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Bill - Here is an article I wrote about 20 years ago. Some of the
sizes may not be available. <br>
<br>
<h3>
TR6 Tire Sizes</h3>
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.
<p>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.
</p>
<p>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.
</p>
<p>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:
</p>
<p> Speed = (dia)(rpm) / (ratio)(336)
<br>
or rpm = (speed)(ratio)(336)/dia
</p>
<p>dia is of the tire
<br>
336 is 63360 in/mi / 60 min/hr / 3.1416
<br>
ratio = overall gear ratio (taken from Bentley)
<br>
sample ratios for my TR6 with a 3.7 differential
<br>
1 11.62
<br>
2 7.44
<br>
3 4.92
<br>
4 3.70
<br>
4od 3.03 (a-type)
</p>
<p>The ratio I used in 4th is 3.7 (U.S.) and 4o/d is 3.03 (A type,
0.82
ratio).
</p>
<p>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.
</p>
<p><b><tt>size dia(in)
dif 3500/4 3000/4od 2078/4od</tt></b>
<br>
<tt>185 82 26.9
0.0 75.9 79.4
55.0</tt>
<br>
<tt>185 70 25.2
-1.7 70.9 74.2
51.4</tt>
<br>
<tt>195 75 26.5
-0.4 74.7 78.1
54.1</tt>
<br>
<tt>195 70 25.7
-1.2 72.5 75.9
52.6</tt>
<br>
<tt>195 65 25.0
-1.9 70.3 73.6
51.0</tt>
<br>
<tt>205 75 27.1
0.2 76.3 79.9
55.3</tt>
<br>
<tt>205 70 26.3
-0.6 74.0 77.5
53.7</tt>
<br>
<tt>205 65 25.5
-1.4 71.8 75.1
52.0</tt>
<br>
<tt>215 65 26.0
-0.9 73.2 76.6
53.1</tt>
<br>
<tt>215 70 26.9
0.0 75.6 79.1
54.8</tt>
</p>
<p><b><tt>sorted by diameter</tt></b>
<br>
<tt>205 75 27.1
0.2 76.3 79.9
55.3</tt>
<br>
<tt>185 82 26.9
0.0 75.9 79.4
55.0</tt>
<br>
<tt>215 70 26.9
0.0 75.6 79.1
54.8</tt>
<br>
<tt>195 75 26.5
-0.4 74.7 78.1
54.1</tt>
<br>
<tt>205 70 26.3
-0.6 74.0 77.5
53.7</tt>
<br>
<tt>215 65 26.0
-0.9 73.2 76.6
53.1</tt>
<br>
<tt>195 70 25.7
-1.2 72.5 75.9
52.6</tt>
<br>
<tt>205 65 25.5
-1.4 71.8 75.1
52.0</tt>
<br>
<tt>185 70 25.2
-1.7 70.9 74.2
51.4</tt>
<br>
<tt>195 65 25.0
-1.9 70.3 73.6
51.0</tt>
</p>
<p>Tire widths in inches, for those of us in the US who are
metrically
challenged:
</p>
<p>185 7.28 in
<br>
195 7.68
<br>
205 8.07
<br>
215 8.46
<br>
225 8.86
</p>
<p>The test results from AAA with 205/65 Dunlop D60 A2 tires at 30
psi
with approx. 3000 miles driven.
</p>
<p>Indicated Test Computed
<br>
Speed Speed
Speed
<br>
30
27 28.4
<br>
40
37 37.9
<br>
50
47 47.4
<br>
60
56 56.9
<br>
70
66 66.4
</p>
<p>Odometer will read 100 for 92 actual miles.
</p>
<p>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).
</p>
<p>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.
</p>
Brian<br>
<br>
<blockquote type="cite"
cite="mid:20190726213032.3291EA1A85@autox.team.net">
<div class="WordSection1">
<div
style="mso-element:para-border-div;border:none;border-top:solid
#E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal" style="border:none;padding:0in"><b>From:
</b><a href="mailto:anabil007@comcast.net"
moz-do-not-send="true">William Pugh</a><br>
<b>Sent: </b>Friday, July 26, 2019 4:07 PM<br>
<b>To: </b><a href="mailto:triumphs@autox.team.net"
moz-do-not-send="true">TR3/6</a><br>
<b>Subject: </b>[TR] Tire Sizes</p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What…if any … is the difference of
185HR-15 … and 185/65/15…??</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks, not trying to start a war…😉</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Bill Pugh</p>
<p class="MsoNormal">PO BOX83</p>
<p class="MsoNormal">Wallace, CA</p>
<p class="MsoNormal">95254</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Life is too short</p>
<p class="MsoNormal">to drive Boring Cars</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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