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Date:         Thu, 11 Jun 1998 03:06:18 EDT
Reply-To:     Modl6971@AOL.COM
Sender:       Vanagon Mailing List <Vanagon@vanagon.com>
From:         Michael Modl <Modl6971@AOL.COM>
Subject:      Re: tire size vs. rpm.  the real answer
Comments: To: dearbo@EROLS.COM, Vanagon@VANAGON.COM
Content-type: text/plain; charset=US-ASCII

In a message dated 98-06-10 23:07:17 EDT, dearbo@EROLS.COM writes:

<< Michael Modl wrote: > Hi Ch. Well now that I thought about it, you are right. Whether or not the > speedo drive is coming from the front diff or the left front wheel the speedo > will always read the same. I dont know why I thought it would make a > difference. I wasn't feeling very well last night. I guess if someone with a > REAR wheel drive vanagon wants to make the RPM lower without having to > recalibrate the speedo, they will have to put 28" diameter tires in the rear > only. RPM at 4000 RPM would equal 78 MPH, (3600 RPM = 70 MPH) and the speedo > will show that speed, since nothing was done at front , right. > Michael Modl. 87 syncro

Hi Michael: You and Ch brought up some good points. Every time I think about it - I almost understand it! The indicated speed (say 50mph) at a certain RPM (say 2000) does not change by putting larger sized tires on the auto but the actual speed does. If I do put on larger tires - say 2" larger in diameter - then at 2000 rpm the larger circumference will be covering more ground than the smaller tires in a time period. Since it covers more distance than the smaller tire in the same unit of time, the car is going faster. How much will the speedometer be off? The TireRack web site has some good info on this.

NEW TIRE DIAMETER/OLD TIRE DIAMETER x MPH at a certain RPM = ACTUAL MPH at that RPM

In my case: 30/28 x 50 = 53.6 So when my speedo indicates 50 I'll really be doing 53.6 so I'll back off a little at higher speeds.

Gary 80 Westy, 88GL

>> Hi Gary, you can also figure out your real speed by timing how long it takes you to travel one mile, on the freeway, between mile markers. Maintain a perfect speed, like 70 MHP, time your car to travel that mile, time a few mile markers because they all aren't exactly one mile appart, take an average, lets say it took 51.5 seconds to travel one mile. Take 3600 seconds and divide it by 51.5, there's your speed, 69.9 MPH....... (You'll never see 30 seconds to travel one mile, unless it's falling over a cliff.) bye, Michael Modl. 87 syncro.


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