

The speedometer cable is driven off the output shaft of transmission by a speedometer pinion gear. The factory had a selection of speedometer pinion gear sizes to compensate for the variety of available tire sizes and axle ratios. The speedometer gear ratio is simply the ratio of the number of teeth on pinion gear compared with the number of teeth on the output shaft. If the factory installed a larger tire, they would use a pinion gear with less teeth. Conversely, if they kept the same tire size but installed a low ratio (high numerically) axle, they would use a pinion gear with more teeth.

At the axle, the differential increases the torque produced by the engine to the rear wheels. Each turn of an axle shaft causes the tire to turn one revolution and the distance travelled in one tire revolution is related to the diameter of the tire.

Mopar speedometer pinions  for  A488, A904, A727, A998, A999 automatic transmissions had the following original part numbers.  Some PNs have been superseded:

Tire Size Calculations

Therefore, using the following values for example:

Speedometer Gear Ratio = 1000 revs/mile
Speedometer Pinion Gear Teeth = 17
Transmission Output Shaft Teeth = 7
Driveshaft Revs per mile = 1000 / 7 x 17
Driveshaft Revs per mile = 2428.571429

Axle Ring Gear Teeth = 41
Axle Pinion Teeth = 14
Axle Gear Ratio = 41 / 14
Axle Gear Ratio = 2.928571429 (ie, 2.93:1)

Tire Turns per mile = Driveshaft Revs / Axle Ratio
Tire Turns per mile = 2428.571429 / 2.928571429
Tire Turns per mile = 829.2682927

Required Circumference = 5280 feet/mile / Tire Turns per Mile
Required Circumference = 5280 / 829.2682927
Required Circumference = 6.367058824 ft

Required Tire Diameter = Required Circumference / Pi x (12 in/ft)

Required Tire Diameter = 6.367058824 / 3.141592654 x 12
Required Tire Diameter = 24.32"

However, the rolling (dynamic) diameter of a tire is about 3% smaller than the calculated static diameter (see Barry's Tire Tech - Tire Dimensions). This means we need a slightly larger tire to keep the odometer accurate. Therefore,

Corrected Required Tire Diameter = 24.32 / 0.97
Corrected Required Tire Diameter = 25.07"

The static diameter of the standard replacement radial-ply tire is the P175/80R13 and its diameter is:

Tire Diameter = rim diameter + 2 x the tire section height
Tire Diameter = 13" + 2 x (175 mm x 0.80) x (1" / 25.4 mm)
Tire Diameter = 24.02"

The P175/80R13, having a static diameter of 24.02", would cause the odometer to record 4.4% more distance than actual. A better choice for this vehicle would be the P185/80R13, which would have a static diameter of 24.65" and an odometer inaccuracy of 1.67%. This compares well with the original tire of the car used in this example (1965 Plymouth Barracuda) is a 6.50-13 bias-ply tire. Assuming it had an aspect ratio of 88%, it would have recorded 1.96% more miles than actually driven.

Speedometer Gear Calculations

If we already have our tires and axle, we need to find the speedometer pinion gear that correctly records distance.  We can also use the speedometer gear to convert our speedometer & odometer to metric.  In this example, let's assume that we have P225/75R15 rear tires on with 2.76:1 axle (47 teeth ring, 17 teeth pinion).

1957 to 1965 transmissions had 7 tooth output shafts and speedometer pinion gears ranging from 16 teeth to 21 teeth.

1966 to 1989 transmissions had 13 tooth output shafts and speedometer pinion gears ranging from 24 teeth to 45 teeth.

The 1000 turns/mile speedometer gear ratio is standard for American cars. Other vehicles (like British cars) often have other gear ratios and these are typically specified on the speedometer.

It is possible to convert your speedometer from US miles to metric kilometers because this requires a speedometer pinion gear with less teeth.  Since many American vehicles had 120 mph speedometers, the net result of this would be that the speedometer would top-out at 75 mph (ie, 120 km/h). There are 1.609344 km per mile, which is, as you can see, the ratio between US and metric pinion teeth.  The only way this could work is if your tire & gear combination required a speedometer pinion gear with 37 or more teeth.  (37 = 23 x 1.609344).  A simpler option would be use a speedometer gear adapter that speeds up the speedometer cable by 61%.

If you're in a metric country like Canada, you're better off getting a GPS rather than changing your speedometer to read in km.  Mechanical speedometers aren't very accurate but I like to have an accurate odometer for planning preventive maintenance.  The GPS accurately displays your speed in km/h (or mph when you're visiting the USA) at eye-level and some have built-in trip odometers, which are handy for calibrating mechanical odometers.

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.

Tires for OEM 13" & 14" Rims

For many popular compact cars of the 1960s (Dart, Lancer, Valiant, Comet, Falcon, Mustang, Chevy II, Nova, Acadian, Corvair, etc) with 13" rims, the correct replacement tire for the OEM 6.50-13 is the P185/80R13.  Barry's Tire Tech reports that this tire had a section width of 6.60" (168 mm) and and aspect ratio of 88% so the overall tire diameter is estimated to have been 24.58". Although this size is still available through specialty tire suppliers (like Coker Tire), it is available at a more reasonable price as the Maxxis P185/80R13 90S RWW MA-1 (free shipping!).

If you had a V8 Valiant or Dart, it would have been equipped with 7.00-14 tires on 14" x 4.5" rims.  Barry's Tire Tech reports that this tire had a section width of 7.00" (178 mm) and aspect ratio of 88%.  The overall tire diameter is estimated to have been 26.32" and would therefore have used the 16-tooth speedometer gear.  The estimated best P-metric replacement for a 7.00-14 tire would be the impossible P205/80R14 size.  Since metric tires only go up to a maximum aspect ratio of 80% and 205 mm is way too wide for a 4.5" wide rim, the cheapest replacement tire option would be to change to the 17-tooth speedometer gear, and use the wider and lower profile P185/75R14 size tire.  Like the 13" tire, this size is still available at a reasonable price as the P185/75R14 89S RWW MA-1 (also free shipping).  The Hankook Optimo H724 is also available in this size.

The Maxxis Tire ships via UPS and Canadians can have it shipped to the UPS Customer Center in border cities in the USA (like Buffalo, NY) by including their phone number in their shipping address.  When the parcel arrives, UPS will call you and they will give you about 1 week to pick up your parcel.  The shipping address for UPS in Buffalo, NY would be: