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How to replace Window Channels and stop the rattle by Brian Kemp
Replacing Triumph TR3 Floors by Bill Brewer
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.
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
Like many of our fine automobiles, this page is constantly
being improved. Suggestions, comments, and questions can be addressed
to Brian at bk13@earthlink.net.
Last updated 6 Jun 00.