Date: Sat, 5 Oct 2002 13:37:26 -0400
Reply-To: Derek Drew <derekdrew@RCN.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Derek Drew <derekdrew@RCN.COM>
Subject: Re: Tire size vs. Engine RPM
In-Reply-To: <OE57ymNaR2AY4RkMc5H0000753a@hotmail.com>
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<font size=3>If you attempt to do these calculations by reference to
circumference you will introduce unavoidable errors into your
calculations because circumference bears only a loose correlation to
revolutions per mile conditions for any given tire. Revolutions per mile
is the only thing that matters as far as your gears are concerned. For
accuracy, it is best to use manufacturers' stated revolutions per mile at
X MPH for each tire and IGNORE both tire height and IGNORE tire
circumference information. A revolutions per mile number that matches
what one would get by calculating that number based on either tire height
or circumference is almost certainly not accurate. <br>
. <br>
At 10:29 AM 10/5/2002 -0400, you wrote:<br>
<blockquote type=cite cite>I plugged the Vanagon gear ratios into a ratio
computer to see what RPM we<br>
are supposed to be turning on the highway. The gear ratios can be
found<br>
here:
<a href="http://volksweb.relitech.com/helmut/GEARBOX.HTM" eudora="autourl">http://volksweb.relitech.com/helmut/GEARBOX.HTM</a>
and the calculator can<br>
be found here: <a href="http://www.xse.com/leres/ss/calculator.html" eudora="autourl">http://www.xse.com/leres/ss/calculator.html</a>. It appears that<br>
the rear end ratio is either 4.83 or 4.86 so I used 4.83 for the<br>
calculations. My old 185R14 tires have a circumference of 25.7 inches<br>
(tirerack.com figure for Yokohama Y356D tires). According to the figures<br>
with the smaller tires the engine was turning 3798 RPM at 70 MPH and 4340<br>
RPM at 80 MPH. The LT27x8.5R14 tires have a circumference of 26.5 inches,<br>
giving me an engine speed of 3669 RPM at 70 MPH (130 RPM less) and 4193 RPM<br>
at 80 MPH (147 RPM less). Theoretical top speed goes up to 103 MPH from 99<br>
MPH.<br>
<br>
Aren't you glad I can now go 4 MPH faster with the larger tires? Ha ha...<br>
<br>
<br>
Chris,<br>
<br>
'85 Westy. </font></blockquote><br>
<div>_______________________________________________</div>
<div>Derek Drew </div>
<div>CEO & Co-Founder </div>
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<div>New York, NY & Washington DC</div>
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<div> PC World Magazine, Aug. 2001,</div>
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<div> Newsweek, Nov. 5, 2001 </div>
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<div>derekdrew@rcn.com</div>
<div>212-580-6486 </div>
<br>
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