Speedometer calibration anyone?
 

So Ive got a about a thousand miles on my restored 84 9$$. It about 6:30am and Ive decided to see how fast I can go on Florida's I-95. I took the car to 110mph and decided to back down to 80 mph (the SL is 70) as i came to up to an overpass. Good thing I did cause the man was on the other side. I looked at my speedo as I passed hI'm an it clearly registered just under 80. The man thought otherwise and gave me a ticket for 89 in a 70. Ouch!!!
Anyway I later GPS'd the car and sure enough when the speedo reads 80 I'm doing 90. (I didn't get caught this tI'me). Actually the speedo reads 40 when the GPS says 45 so the variation is progressive across the whole speed range.

Has anyone ever tried to recalibrate a 944 speedometer? Is this something I can do on a bench or do i need to send the unit off for rework?

Thanks for the assist.

CDB

I always use my GPS for speed readouts anyway. I don't have a vehicle that the speedo and the GPS agree. But, to answer your question, I don't know if they can be calibrated. Seems like a lot of variables for 80's technology to cope with.

The speedo is driven off of the front wheels - have you changed the size of the front tires. Never known of a speedo to be that far off with the correct size tires on the front.

There are places to send the speedometer out to to have them re-calibrated.

Hey, you're back! I was about to shoot you a PM and see how things are going with your 44.

Like colakidd said, the tires are what will vary your speedo reading.

Jackson

I think (and it might depend on the laws in your area) that if you have a demonstrable mechanical descrepancy, you might have a (partial?) defense to your speeding ticket. You might want to check into this.

BTW, did you have the needle off the speedo at some point in the resto? Like replacing the dial faces with white faces or something? If so, it is possible that the needle wasn't replaced correctly and that can cause underreading as well...

mine does the exact same thing...stock wheels and tires.

I had the same problem in my 83 NA because I run 55's. (rather than 60's).

What I did was use a GPS to establish the margin of error at different speeds, then pulled the speedo out and took the needle off and reset it (slightly lower than zero, if I remember correctly) so that it read dead accurate at about 60 MPH (kinda the mid-point - a compromise speed).

I had to do it a couple of times to get it just right - checking it against the GPS.

And now as a bonus, I can now pull out my instrument cluster in under 5 minutes!

But it was worth the effort, because i like having an accurate speedometer.

A caveat: be very careful taking the needle off because they're very fragile! Easy to bend or break...

Anyone know of any electric ways to go through this? By electric I mean having an eletric speedometer... anything other than a bouncy stock needle!

By far the cheapest and easiest way to fix this is replace the speedo cable.

Jackson