Speedometer vs Real Speed

[RobertM]

Vehicle : 1999 Ranger, V6 3.0L Flex (VIN V) Automatic (4R44E). Mfr date - 9/98
Speedometer is off on it's reading in comparison to true speed. The tires are per the door label specs. The problem existed before and after replacing the transmission, with the same reading, no change.
Here is the scenario. Read 30 mph .... GPS and Radar 25 mph
Read 34 mph .... GPS and Radar 30 mph
" 43 mph .... " " 40 mph
" 47 mph .... " " 45 mph
" 50 mph .... " " 51 mph
" 55 mph 55 mph
61 mph 60 mph
67 mph 65 mph
73 mph 70 mph
I cannot figure out why this would be occuring. The gear cannot be the issue, since each tranny had their own and they were not swapped. I have no codes, runs good. I just have to do manual calculations to avoid tickets, you might say.
Any advice on what the problem might be?

 

[ab_slack]

That is small differences in all instances except for the 5mph down around 25mph, but the linearity goes to heck at the low speeds so big error there is no surprise to me.

Every vehicle where I have compared GPS speed indications with the speedometer in the past 15 years shows the speedometer indicating 3% to 5% than the GPS. While GPS can get flakey I tend to think it is fairly accurate.

I don't know this to be a fact, but I suspect regulations require a certain accuracy that allows a speedometer tolerance of something like -0% to + 7%. Thus a properly working speedometer will never read lower than real speed. That they always will read slightly higher and even not be far off even if slightly larger tires used.

 

[Road Angels]

speedo

Do a mile marker check see if your odometer is correct, if not start looking at the speed sensor

 

[oldcrowalki]

You say you have to manually calculate to avoid tickets, but except for 50 mph, the speedometer is reading higher mph than what you're really going----so if it were me I'd probably just leave it because it's actually helping you avoid tickets.

 

[kryptonitecb]

All speedometers are off even when band new. If you have a gps use it to verify your actual speed. Unless you have the tools to calibrate the speedo it will always be that way.

Sent from my SAMSUNG-SGH-I547 using Tapatalk

 

[RonD]

GPS works by time and distance, so is more accurate in general than a speedometer and is more accurate at higher speeds(greater distance traveled)
But it can be off when going up or down hills, or lots of turns, because you are moving in two directions.

General speedometer design is that the speedometer should always read higher than actual speed, so any error between real speed and shown speed will not lead to a ticket or going into a corner too fast.
So what you see is "normal" on all speedometers.
You can pay to have speedometers calibrated, auto makers won't, but you can

 

[pcollins]

My trucks only got 20k miles. Same thing. Thats just the way they make 'em.