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?
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"
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