Speedo Issue

[Martyn]

Speedo flickers erratically (and noisy) up to 50 mph and then stabilises. One garage says I need a new speedo, another says I need a new speedo drive. Their diagnosis solely on my description of problem.

How do I identify the real cause of the issue? If it is speedo drive, is it easy to install in '85 model with transmission in?

Thanks

[18Fiatsandcounting]

One possible cause of this flickering is a bad speedometer cable, either because the cable is kinked or doesn't have any lubrication. The inner metal cable can repeatedly "grab and release", causing the speedometer needle to jump around, especially at low speeds. At higher speeds, this seems to be less of an effect.

I've seen this on several cars. A new speedometer cable, or a relubrication of the old cable and checking to make sure it's not bent at too tight an angle, usually fixed the problem.

-Bryan

[Martyn]

Thanks Brian

[Nicketts]

I have exactly the same issue, plus it reads exactly 10 mph high at all speeds. Is there a description of how to disconnect and then lube it on this site? On mine, it is so tight I can barely get the wood dash away from its mount. I bought some white lithium but have been afraid to disconnect.

I have 14” turbo chromadoras. Strangely, the spare is an ancient Pirelli on a 13” steel wheel(?!) it’s useless, I know.

Basically, I’ve learned to live with these issues four years now. Where do I start?

[18Fiatsandcounting]

My two current spiders are both very early model years, so I can't speak for later cars, but I'm pretty sure you need to remove the dash to get to the speedometer cable. If you do that and remove the cable from the rear of the speedometer, you can shoot some lithium grease (the spray type with the little red straw) into the housing opening, and then work the cable around.

If you're adventuresome, you could pull the inner metal cable out all the way and just directly lubricate it with a light oil or lithium grease or the like. You could also pull out the inner cable by disconnecting the connection on the transmission and pulling it out from under the car. Either direction, you need to be sure you can rotate it to get the square shaped head on the ends of the cable to correctly engage both the speedometer and the transmission housings when you tighten it back up.

As for speedometers that read too high or too low, the usual culprit is new tires with a different overall diameter than the originals, but this would give maybe a few percent high at all speeds, but not a constant 10 MPH too high. I have to admit I'm kinda stuck on that one... Sorry!

-Bryan

[DieselSpider]

You do have to do the math in order to vet things out.

The original tire size for most of our Spiders is 165/82-13 which have a circumference of 74.30979788 inches so it takes 852.6466469 wheel revolutions to travel a mile while for example a 185/75-14 has a circumference of 78.30481531 inches and takes 809.1456412 wheel revolutions to travel a mile. At 5,000 rpm with the stock differential with the car in fifth gear you will be going about 93 mph with new 13 inch tires and about 98 mph with new 14 inch tires however if the differential has been changed to a 3.9 over time and owners then the 185/75-14 tires combined with the 3.9 differential will have you going about 108 mph at 5000 rpm in fifth gear instead of the original 93 mph that the speedometer was originally set up for. This is based on fifth gear having a 0.881 to 1 overdrive ratio.

There are calculators on line or you can setup an Excel Spreadsheet to do the calculations so you can have a list of "What Ifs" to cogitate over when considering tire size changes. I have a "Pet" spreadsheet that I use for these comparisons.

A 175/70-14 tire is probably the best match to the circumference of the original 165/82-13 tires being just 0.025 inches smaller in circumference.