Date: Tue, 9 Jun 1998 20:43:40 -0700
Reply-To: David Marshall <vanagon@VOLKSWAGEN.ORG>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: David Marshall <vanagon@VOLKSWAGEN.ORG>
Subject: Re: tire size vs. rpm. the real answer
In-Reply-To: <l03130300b1a31e6357fc@[206.11.2.206]>
Content-Type: text/plain; charset="us-ascii"
If you are all interested in playing with RPMs and tire sizes check out:
http://www.volkswagen.org/tiresize.htm
At 12:33 6/9/98 -0500, Carl Hansen wrote:
>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>>
>
>
-- David Marshall, Quesnel BC, mailto:david@volkswagen.org --
-- 78 1.8L VW Rabbit, 80 2.0L VW Caddy, 87 Audi 5KQ --
-- 85 1.8L VW Cabrio, 88 1.6L VW Syncro Double Cab --
-- Volkswagen Homepage http://www.volkswagen.org --
-- USE DAVID@VOLKSWAGEN.ORG WHEN SENDING EMAIL --
| 
