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Date:         Thu, 16 Dec 1999 09:40:09 -0800
Reply-To:     Doran Nugent <doranosaurus@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Doran Nugent <doranosaurus@YAHOO.COM>
Subject:      RPM vs MPH vs ACTUAL SPEED
Comments: To: "Fitz-Randolph, Douglas" <dfrandolph@TALKAM.NET>
Content-Type: text/plain; charset=us-ascii

Doug and All,

I believe, correct me if I'm wrong, that the speedo is indeed affected by "bigger shoes." If the speedo measures the revolution of the front wheel, not the actual speed of travel, then it would display a SLOWER SPEED on the needle with bigger tires.

Lets see if this is right; Normal tires would travel, lets say, 10 feet in one revolution. The vehicle would have traveled ten feet in, lets say, 60 seconds. (I know that would be very slow but this it's hypothetical).

The speedo is made to show revolutions per minute of the wheel, I think. And it is calibrated for a certain sized tire. So if the wheel travels 10 feet in one minute the speedo would show a speed 10ft/minute with normal, stock tires. Larger tires would travel, lets say, 12 feet in one revolution. So a larger tire van would travel FARTHER in one minute and the speedo would display 12ft/minute.

If your actual speed was the same in both cases, The speedo would show 10ft/minute with tires that are ten feet around, and would show aprox. 8ft/minute.

They way I see it in my head makes sense, though I'm sure you math people could describe it more succinctly with an equation or two.

What do you all think?

Doran, '90 Weekender Carat

--- "Fitz-Randolph, Douglas" <dfrandolph@TALKAM.NET> wrote: > John Wrote: <<I've always kept it at 65-70, but > would like to > cruise around 75 MPH. >> > > My philosophy is that lower is better, but as long > as I am in > the "solid green" zone on the tach things'll be OK. > I shared > John's feeling, though - that I'd like to be able to > cruise @ > 75 AND be in the "safe zone", which was my primary > motivation > for going to 15" wheels with larger diameter tires. > Now the > same engine RPM that produced 70 mph with my > 185R14's produces > 77.7 mph with my 215/75R15's - this is calculated, > not > measured. And of course, the speedo still reads 70 > mph because > it has no idea it's wearing bigger shoes. > > Doug Fitz-Randolph > Yarmouth, ME > '90 Syncro >

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