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Discussion Starter #1
I was driving with my GPS the other day and I noticed that my speedometer read about 12-15% slower than my actual speed based on GPS. It was pretty consistent up to about 80 mph.

I know that by changing the tire size it could mess with the speedo reading being accurate but I have an 86 Spider and my tires are 185/70~14, which I believe are stock size.

I read the post about "how to fix a slow reading tach/speedo" but I don't think thats the issue. All the lights and gauges work (with the exception of the fuel gauge-prob sending unit). The speedo and tach "bounce"' a little upon initial acceleration, but I can deal with that... Any thoughts?
 

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Discussion Starter #3
So my next question is... How do I calibrate or adjust the speedo? Does it have the same type of adjustment screw. If so, how far would I have to turn it? and in which direction?

Sorry.. Just had an AAGGHHHH moment....
 

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very common on spiders. Youre lucky- my spider is off by say, 25 mph at 55 mph. yes, speeding tickets.......dang.
 

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Discussion Starter #6

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Discussion Starter #8
I couldn't wait till the weekend so I did it this afternoon... The speedo adjustment screw was already almost to the end (counter clockwise) so I turned it about 10 degrees. While I was in there I replaced all of the bulbs with slightly brighter ones (Sylvania #74 LL 1.4 watt) and lined the inside casing with aluminum duct tape (highly reflective). The bulbs were just a little bigger than the ones in there and it took a little more force to get them in, but they worked great. The speedo reads pretty close to correct! and the gauges are substantially brighter. I took before and after photos, but in the pics, you really can't tell the difference. As for the curved needles.... they really do seem fragile, so I left them alone! Thanks again to this awesome board!!!
 

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The GPS speedometer app on my iPhone (the aptly named GPS Speed) gives me almost exactly the same speed as my speedometer, problem is, both of them are 10-15 miles an hour fast. Have to stick with (RPMs in fifth gear x 2) / 100 = actual speed (more or less).
 

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But Mad North-Northwest
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Er, GPS-based speed calcs are pretty darned accurate (unless there's an issue with that particular app). How can you tell the GPS is off?

I would suspect your tach is probably slow: most of them are.
 

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Discussion Starter #12
After the adjustment, it's reading a little fast now... not that big of a deal. The biggest pain in the but is that **** trim ring around the monopod. I feel like its going to snap when putting it back on or else I'd adjust it some more. I'd rather the speedo read fast by 5mph than too slow by 10mph. Saves money on speeding tickets, lol.

GPS is a pretty solid solution. Haven't tried the iphone app. You can actually get a cool heads up GPS speedo for under $200...
 

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But Mad North-Northwest
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Yeah, don't worry, it'll read slow again eventually :D

Apparently the issue is that the adjustment pot degrades over time. You can just re-adjust it every year or so, but I finally pulled my speedo out and gave it to a local speedometer guy who just soldered in a replacement potentiometer and calibrated it on the bench with a signal generator. It's been working well since.
 
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