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.