I have gotten my rich running problem fixed on my 1991 F-150. Now we have another problem. When it is cold the speedo reads really off. I was running "80" at 2,000 RPM's. I was on a highway so i set the cruise since i was keeping up with everyone and over the next 10 miles the speed slowly dropped to about 65. What kind of sending unit is used to send the speed to the dash?? Is it a cable or wired sending unit? and does anyone think that is the problem?? Oh yeah it is a 91 f150 5.8 efi e4do tranny and I am not sure about the rear gearing. there is a foot of snow on the ground and can''t get it in the garage to jack up and check.

 

A 1991 truck uses a good old fashioned mechanical speedo cable and speedo head. The cruise control and computer get vehicle speed input from a separate VSS. If you set the cruise control, and the actual vehicle speed was constant while the indicated speed varied, the fault is most likely the speedo head.

There is a Ford TSB about exactly this problem "Speedometer reads too high when cold" The "cure" is a re-designed speedometer head.

I have reproduced a portion of it below.

Article No. 92-12-9

DATE: 06-03-92

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.

ACTION:
Replace the speedometer head with a service speedometer assembly.