Alright, why is my speedo off?
 

This is on my new(to me) '90 F150 Lariat (4x2). It has stock sized (235/75/15) Michelin LTX M/S on it (that is stock size, isn't it?). The speedo is reading faster than I'm actually going. By gauging against mile markers along the interstate, it appears that when reading 70mph, I'm actually going just under 65 (takes ~52sec to eclipse one mile - If my math is correct, @ 65mph, it should take 50sec to go one mile).
Going slower, speedo reading 65, it seems to be closer, but still not accurate - takes 54sec to go one mile which should be between 60 and 65.

I got Pkrwud to translate my door codes, and I am supposed to have 3.55 gears. Anyone know what driven speedo gear is correct for 3.55s and an AOD?

Alternatively, I remember reading in the owners manual of my '94 F150 that I had some years ago that the speedo can be recalibrated (like when changing tire sizes) - Is it possible that my truck had bigger tires on it prior to my purchasing it and the speedo was re-cal'd for them, but now needs to be re-cal'd back? (The tires on it now are very new - apparently replaced only a few thousand miles before I got it)

Am I anywhere close with my ideas? Anyone have any others for me, or can verify mine?

TIA,

Wade

Hmmmm, maybe its not off.........
 

I just found this info on the net;

The table below shows some examples with time in seconds:

Time in seconds to travel:
1 mile 2 miles
55 mph 65.45 130.0
60 mph 60.0 120.0
65 mph 55.38 110.77
70 mph 51.43 102.86
75 mph 48.0 96.0
Example: You passenger measures 56 seconds between mile post signs as you drive at 60 mph as indicated by your speedometer. Your actual speed is 3600 / 56 = 64.28 mph.

Those numbers are actually near-dead-on what I was getting while driving....... I guess maybe I'm just not used to the difference in ride of the F150 vs my Mustang and the wife's Sable. :confused: :D

Wade

The formula is:

3600/ time to drive 1 mile = MPH

or

3600/mph = time to drive 1 mile