I've been looking at this issue for a while now trying to figure out how to work with this problem. I've verified all this with two GPS units. GPS units agree with each other, so I'll take them as being correct.

If I want to go 35MPH, my speedometer needs to say 39MPH.
If I want to go 40MPH, my speedometer needs to say 43MPH.
If I want to go 50MPH, my speedometer needs to say 53MPH.
Once I hit 60MPH, it is pretty much dead on.

I have stock 3.73 gears rolling on stock 235/75r16 tires. No modifications to the drive train that I know of.

This past weekend I put a new speed sensor in, but that has not changed anything.

I've tried connecting my scan tool to see if I can read my speed from the computer, but that data is not available.

If I were going faster than what my speedometer was saying, I might have an argument that my gauge was off and was having a hard time raising the needle...but that's not happening.

I need some more ideas.

 

I've had the similar issue of my speedometer being off but by a much larger range and from my experience if your only off by 4MPH then it is probably the gps someone correct me if I'm wrong but your car speedometer is based off wheel rotations on the pavement where as your gps is based off of satelites which are not as accurate

 

I've had three different gps' in my truck as well as my other vehicles. All of them have told me that my Ranger's speedo is off when I get above 50mph and the rest of the vehicles are correct. I just try to remember that when I'm driving over 50 so that I'm still doing the correct speed and not pissing off other drivers by driving too slow.

 

I think that is about normal for a Ranger because mine is very similar. The faster I go the closer it is to being correct. I have 4.10 gears with 31" tires as it came from the factory. With 32" tires the speedometer is almost dead on at 70 mph but reading a little fast with 31" tires. At slower speeds the 32" the speedometer is still fast with 32" tires but not as bad as with 31" tires.

Mine was also verified with GPS.

 

tire wear is also another thing to consider too. also 3MPH off isnt bad at all, hell, alot of vehicles come from the factory that way

 

tires aren't new, but still have quite a bit of tread on them yet.

Most of my driving is in the city, so 35 is usually where I'm at and there are two speed traps I go past every day. Very nerve racking to run past a cop looking for speeders and you are pushing 40. (I've had LOTS of tickets in the past and just don't want any more)

 

Remind me to stay away from Kansas if you get stopped for 5MPH over there

 

any wear at all will leave youre speedo off by a percentage. 99% of police allow a little leaway to make up for speedos that arent dead on. its very common. when my last set of tires were new i calibrated the speedo with my GPS and my tuner, it was within .5 MPH but right before i replaced them the speedo was off almost 5 MPH due to the tire wear

 

I've been pulled over 3 years ago for 7 over


what is really getting me is why it is off on one end of the gauge...but not the other. It just doesn't seem right.

 

Vehicles (in general) usually come within 2~5% fast from the factory. So you should not be worrying about anything. You should worry when you speedometer reads slow (such as a change to a larger tire), because you are going faster than read.

Also FYI, speedometers work off of the rotation of the tire as mentioned before, and they are not accurate at all speeds. They are calibrated at highway speeds, so reading fast is completely acceptable. Calibrating it at a lower speed will only mess up the calibration at a higher speed, and you would only be looking for trouble then.

My recommendation is to live with it. You are better off this way.