Date: Sat, 24 Jan 2004 00:18:56 -0800
Reply-To: David Marshall <vanagon@VOLKSWAGEN.ORG>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Marshall <vanagon@VOLKSWAGEN.ORG>
Subject: Tires sizes and speedometer errors
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I updated http://www.syncro.ca/tiresize to include the revolutions per km or
per mile with you click on the diameter of the tire. While doing this, I
realized that our speedometers are calibrated correctly.
Looking at ETKA on the 2WD Vanagons there is a cable that goes from the
wheel to the speedometer with no gearing. On Canadian / European
speedometers there is 500 listed on the bottom of the speedometer (805 in
the USA). Meaning that you need 500 revolutions to make 1km on the odometer
(805 rev to make a mile). Well, if you look at the stock 185R14 tires they
need 483 revolutions to make a kilometer or they are off 3.5% which is about
how much our speedometers are 'designed' to be off according to the Bentley
manual. For those people who are unfortunate enough to have P rated
185/75R14 tires on their van, they'll find they are doing the correct speed
at 497 r/km or less than 1% off! Still no reason to have P tires on there!
There is a neat little box on the Euro ETKA that shows a 1:0.967 gearing
reduction (171 957 501 - Golf part number).... let's do the math - 0.967 / 2
= 483.5 which is basically the correct amount of turns for the 185R14 tire.
Interesting!
With all this being said, I can make my Syncro with 215/85R16 tires read
correctly if I replace it with a MPH speedometer and pretend they are
kilometers. These tires are 408 rev per kilometer. The Syncro speedometer
needs 2x as many revolutions of the cable to make a 1km as compared to a 2WD
speedometer. The 2WD MPH speedometer is 805 which almost double 408!
With this being said, Syncro owners have some play in their sides as the
output of the front differential has different gears for different tire
sizes and different final drives. Stock North American ratios are 8:19 (094
409 181 B), but 8:20 (094 409 181 C), 9:20 (094 409 181 D), 9:21 (094 409
181 F), 9:22 (094 409 181 G), 7:19 (094 409 181 H), 8:18 (094 409 181 J) are
available. Strange that they list the tire size 215R16 as an option! With
the 8:19 in there (0.421) I guess that this is why when you have 29" tall
tires on a Syncro the speedometer is pretty close to being correct!
Are we asleep yet!? :)
David Marshall
Fast Forward Automotive Inc.
4356 Quesnel-Hixon Road
Quesnel BC Canada V2J 6Z3
http://www.fastforward.ca mailto:sales@fastforward.ca
Phone: (250) 992 7775 FAX: (250) 992 1160
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