Tyre Sizing

When changing either tyres or rim sizes it can be confusing as to which tyre specification to get to make sure the speedo is not put out. Some tyre sales people will sell you whatever they want without a care to the accuracy of the speedo afterwards, thinking ‘close enough is good enough’. To avoid unwanted speeding fines when your speedo is telling you you are on the legal limit, you must be prepared come tyre shopping time.

Rolling Diameter

The tyres rolling diameter (or circumference) is the measurement of the part of the surface that touches the road. Trying to measure this with a ruler is silly, due the the curving shoulders of a tyre, therefore we use mathematics to determine the closest number possible. Unfortunately this number can never be absolute in terms of its accuracy, as factors such as tyre pressure can alter the figure slightly, but it is still the best way to do it.

Tyre Specifications

All road tyres are specified using three numbers, which represent the width, the profile and the size of rim it is meant to fit. An example comes with the factory tyre size for the S13.

205/60 R15

(Width) / (Profile) R (Wheel Size)

The tyre is 205mm wide, has a profile of 60 and suits 15 inch rim. The width and profile is straight forward, but the profile can use some explaining. The profile represents the side wall height, and is a mathematical ratio number. This means that the profile is never a set figure (ie 60 doesn’t always equal 60mm), but a determining figure of the rolling diameter in relation to the width and rim size. The larger the number the fatter the sidewall, and the lower the more they look like black rubber bands.

Front and Rear Tyre Diameter

Ideally it is nice to match all four tyres to the factory size, but clearly the two wheels that the speedo are driven from are more important. These will always be the driven wheels, such as the rears in an S13 because it is rear wheel drive. Therefore, the front wheels can be slightly out compared to the rears, but not too much as problems such as guard scrubbing can occur. For the rears, it is best to stay within only a few millimetres difference in rolling diameter to keep the speedo accurate.

Consequences of Tyre Inaccuracy

How much the speedo is altered depends on the difference in rolling diameter. Speedo inaccuracy will always be in a percentage however, rather than a fixed gap. By this I mean that the speedo may under read by 10%, instead of always be under reading by a fixed amount of 10km/h for example.

When the rolling diameter is larger than factory, it makes the speedo under read. That is, you will be travelling faster than your speedo is telling you, which is very nasty for speeding tickets.

When the rolling diameter is smaller than factory, it makes the speedo over read. That is, you will be travelling slower than your speedo is telling you, a much more conservative and safe proposition.

Obviously the tyre diameter also has a direct bearing on vehicle gearing, and altering the tyre size can be like altering the final drive ratio in the differential. When the diameter is larger, the vehicle will have a slight advantage in acceleration at the expense of fuel economy and reduced top speed, and vice versa.

How to Match the Correct Tyre for a Non Factory Size

Handy programs exist to tell you the rolling diameter for a given tyre specification, by doing the maths for you. I use Gearcalc, a free application I found some time ago. It has many more functions to do with the gearing of a vehicle, but can be used simply to tell a tyre’s rolling diameter and circumference.

It is available for download here: Gearcalc

Below is table generated by using Gearcalc, to assist with selecting the correct tyre size. As an example of how to use it, highlighted and in bold is the factory tyre size for the S13: 205/60 R15. Also highlighted are the closest matches to this in diameter.

  1. Say you were going drag racing with stock 15×6 inch rim. You want some wider rubber under the rear to assist launching, yet you want to keep it for the street so the speedo is accurate. The wisest choice would probably be the 245/50 R15. Of course if they were only going on at the track then it might be wiser to choose a tyre with a slightly larger diameter to assist acceleration, and higher profile to ‘bag’ better on launch. Maybe then a 255/50 R15 would be good.
  2. Next we might want to upgrade to a larger rim for the street that maximizes a good balance of size and performance. For a 16 inch wheel 225/50 R16 on the front and 245/45 R16 would be near perfect. For a 17 inch rim a 215/45 R17 for the front and 245/40 R17 for the rear is quite good.
  3. Now perhaps you wanted some pimp style 19 inch chromies. 205/25 R19 would be a good bet all round.

Clearly the table can be used for other cars, just look up your factory size and then search for those closest. Notice the difference in diameter one size up in profile can make, so take care to get the exact tyre you want. Anything not listed here can be found by using gearcalc. Enjoy!

R 15
195
205
215
225
235
245
255
265
275
40
537.00
545.00
553.00
561.00
569.00
577.00
585.00
593.00
601.00
45
556.50

565.50

574.50
583.50
592.50
601.50
610.50
619.50
628.50
50
576.00
586.00
596.00
606.00
616.00
626.00
636.00
646.00
656.00
55
595.50
606.50
617.50
628.50
639.50
650.50
661.50
672.50
683.50
60

615.00

627.00
639.00
651.00
663.00
675.00
687.00
699.00
711.00
65
634.50
647.50
660.50
673.50
686.50
699.50
712.50
725.50
738.50
R 16
195
205
215
225
235
245
255
265
275
35
542.90
549.90
556.90
563.90
570.90
577.90
584.90
591.90
598.90
40

562.40

570.40
578.40
585.40
593.40
601.40
609.40
617.40
624.40
45
581.90
590.90
599.90
608.90
617.90
626.90
635.90
644.90
653.90
50
601.40
611.40
621.40
631.40
641.40
651.40
661.40
671.40
681.40
55
620.90
631.90
642.90
653.90
664.90
675.90
686.90
697.90
708.90
60
640.40
652.40
664.40
676.40
688.40
700.40
712.40
724.40
736.40
R 17
195
205
215
225
235
245
255
265
275
35
568.30
575.30
582.30
589.30
596.30
603.30
610.30
617.30
624.30
40
587.80
595.80
603.80
611.80
619.80
627.80
635.80
643.80
651.80
45
607.30
616.30
625.30
634.30
643.30
652.30
661.30
670.30
679.30
50
626.80
636.80
646.80
656.80
666.80
676.80
686.80
696.80
706.80
55
646.30
657.30
668.30
679.30
690.30
701.30
712.30
723.30
734.30
R 18
195
205
215
225
235
245
255
265
275
30
574.20
580.20
586.20
592.20
598.20
604.20
610.20
616.20
622.20
35
600.70
607.70
614.70
621.70
628.70
635.70
642.70
649.70
656.70
40
613.20
621.20
629.20
637.20
645.20
653.20
661.20
669.20
677.20
45
632.70
641.70
650.70
659.70
668.70
677.70
686.70
695.70
704.70
R 19
195
205
215
225
235
245
255
265
275
30
599.60
605.60
611.60
617.60
623.60
629.60
635.60
641.60
647.60
35
619.10
626.10
633.10
640.10
647.10
654.10
661.10
668.10
675.10
40
638.60
646.60
654.60

658.60

670.60
678.60
686.60
694.60
702.60

 

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