Speedometer fun

Ok folks, here is where I got completely confused yesterday.

Most Vanagon owners who have been in their dash know there is an early type speedometer with a screw on speedo cable, and a later style with a push on cable that clips on the back of the speedo housing.  

Ever wondered what that number on the face of the speedo means?  The later US release MPH style 2wd models came with an 805, an 820, a 1600 and maybe others.  The number refers to how many revolutions of the speedo cable * 100 to travel 1/100th of a mile (52 feet 9.5 inches).  At that point in time, that was also the tire size (revs per mile) they were using.

The 1600 version was a Syncro speedometer as its drive did not feed from the front wheel, hence the higher number.

I am going to focus on the later style, as that is what I have.  In the models released in 1984 and 1985, some of the speedometers had a metal casing that housed the guts of the speedo, many of these are still going as the pieces prone to failure were plastic related.

The failure happens when the plastic housing has worn to a point, or the clip on the end of the dowel that runs though the number spool fails.  I have seen people do all sorts of things to try and repair the speedo - JB weld, add a tab to retain the dowel, paper clips used as springs to put constant pressure but I have yet to see any repair other than a VDO rebuild last longer than a year.

Back to the function of the Speedometer.  The earlier style 84-85 speedometers were 'intended' to be off.  What's that you say?  Yes, the speed indicated on the speedometer is intentionally higher than the actual speed when the vehicle came from the factory floor. Some sort of government conspiracy? Maybe, I don't happen to subscribe. 

But to determine speed correctly, use this formula: (from the Bentley)

Indicated Speed = Actual speed + 6.2mph + (.05* actual speed)

So, given this formula; my 1984 Vanagon (built in September '83) traveling at 50mph will actually read 50+6.2+(50*.05).  = 58.7 mph.  This is assuming stock diameter tires.

The later style speedometer had a difference of only 3 mph, so a 1986 Vanagon traveling at 50mph will actually read 53 on the speedometer with stock tires.  That's why you see folks who replace their speedos state their mileage is closer to being correct when they go with a later speedo (usually due to lower mileage) because it actually is more accurate.

Most of us are using different than stock diameter tires, so adjust your calculations accordingly.  Or use the speed box app on your iPhone. 

So where is the confusion?

Look at the two speedometers above, actually their faceplates.  At roughly 3o'clock the earlier speedo is 72mph, while the later one is 82mph.  With a same 805 designation, same vehicle with same tires and cable - what makes one read higher than the other?  Electromagnetic field difference.

The increase or decrease of electromagnetic field and a hairspring inside the speedometer controls the indicated speed of the speedometer.  So when we send the speedometer off to VDO and we tell them it needs to be calibrated because its 8.7 miles per hour off, VDO will either increase or decrease the magnet strength within the speedometer so that the spinning from the cable will create an appropriate electromagnetic field and read the appropriate speed.

Took me a bit to get my answers and wrap my head around that.  Now I want to know how I calibrate my own speedo so I can continue to use the metal housing speedo and indicate the correct speed?

Note: The speedometer function is independent of the odometer function, so an increase in speed does not mean your odometer is reading more than what it actually travelled.

Make sense?

Now in my case: my tires are actually too big for the van - they are a 769 rev per mile. Which actually negates the built in difference in the later style speedo I was playing with today.  I had the iPhone out with speed box app going to do a quick check on what the speedo was telling me.