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rtheriaque

Speedo reads fast... Tried a few things...

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Greetings, All,

I have a '73 2002. I have little knowledge of the car's history, unfortunately. I know there are modifications, given the tii distributor and other "little" things here and there.

The speedo consistently reads faster than the car is traveling. I've covered a few of what I'd think would be the "usual" suspects, but I can't get it to read correctly.

Thinking the car may have been fitted with a later differential, I threw the speedo from my correctly-reading '76 in there. It read similarly (if not identically) fast. For giggles (and to check a balance issue) I threw the four wheels/tires from the same '76 onto the car (they're 13's vs. the 14's I have on the '73). No change in the speedo output.

The speedometer is rock steady without any wobble. There must be something funky in gearing somewhere, but I'm unsure of how to check. The differential is a long nose (I think... It's identical to the '76's, both driveshafts are identical length with different transmission splines).

I'm obviously overlooking something, and it's probably painfully obvious. Would anybody care to berate me and point out the answer that's right in front of my nose?

Thanks!

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When I changed the diff in my car from the stock 3.64 to a 3.91 LSD, my speedo read 10% faster than I was going (so - when it read 55, I was really going 50). My guess is that someone has done a diff swap, and your speedo needs to be re calibrated. I sent mine to PA Speedo just for this.

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before you get all wacky changing and pulling

more things apart - confirm exactly what your speed is

using a road side POLICE radar trailer with the HUGE

readout of your speed as you approach,

or with one of those carry on Navigation units.

Most likely your diff is the cause given all that

you have already touched on.

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Been there! 55 MPH on the speedo showed 42 on the "You're a speeding idiot!!!" police board. Incidentally, my wife's digital readout Civic was dead-on (following behind).

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See the bottom of my coincidentally very recent post at http://www.bmw2002faq.com/component/option,com_forum/Itemid,50/page,viewtopic/topic_view,threads/p,519071/t,308131/

In summary, speedos are marked on the back housing to indicate which diff they are calibrated for. They can be recalibrated. If you want accuracy, I suggest professional calibration. However, I had a situation very similar to yours with my car when I first bought it. My speedo was off by about 10 MPH at 60 MPH. I am pretty sure I had the original speedo and diff in the car. The speedo error was not linear- worse at higher speeds. I took the speedo apart and adjusted the spring a bit. IIRC, I did this by twisting toward the upper range and stretching the spring a little. This corrected about 90% of the error. Unfortunately, I also had the "flutter" problem, so I ultimately bought a new speedo.

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FYI. Those speed signs are for the DOT's traffic and engineering surveys. They use the data pulled from those signs to estimate what speed should be set as the limit for the road. They aren't there to tell you you're speeding, although that is the effect. They are there so they know how fast to make the limit for that road. They must update these surveys every 3 years, and they also use them when you get speeding tickets.

So, the next time you see one of those speed signs, mash the gas.

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remove the speedo's back cover (take that big nut off the back side) and then look at the input side of the speedo needle shaft--the side where the cable plugs in. See if you can move it in and out more than a half mm or so. If so, it's loose, and will cause the speedo to read fast.

I discovered that accidentally when mine went completely bonkers and speeds above 30 mph. When I fixed that problem by tightening the shaft, I also fixed a consistent 7.5 mph error that I had been living with for many years.

If you want details, drop me an e-mail and I'll send you the Roundel column I wrote on the subject.

cheers

mike

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When my speedo read fast I thought my car was REALLY fast! Then I replaced it and found out my car was really slow.

-Matt

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