TR6 Tire Sizes

I bought my TR6 in Oct 96 with worn and flat spotted tires.  I wanted to install a wider tire, without creating too great an error in the speedometer reading.  To compare sizes, I made a spreadsheet summarized below.  It shows indicated speed at a given rpm.

I was told by two local tire stores that the old 185/15 tire is 185/82/15 in current sizing.  Others on the list have said it is 185/78.  Either was, the results are similar.

The table shows tire size, diameter difference from stock size, and actual speed with the given tire size in 4th gear and overdrive at 3500 rpm.  The formula used for the actual speeds is:

        Speed = (dia)(rpm) / (ratio)(336)
        or rpm = (speed)(ratio)(336)/dia

dia is of the tire
336 is 63360 in/mi / 60 min/hr / 3.1416
ratio = overall gear ratio (taken from Bentley)
    sample ratios for my TR6 with a 3.7 differential
    1     11.62
    2     7.44
    3     4.92
    4     3.70
    4od 3.03 (a-type)

The ratio I used in 4th is 3.7 (U.S.) and 4o/d is 3.03 (A type, 0.82 ratio).

The table below shows tire size, diameter, diameter difference from stock, speed at 3500 rpm in 4th gear, 3000 rpm in 4th gear with A-type overdrive, and a comparison at 55 mph.

size       dia(in)    dif    3500/4   3000/4od  2078/4od
185 82      26.9      0.0     75.9     79.4      55.0
185 70      25.2     -1.7     70.9     74.2      51.4
195 75      26.5     -0.4     74.7     78.1      54.1
195 70      25.7     -1.2     72.5     75.9      52.6
195 65      25.0     -1.9     70.3     73.6      51.0
205 75      27.1      0.2     76.3     79.9      55.3
205 70      26.3     -0.6     74.0     77.5      53.7
205 65      25.5     -1.4     71.8     75.1      52.0
215 65      26.0     -0.9     73.2     76.6      53.1
215 70      26.9      0.0     75.6     79.1      54.8

sorted by diameter
205 75      27.1      0.2     76.3     79.9      55.3
185 82      26.9      0.0     75.9     79.4      55.0
215 70      26.9      0.0     75.6     79.1      54.8
195 75      26.5     -0.4     74.7     78.1      54.1
205 70      26.3     -0.6     74.0     77.5      53.7
215 65      26.0     -0.9     73.2     76.6      53.1
195 70      25.7     -1.2     72.5     75.9      52.6
205 65      25.5     -1.4     71.8     75.1      52.0
185 70      25.2     -1.7     70.9     74.2      51.4
195 65      25.0     -1.9     70.3     73.6      51.0

Tire widths in inches, for those of us in the US who are metrically challenged:

185    7.28 in
195    7.68
205    8.07
215    8.46
225    8.86

The test results from AAA with 205/65 Dunlop D60 A2 tires at 30 psi with approx. 3000 miles driven.

Indicated     Test     Computed
Speed        Speed      Speed
30                27           28.4
40                37           37.9
50                47           47.4
60                56           56.9
70                66           66.4

Odometer will read 100 for 92 actual miles.

I ended up with 205/65 for the improved performance, though 205/70 might have been a better choice.  Going with a smaller tire can be a benefit.  If you like to drive fast (above the posted speed limit), just pretend you don't have any speedometer error.  With my 205/65's my speedometer shows about 70 mph when I'm actually doing only 66 mph.  This gives me a slight safety margin against speeding tickets.  This also gives you slightly faster acceleration due to the overall ratio change (at the expense of mpg).

I caution you think about clearance regarding 215 size tires.  I have a rear tube shock conversion and they will not fit.  Some conversions I've seen may also rub a 205 tire.  The 195/75 is a difficult tire to find.

If you want to play with the numbers yourself, try one of the spreadsheets below.

 Excel, Office 97 version

 Triumph RPM to MPH

I also used the spreadsheet to find a lower profile tire for other cars, just changed the formulas.  Hope you can make some use of it.

Brian Kemp
bk13@earthlink.net
72 TR6
 { from approx. June 2000 }