I couldn't figure this out with the tire calculater. I followed a friend that has a new car. He drove at 65 mph, my speedo read 50ish.
My tires are 205x55x16

Is this a tire issue or a speedo spring tension issue ?

Tire size calculator

Stock tire sizes taken from a previous posting by Jeff Bowlsby:
55SR15 (4-1/2J x 15)
165SR15 (4-1/2J x 15 or 5-1/2J x 15)
165HR15 (4-1/2J x 15 or 5-1/2J x 15)

Jim, a 205/55x16 will turn more revolutions than a stock 165SR15 at any given speed. IIRC, a 165SR15 is really 165/80x15.

Revolutions per mile:

205/55x16 = 810.68
165/80x15 = 794.22

With the 205s, at a true 65 mph, your speedo should be reading about 63~64 mph. Check your speed with a GPS to see how far off you are.

Here's the tire size calculator I use Click here

At true 65 mph, my speedo reads 50. When my speedo reads 65 I'm doing a true 75+

Is this something NorthHollywod Speedometer needs to repair ?

Are you getting any noise or squeaking from the cable? If not it has to be either the gauge or the speedo drive attached to the trans.

Actually, I believe the cable is out of the equation. Think about it, the tip of the cable turns exactly as fast as the drive moves it, no more, no less. The drive itself is a direct mechanical connection to the pinion drive gear, which in turn is part of the pinion bolt. Gotta be in the speedo itself.

The Cap'n

I'm with the Captain on this one. If the one end of the cable turns slower that means it is constantly winding and would have snapped long ago. Could be things are just a little gummed up in the speedo or the spring tension is too tight. Something (force) is keeping it from rotating the correct amount

Do you know the history of the car/speedo?

I had a 6 that I sent the speedo to N. Hollywood to get it recalibrated for the tires I had. That car got wrecked and all the parts put into a 4 chassis. If the tires are different than the ones I had, you would have that problem if you had my old speedo!!!

Just a thought !

Speedo has been in the car since '83 when I acquired it. I changed wheels 3x; Riverias, 2L Fuchs, 15" Fuchs, and now 16" Fuchs

How is the spring tension adjusted ??

The problem is, you just don't drive fast enough.

After all these years, now we know why, Elliot.


Keep it the way it is and delete this thread.

If and/or when you receive your first speeding ticket, the judge will order you to have it fixed and will eliminate or reduce the fine

From what I have read, there are 2 ways for adjusting the speedometer. The first way, and they say is the most preferred way, would be changing the strength of the magnets. This would require some special equipment. First, you would need to measure the current strength of the magnet with a special tool. Then with another special tool, you would need to degauss the magnet. Next, another tool to magnetize the magnets to a new certain strength. Seems to me like alot of mickey mousing around for calibrating your speedo.

The 2nd way deals with changing the spring constant. This would require you to pull the needle (not easy to do) and spin the cable with a drill. I have read somewhere, but I tend to forget the exact intel, but if you spin the cable at 1000 rpm's, it should show 60mph. You could buy a cheap non contact photo tachometer to ensure your drill is spinning at 1k. Then, I would see what speed the needle is pointing at. From there, you could reset the needle.

Ok, so its not original and it may not be appropriate for this discussion, but then not much on my car is. When it finally gets running I am going to use a speedhut GPS speedo. Installs normally in the dash, sensor sits up front, 12 volt hook up, off you go. Start at 169$
No more cables, calibration etc.

speedhut

.....Answer to your ?,,,it is a tire size issue!
(if not this, then look to speedo?)

The outside diameter isn't that much different between the two sizes of tires. I think it's probably the speedo needing recalibrated, unless it's an issue with the ratio of the angle drive (is that a possibility, Cap'n?)

Thanks for the info I'll call North Hollywood Monday. I did get a warning driving from Carmel to Marina. My speedo was reading around 70 and I was clocked by radar at 85

Your putting less miles on you car.