Date: Tue, 9 Jun 1998 12:33:05 -0500
Reply-To: Carl Hansen <carl_hansen@IEEE.ORG>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Carl Hansen <carl_hansen@IEEE.ORG>
Subject: tire size vs. rpm. the real answer
Content-Type: text/plain; charset="us-ascii"
Well, sorta, but not quite. Let's take Michael's example of a van on
jacks, and extend it a bit. Imagine the van is a NON-Syncro, attach
rubberbands, big ones, so that the rear wheels drive the front tires, and
the tires front and back are the same size. You will again get a
speedometer reading, and no matter what size the wheels are (as long as
front and back are the same), at 4000 RPM, you will get the same reading.
So what did that prove, well, nothing actually, IN EITHER CASE.
The actual effect of larger or smaller tires CAN ONLY be tested on the
road. And as Gary noted, bigger tires, means 70 mph comes up at a slower
RPM, and with smaller tires, 70 mph comes up at a higher RPM. It doesn't
matter is you have a 1, 2, 3, or 4 wheel drive vehicle, it doesn't matter
if you attach to a driven wheel, or an undriven wheel, the same is true.
Now if you are a rally diver, you might care about undriven vs. driven
because of wheel spin. I can remember a REAL deap and muddy flat section
that I knew was no longer than 1/2 mile long, but Tom and I had an
indicated 1.2 mile on the odometer. Lots of wheel spin, and a little fear
that we wouldn't get to the other end and would have to have some farmer
pull us out.
Ch
'89 SyncroBeast
Prior Lake, MinneSnowta
well, ok, it's summer
Land of Sky Blue Waters
<<SNIP>>
In a message dated 98-06-08 21:32:10 EDT, dearbo@EROLS.COM writes:
<< Hmmm - I just did the same (increased tire diameter by 2") on a 4x4 that
I have and got the same results. Quiet on the flats, downshifts more on
hills - less torque. I haven't checked the speedo yet. It should be going
faster than the speedo indicates - right? Also the milage trip meter
will be off so its going to be difficult to determine gas milage. I was
going to do some conversions of RPM to mph as I made up a little chart of
"before" readings: 2000rpm = 50 mph in top gear but I'm getting the same
readings after the tire diameter increase. Any ideas on this?
Gary
88GL, 80Westy
>>
Gary, no matter what tire size you put your sycro, the speedo will always
read the same. That's because your speedo cable runs to your front
differential. Imagine your car up in the air on a hoist, the wheels are off,
the car is running and you put it in 4th gear and bring it up to 4000 RPM,
you'll will see about 70 MPH. Now imagine big huge monster tuck tires on the
syncro with the same test as above, nothing changes right. Non syncro
vanagons have the speedo cable running to the left front wheel, so any
dramatic tire size changes will really effect the indicated speed on the
speedo. The bigger the tire, the less speed they will indicate, the smaller
the tire, the faster it'll indicate. Either way, on both cars you'll need to
figure out actuall speed and adjust the speedo's accordingly. Have fun.
Michael Modl. 87 syncro. ( 16" rims, with 215-65-16 tires, at 4000 RPM =
72MHP, actual speed = 75 MHP. )
Michael Modl
<<UNSNIP>>
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