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Old Size - New Size

It has been years since tire manufacturers changed the numbering system on rear tractor tires. For those who have the older tires on your antique tractors, the change to the new system can be confusing.

When rubber tires were first used on tractors (1932-1938), the widest rear rim produced was 8". With larger tires, the sidewalls were pulled in, causing the tire to be very rounded.

 

TIRE SIZE CONVERSION CHART

I

II

III

1932-1940

1938-I960

1955-Today

Old Conventional Size

Original Rim Width

Old Wheel Basel Size

Rim Width

Today's Size

Rim Width

7.00

5.5

8

7

8.3

7

7.50

5.5

9

8

9.5

8

8.25

6

10

10

11.2

10

9.00

6

11

11

12.4

11

10.00

6

12

12

13.6

12

11.25

8

13

13

14.9

13

12.75

8

14

15

16.9

15

13.50

8

15

16

18.4

16

9.00-40 (1) and 9-40 (II) are NOT the same size tire!

Add your rim size to the tire size. For example: If you have a 24" rim...750-24 = 9-24 = 9.5-24

 

The first tire sizes (I) were the approximate tread width measured to the closest one-fourth inch.

In 1938, wider rims were used for better tire performance. With wider rims the tire beads were not pulled in as far, which allowed the tread to flatten and with a deeper tread the tires became more efficient. With the wider rim the same size tire has a wider tread width.

The second sizes (II) still used the approximate tread width. This method measured to the closest inch with no decimal points.

In the mid-50s, the third number (III) method began. The new numbers referred to the overall width of the tire (sidewall to sidewall). This method measured to the closest tenth of an inch. The tires basically still had the same overall diameter and width as method (II), but a new number size.

Frequently Asked Questions

Q. Can I use the new tire sizes (III) on my old narrow rims?

A. Yes, if you are restoring your tractor for show. The tire beads will be pulled in and make your tire look more rounded and balloon-like, resembling the old tire (I) look. We recommend wider rims for farming or tractor pulling.

Q. Why don't my new tires (III) look like the tire beads fit correctly when mounted on my old rims (I)?

A. The early rim, produced before 1940, had a 1 .4" flange height while today's rim has a 1" flange height. Using new tires on old rims was not a problem until the 1980s when tire makers started building tires with beads that fit over the lip (flange) of the rim. Those tires will work on the old rims, but will not look right. On some tires the higher flange covers part of the wording on the sidewalk You have three choices...

1. Use tires the way they are even though you don't like the look

2. Use tires with old style beads, such as BF Goodrich (Most new U.S. tires have new style beads)

3. Change to new style rims IF available

Q. How wide should my rims be for tractor pulling?

A. We recommend a rim that is approximately the same as the sidewall width of your tire. Example: 12.4 on 12", 14.9 on 14", 18.4 on 18". You want to keep your sidewalls as straight as possible. This allows you to use lower air pressure without the sidewalls buckling. Lowering the PSI gives a larger footprint for better traction. When you put tires on rims that are wider than the sidewalls of the tire, the tread will tend to concave and will need higher PSI to keep the tread flat.

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