speedometer is too fast?

[E30Mracer]

Hello all, Happy 4th of July!
Quick question. Is it possible for the speedo to get crazy and show about +10-20 mph fault?
I noticed it being strange when driving from NY city a few days ago. While showing about 90mph I was still keeping up with steady traffic, which makes me wonder if the speedo is wrong since most people don't drive as fast even on the interstate highway.
My wheels are right for the period 15" Fuchs and the car is of course a 75 911S. Any ideas?
Thanks all!

[project911]

What's the height of your tire profile?

If it's different than the tires it left the factory with that could throw it off.

Bigger = faster speedo than actual
Smaller = slower speedo than actual.

I've also heard of speedo cables stretching over time, but I've not sure what years it affects.

[E30Mracer]

it's 55.

[project911]

Cables were used through 77.

My speedo acts the same way, but I haven't investigated it yet.

[project911]

The stock size for your year and model is 185 70 15

But your 55's would make the speedo seem a little less than actual, and I don't think tire profile alone can account for a 10-20 mph difference.

Hopefully others will chime in with their success fixes.

[Wil Ferch]

Speedo's of that time were delivered -0% / up to +10% optimisitic due to German law not allowing error the other way. However, this wouldn't account for the +/- variation you're seeing...
--Wil

[E30Mracer]

is this size for front and rear?

[E30Mracer]

Another thing, I know for a fact that the speedo was changed some time ago.

[project911]

This is a good read:

http://www.pelicanparts.com/techarticles/Mult_gauge_repair/mult_gauge_repair.htm


I didn't know they could become uncalibrated.
I'm going to use the drill method and test mine out.

[project911]

I got the tire sizes from here:

And yet again another tire question!

[sundaypunch]

When I got my 911 I swore the speedo read 10-15 mpg too fast. My tires were the correct size but I assumed it was old and needed recalibration. My mistake was in not taking the time to check it with one of my other vehicles.

To make a long story short, it was reading correctly and I ended up getting a speeding ticket driving much faster than I thought I was.

[Rufblackbird]

My '72 reads about 5mph too fast. This was done using GPS, but I'm not too sure how accurate that is. This is usually one of the first things I check when I get a car, as my VW rabbit pickup reads about 7-8mph fast, but that's probably due to a different trans, speedo cable, or gauge cluster. For a while I was wondering why I seemed to be hauling ass but everyone was passing me.

[MAS]

Sounds familiar... the speedo in my 77 reads about 6-7 kph too fast, as well.

-MAS

[pwd72s]

Before GPS, the method was stopwatch and mile posts...

[klaucke]

Bigger = faster speedo than actual
Smaller = slower speedo than actual

Actually, bigger tires will make the speedo read slower than you are actually going, and smaller tires will make the speed read faster than you are really going.
Bigger tires effectively gear your car up so that for a given revolution of the engine, the car goes farther.

[Rufblackbird]

Originally posted by pwd72s
Before GPS, the method was stopwatch and mile posts...

[E30Mracer]

OK, i actually got up and checked the tire size - rear is 225x50/15. So i guess it contributes to the faulty speedo.