speedometer calibration

[MrWampus]

Hey folks, I just figured out that my speedo is reading almost 10% fast (when my real speed is 65, the speedo says 71). This should keep me out of trouble with smokey, but I'm also racking up 10% more miles on the odo than actual. Is there a way to calibrate the speedo in an 81 spider? Is it possible to DIY? Is it calibrated in the speedo unit or in the transmission? Should I even care about it?

(Inquiring minds... etc, etc)

[124JOE]

use stock size tires

[joelbert2k]

I have the exact same problem, and I have stock size tires.
I just figured I'd live with it and get taller tires next time.
Joel

[124JOE]

i think its in the gear on the end of the speedo cable then

[vandor]

Hi,

> I just figured out that my speedo is reading almost 10% fast (when my real speed is 65, the speedo says 71).

Pretty common with Spiders. To correct this I put a speedo drive and gear from a car with a 4.1:1 rear ends, but my gears are actually 3.9:1. So the speedo is ~5% slower than it would be with the 'correct' speedo drive.

Apparently there is an easier way of doing it, there are companies that make little boxes that go in the speedo cable to change its speed. You just tell them what the speedo is showing at what speed, and they make a custom box that will reduced the speedo cable speed by the required percentage.

> I'm also racking up 10% more miles on the odo than actual.

Not necessarily! Just because the speedo is 10% off does not mean the odometer is aswell. You need to test it separately by driving 10 miles exactly (use mileage markers to check distance), and see what the odometer shows.
On the Spiders I tested the Speedo was off by 7-9%, while the odo was only off 2-3%.

>Is there a way to calibrate the speedo in an 81 spider? Is it possible to DIY? Is it calibrated in the speedo unit or in the >transmission?

I do not know a DIY way to do it. I suppose you could try to move the spring in the speedo that acts on the needle, so it is stiffer, but it would take a lot of experimentation, if it could even be done. Not worth the hassle, IMO.

[boogiedude]

If I'm not mistaken, don't all cars of this vintage have an approximate 5% deviation at high speed? I seem to remember reading somewhere about an acceptable margin of error

[ctb1025]

All,

I was wondering if you can help me. I just put in a new speedometer cable. Does the speedometer typically bounce around a bit when you are accelerating up to 30 MPH. Once I get past 30 MPH the speedometer stabilizes. When I got the car the cable was broken so I don't have anything to compare it to. Hopefully you all can help.

Thanks,
Chris

[124ADDHE]

I bought 81 US spider, 85mph speedo read about 10% high and i wanted KMPH anyways, took 220kmph speedo from my 74' and whadyaknow she reads within readable perfection according to my gps...isnt that some luck; so maybe each speed is just a bit different.

[vandor]

Does the speedometer typically bounce around a bit when you are accelerating up to 30 MPH. Once I get past 30 MPH the speedometer stabilizes.

It's not normal. Sounds like the cable is binding. Be sure it is not making too tight a radius anywhere. I suppose it could be the speedometer, but it's not likely.
bye,

Csaba