Today I had my speedometer calibration checked at a local auto club
office. They have a portable setup of rollers that your car is driven
on (like a dynamometer) while the technician and the equipment make
measurements. The actual results follow the math.
I bought my TR6 1 1/2 years ago, 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 as listed in Moss (ex:
dia*3500/3.7/346 for the 73.7 mph of the 185/82 at 3500 rpm in 4th gear.
(dia)(rpm) / (ratio)(346) is a formula presented in the Moss magazine
regarding TR6 tire sizes. The ratio I used in 4th is 3.7 (US) and 4o/d
is 3.03 (A type, 0.82 ratio).
dia (in) dif 3.5/4 rpm 3.5/4od 2140rpm/oc
185 82 26.9 0.0 73.7 90.0 55.0
185 70 25.2 -1.7 68.9 84.1 51.4
195 75 26.5 -0.4 72.5 88.5 54.1
195 70 25.7 -1.2 70.4 86.0 52.6
195 65 25.0 -1.9 68.3 83.4 51.0
205 75 27.1 0.2 74.1 90.5 55.3
205 70 26.3 -0.6 71.9 87.8 53.7
205 65 25.5 -1.4 69.7 85.1 52.0
215 70 26.9 0.0 73.4 89.6 54.8
or sorted by size,
dia (in) dif 3.5/4 rpm 3.5/4od
205 75 27.1 0.2 74.1 90.5
185 82 26.9 0.0 73.7 90.0
215 70 26.9 0.0 73.4 89.6
195 75 26.5 -0.4 72.5 88.5
205 70 26.3 -0.6 71.9 87.8
195 70 25.7 -1.2 70.4 86.0
205 65 25.5 -1.4 69.7 85.1
185 70 25.2 -1.7 68.9 84.1
195 65 25.0 -1.9 68.3 83.4
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 Actual
Speed Speed
30 27
40 37
50 47
60 56
70 66
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 74 mph when I'm actually doing only 70 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, you can download the
spreadsheet at the tech section of the Southern California Triumph
Owners Association (SCTOA) web site.
http://home.earthlink.net/~bk13/tech.htm
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
72 TR6
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