Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (November 2007, week 3)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Sat, 17 Nov 2007 10:09:58 -0800
Reply-To:     Michael Elliott <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Michael Elliott <camping.elliott@GMAIL.COM>
Subject:      Re: Speed vs rpms w/ AT?
Comments: To: John Bange <jbange@GMAIL.COM>
In-Reply-To:  <6da579340711170852o6a163831u7bc680fd85d3ce7d@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Bitchin'. Thanks, John!

-- Mike "Rocket J Squirrel" Elliott 71 Type 2: the Wonderbus 84 Westfalia: Mellow Yellow ("The Electrical Banana") 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano KG6RCR

On 11/17/2007 8:52 AM John Bange wrote:

>> I'd like to check my tach's accuracy. With the NH type automatic >> transmission (1984 Vanagon), what the math to find what I should expect to >> >> read on the tach if I'm doing 60mph? > > > Well, the 3rd gear ouput on the auto is 1:1, so that makes it a little > easier. The differential in the final drive section is 4.09, so divide RPM > by 4.09 to get the number of times per minute your real wheels are (ideally) > rotating. Then reduce that by a little to account for the stupid non-locking > torque converter---- I'd say a conservative 10% is about right for the far > end. Now, just off the top of my head, "regular" vanagon tires are like 26" > in diameter? So that's about 81 inches around (2 * pi * 13). There are 63360 > inches per mile, 60 minutes per hour, so (conveniently) 60mph = 63360 > inches/minute. 63360 divided by 81 gives you 782 wheel RPM, which multiplied > times 4.09 for the differential gives you about 3200RPM ideally. Torque > converter slippage will trend that a little higher, how much being > determined by headwinds, hills, air resistance which increases at higher > speeds, etc., but 10% makes a good high end guess, so divide by .9 for 3555 > RPM. That means you should be indicating 3200-3500 RPM at 60mph. > > So the final formula for ideal MPH->RPM is: > > (MPH / 60 * 63360) / (pi * TireDiameter) * 4.09 > > The reverse, RPM->MPH is: > > ( RPM / 4.09 ) * (pi * TireDiameter) * 60 / 63360 > > The torque converter can really monkeywrench any attempt to get an accurate > reading, but the ideal calculation at least gives you a place to start, > particularly if you're considering a tire diameter change. Just copy-paste > the appropriate formula into google's search box, replace > MPH or RPM and TireDiameter with the appropriate values (google will fill in > pi for you if you just type "pi"!) and click the search button. > > -- > John Bange > '90 Vanagon - "Geldsauger"


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.