speedometer runs fast when cold

The_Dude · Oct 31, 2011

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 · Oct 31, 2011

Makes no Damn sense to me seeing how its mechanical

Shaolin Crane · Oct 31, 2011

Lay off the pipe?

Mintsick · Nov 1, 2011

And you determined this how?

The_Dude · Nov 1, 2011

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 · Nov 1, 2011

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

That could explain the diff between rpms/speed.

The_Dude · Nov 1, 2011

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 · Nov 1, 2011

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 · Nov 1, 2011

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

Adams91LX · Nov 1, 2011

swap the cable... repeat test?

stykthyn · Nov 1, 2011

The_Dude said:
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 · Nov 1, 2011

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 · Nov 1, 2011

Shaolin Crane said:
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 · Nov 1, 2011

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

jrichker · Nov 2, 2011

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 · Nov 2, 2011

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

Shaolin Crane · Nov 2, 2011

Yeah, why?

cardudeusa · Nov 2, 2011

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

jrichker · Nov 2, 2011

cardudeusa said:
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.

1991notchbackLX said:
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 · Nov 2, 2011

jrichker said:
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.

speedometer runs fast when cold

The_Dude

Member

srtthis

the guy doing it does every local racers rear end

Shaolin Crane

Banned

Mintsick

The_Dude

Member

Boosted92LX

It's only an inch or two. What's the big deal?

The_Dude

Member

Mintsick

Shaolin Crane

Banned

Adams91LX

Active Member

stykthyn

I want to measure mine. It doesn't look that tall.

BlackGT89

Member

Mintsick

Shaolin Crane

Banned

jrichker

StangNet's favorite TOOL

1991notchbackLX

Active Member

Shaolin Crane

Banned

cardudeusa

jrichker

StangNet's favorite TOOL

The_Dude

Member

Similar threads

speedometer runs fast when cold

Member

the guy doing it does every local racers rear end

Banned

Member

It's only an inch or two. What's the big deal?

Member

Banned

Active Member

I want to measure mine. It doesn't look that tall.

Member

Banned

StangNet's favorite TOOL

Active Member

Banned

StangNet's favorite TOOL

Member

Similar threads

Privacy & Transparency

Privacy & Transparency