speedometer runs fast when cold

[The_Dude]

Why does my speedometer run about %15 fast first thing in the morning. The odometer seems to still be correct. After the car warms up, the speedometer goes back to normal.

 

[srtthis]

Makes no Damn sense to me seeing how its mechanical

 

[Shaolin Crane]

Lay off the pipe?

 

[Mintsick]

And you determined this how?

 

[The_Dude]

When cold, 1400 rpms = 60 - 65 mph
When warm, 1400 rpms = 55 mph
I can tell by the way the engine reacts that my tachometer is not off. I checked the odo against the mileage markers on the highway and it appears to be correct.

 

[Boosted92LX]

Have you got an auto trans that's not shifting into overdrive?

That could explain the diff between rpms/speed.

 

[The_Dude]

No, 5-speed, and I'm not forgetting to upshift. 55mph at 1400 rpms in 5th gear is correct. I've checked it with a GPS.

 

[Mintsick]

How cold are we talking here? The only off the wall thing I can come up with is that...actually no I got nothing!

 

[Shaolin Crane]

Possibly the cable expands when it gets warmer but thats a real stretch

 

[Adams91LX]

swap the cable... repeat test?

 

[stykthyn]

Why does my speedometer run about %15 fast first thing in the morning. The odometer seems to still be correct. After the car warms up, the speedometer goes back to normal.

its all the extra hp from dense cold air.

 

[BlackGT89]

Actually, I think my speedometer does this a bit as well. I have no explaination for it either besides just being old and having about 1/4 million miles on it. I notice it the most when I'm going to work in the morning and there is a little chill in the air. On mine though, I can almost watch it rise by 3 or 4 mph and then slowly fall as it warms up, all the while my cruise is set to a constant speed.

 

[Mintsick]

Possibly the cable expands when it gets warmer but thats a real stretch

This is how off the wall I was thinking too.

 

[Shaolin Crane]

Maybe the magnets in the speedo something something when its cold?

 

[jrichker]

How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.


A Google search turnd up this gem::

SPEEDOMETER-MECHANICAL (ANALOG)-INDICATES HIGH SPEED AT LOW AMBIENT TEMPERATURES (ABOUT 20 DEGREES F OR -7 DEGREES C OR BELOW)- VEHICLES BUILT AFTER 4/1/90

FORD:
1990-92 ESCORT, MUSTANG, TAURUS, TEMPO

LINCOLN-MERCURY:
1990-92 SABLE, TOPAZ, TRACER

LIGHT TRUCK:
1990-91 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES
1990-92 RANGER
1991-92 EXPLORER

ISSUE:
Mechanical speedometers may indicate higher than actual vehicle speed at low ambient temperatures (about 20~ F or -7~ C or below). This occurs because of insufficient lubrication at lower temperatures. The speedometer will indicate speed correctly after the vehicle warms up.

Replace the speedometer head with a service speedometer assembly. Speedometer part numbers are identified in the Parts Block/Application Chart. Refer to the appropriate Service Manual for removal and installation details."

Quted from said TSB on AllData

 

[1991notchbackLX]

I hope you guys aren't relying on the stock tachometer for accurate RPM readings...

 

[Shaolin Crane]

Yeah, why?

 

[cardudeusa]

Mine does this aswell. So it's not the cable but a part in the dash?

 

[jrichker]

Mine does this aswell. So it's not the cable but a part in the dash?

It's the speedo head that's the problem.

I hope you guys aren't relying on the stock tachometer for accurate RPM readings...

He's right.
The stock tach isn't known for accuracy. I have a calibration test jig that proved that the stock tach may be off by 200 RPM or more.

 

[The_Dude]

How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.


A Google search turnd up this gem::

SPEEDOMETER-MECHANICAL (ANALOG)-INDICATES HIGH SPEED AT LOW AMBIENT TEMPERATURES (ABOUT 20 DEGREES F OR -7 DEGREES C OR BELOW)- VEHICLES BUILT AFTER 4/1/90

FORD:
1990-92 ESCORT, MUSTANG, TAURUS, TEMPO

LINCOLN-MERCURY:
1990-92 SABLE, TOPAZ, TRACER

LIGHT TRUCK:
1990-91 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES
1990-92 RANGER
1991-92 EXPLORER

ISSUE:
Mechanical speedometers may indicate higher than actual vehicle speed at low ambient temperatures (about 20~ F or -7~ C or below). This occurs because of insufficient lubrication at lower temperatures. The speedometer will indicate speed correctly after the vehicle warms up.

Replace the speedometer head with a service speedometer assembly. Speedometer part numbers are identified in the Parts Block/Application Chart. Refer to the appropriate Service Manual for removal and installation details."

Quted from said TSB on AllData

Very good info. Since I don't really want to change the speedo head, I'll just live with it.