Jeff,

Tire diameters vary by manf and model, you can't
compare or calculate with accuracy. You really should
use the revs/mile for a particular tire, as stated by
the tire company

With that caveat, search the www.syncro.org site for
the final drive calculation spreadsheet. Its set up
for syncros, but you can download it and customize it
for your vehicle, lots of info about not stock gears
and final drives is included with the spreadsheet,
originally put together by Tim Smith.

I can't remember exactly where it is and what its
called, but it will allow you to work these problems
til the cows come home.

Good luck

Tom F.

--- Jeffrey Schwaia <jeff@TSSGI.COM> wrote:
> I posted this question a week ago and didn't receive
> any responses so I
> thought I would try again.  Any input is
> appreciated.
>
> Thanks,
>
> Jeff
>
> -----Original Message-----
> From: Jeffrey Schwaia [mailto:jeff@tssgi.com]
> Sent: Friday, December 13, 2002 11:34 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: RPMs
>
> The previous thread about engine RPMs and top speed
> got me thinking, and I
> was hoping that some of you gear heads out there
> could confirm or correct my
> calculations.
>
> Here's what I've come up with:
>
> GIVEN....
>
> PI = 3.14159
> MILE = 5280'
>
> SPEED = ((((RPM /(Gear * Final Drive)) * (2 * PI *
> Tire Radius)) / 12 ) *
> 60) / MILE
>
> Using this formula with the gear ratios (4th gear =
> 0.85, Final Drive =
> 4.83) and tire radius (~12") in my 4 speed Vanagon I
> come up with: ~71 mph @
> 4,000 RPM.  This sounds dead on for my van.
>
> It also gives me:
>
> ~18mph for 1st gear @ 4,000 RPM
> ~34mph for 2nd gear @ 4,000 RPM
> ~56mph for 3rd gear @ 4,000 RPM
>
> All these speeds sound about right.
>
> Am I missing something?
>
> I'm looking to build a freeway flyer by mixing gear
> ratios from different
> trannys, so this formula could be of great help...
> if it's correct.
>
> Cheers,
>
> Jeff

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