# 300SD Speedometer Calibration

I just got a speeding ticket in my 300SD.. 75 in a 65....
I had the cruise set at 65 or 66..
Does anyone know if it's possible to adujust the calibration of the
speedomter? If so, how?
Thanks
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"Does anyone know if it's possible to adujust the calibration of the speedomter? If so, how?"
Only way that I know of is to change the tire sizes, which is the most common way they wind up off in the first place. There is a calculator available on the internet that I've used where you can put in the original tire size, then put in another tire size and it will give you speedometer error. You can find the calc by searching.
I'd also do a measured mile timing to verify the speedo is off.
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Unless you're running huge tires on that SD I would suggest there is something other than the speedo that's out of whack, maybe the cop. Here's your calculator: http://www.miata.net/garage/tirecalc.html Good luck, Doug
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On Fri, 01 Apr 2005 07:01:24 -0800, dougmcp wrote:

There are a couple of possibilities. Given the vintage of your car, it's possible that there has been an incorrect replacement of one part or another, throwing off the overall speedometer drive ratios. This one is dead simple to check. Find a highway. Note your odometer reading. Drive about 20 miles, watching the mile markers. Watch the odometer to see if it's clicking over about the same time after each mile marker. And, oh yeah - do watch the road. If your're odometer is turning over nicely at the same point from each mile marker, then the drive ratio's are correct. (Note: this is for the mechanical speedos - you know, the one's with a spinning drive cable and geers - the newest electronic units "drive ratio" is done by a counter in a chip - which in some cases, can also be "adjusted")
If your odometer is NOT kicking over at the same point after each mile marker, your overall ratio is off. Take note of how far off it is after 20 miles - the speedo shop may want this info.
In either case, any good speedo shop should be able to help. There are a variety of reasons a speedo can be reading wrong, depending much on the design of the speedo and whether it's the older mechanical rotating magnet design or one of the slightly newer electronic coil designs, or the latest digital designs.
http://www.nhspeedometer.com has been in business longer than a lot of us. I won't claim that they have bargain basement prices, but I've used them twice and they did what they were supposed to do.
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On 1 Apr 2005 06:49:39 -0800, " snipped-for-privacy@optonline.net"

Far easier just to grab a GPS and check the speed via it's speedo. GPS's have very accurate speedos.
To view the day to day life of a loser go here. http://members.iinet.net.au/~farmerjim/log/log.html
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I believe W126 speedometer 'counts' pulses to determine distance and speed. In another word, there is no steel cable connecting transmission to speedometer. It is just a wire. There is nothing inside the speedometer to adjust.
Is the cluster original? Different engine/tranny configurations use different parameters to determine a 'mile'. Clusters are not inter-changable between different configurations, even if they all come from W126.
Paul Valois wrote:

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It may count pulses for speed... but for odometer, there are three different 3rd gear for different cars. I once put in a 500SEL speedo into my 380SE and it is too fast by 5 MPH all the way... same for odometer... too fast too.
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