I own a 93 Accord EX with automatic transmission. Lately, the speedometer is not consistently indicating the cars speed. Most of the time, it's showing I'm going about 10-12 mph slower than actual, but sometimes, it shows the correct speed. I've correlated the speed discrepancies with the tachometer, and with my "speed box" app on my phone. Also, the speedometer reading is off by the same amount regardless of speed, and the needle does not jump around.
The odometer seems to be functioning correctly, and the #1 fuse is OK.
Any ideas on how to get my speedometer to work properly?

 

Given you have an automatic transmission any correlation between engine RPMs and vehicle speed are coincidental at best so you may as well throw that metric out the window. That said, correlating indicated speed to a GPS type of device is a good test; I have no idea what a "speed box" app is, but many phone based apps have significant error.

Probably the easiest thing to do would be to drive along next to a friend and call him or her and relate indicated speed back and forth.

 

1. The odometer drive circuit and the speedometer drive circuit receive pulses from the vehicle speed sensor (VSS) located on the transmission.

The pulse rate increases as the car accelerates. The frequency and duration of these input pulses are measured and displayed by the speedometer. Pulses from the VSS drive the odometer stepper motor.

2. As mentioned previously, any correlation between rpm and vehicle speed are coincidental.

The tachometer senses ignition pulses from the distributor through the ignition control module (ICM). The solid-state tachometer displays these pulses as engine speed. For each 200 pulses per minute from the module, the tachometer displays 100 rpm.

3. On most older Hondas the speedometer were a little off, within 5 mph from the actual speed. There are people that asked similar questions regarding their older Hondas on other Honda forums, and the consensus is that most older Hondas' speedometer reading were a little off, generally showing faster than actual. Newer generation (e.g., 8th gen, 9th gen) Honda Accord's speedometers are more accurate.

If it was actually 10-12 mph off, then something is wrong. Are your wheel and tire sizes stock? If the wheel and sizes are proper; then it may be something wrong with the speedometer drive circuit if it was 10-12 mph off and the odometer reads accurately.