I happened to glance at my GPS today on a 60 mile roadtrip and noticed my Gps was reporting 71MPH, though my cruise control was locked on 75.

Assuming the GPS was off, I finally clocked myself for 10 miles, did the math and yes, indeed- I was travelling at 71.1 MPh. The error was relative to speed as I also tested 40 MPH and saw a 2 MPh delta.

Has anyone ever noted this discrepancy? Im am being to ****?

Id recently flashed my car from Brenspeed- so I called them and they had me confirm my revs/mile setting- but everything seemed right: 235/55ZR17 Pirellis.... 767.5 revs/min from the pirelli site - set for 770.

I havent changed anything else gears, axles, etc...

 

My speedo runs about 2MPH higher at highway speeds. I have the 18" Bullitts.Most American cars read higher by a little. I guess the tune doesn't change the speedo error that is in the mechanical system. Can you re-adjust to match your gps with the tuner?

 

could this be a difference in tire pressure or tread wear? larger/smaller tires have the same effect on your speedo as gears do, right?

 

I could go in a change the revs/mile until it matches up with the GPS.... and Im really ASSuming the GPS is more accurate.... you'd think something hard-wired to the axle would be dead nuts accurate...

 

Only if they were designed it to be. Traditionally they were always designed to read high (that way you thought you were going faster, good for sales and safety). Over a flat open road with cruise the gps is better.

 

I don't know how technical you are but this slightly complicated article might explain your GPS error...

http://www.aprs.net/vm/gps_cs.htm

And this one too...

http://www.gpsnuts.com/myGPS/GPS/Tec...uracy_by_c.htm

 

Not enough to explain 4 mph at highway speeds.
I know my gps is within 1 mph at 500mph.
There is a speedo error, not gps.

 

I noticed my speedo is 2 mph higher across the board than my Garmin C330 GPS. The GPS matches perfect to my other cars.

The strange thing is, when I'm at a stop, my speedo is not at 0, but rather at about 2mph. Almost as if the needle was slightly off when it was installed on the speedo.

No wonder my 0-60 times are as quick as the GT500....jk!!!

 

That was my first guess- but after I clocked the car over a 10 mile distance (measured the time- then extrapolated average speed) - I found the GPS was dead-on.

 

Now that you mention it, I'm thinking my speedo is reading too high, too. From the moment I got my 06 GT (about a month ago) it seemed obvoius, although I just told myself that it's such a great car that it feels like it's idling at 70 mph (except that the Saturns were passing me on the freeway).

 

Every day I pass one of those speed limit signs that say "Your speed is XX" It picks up at 40mph, the posted speed limit, and at 46 it reads SLOW DOWN"
I always happen to check my speedo to see if the two speeds match and they always do.

Maybe 40-45 isn't fast enough to pick up any descrepancy.

 

...and what exactly are you flying?

 

i've noticed exactly the same thing
my GPS is always 10KM/h slower than my speedo, regardless of the location or actual speed.
i wanna try to set cruise along a friend in another car and see if our speed matches in reality

 

Yeah, wondering the same thing. My helo won't do that...

I think the guy above was right when he said they do that intentionally. I think if it's going to be off, I'd rather it be off that way than the other. If it were reading slow, you might end up with a ticket for speeding when you thought you were fine, and then you'd have a case to argue and blame Ford.

 

thats a drag we are putting more miles on our cars that we are driving them. Not right

 

I have checked my speedo w/ 2 differnt GPS'
W/ Speedo reading 60, I am actually going 56 mph. My speedo is about 4 mph slow from 45 to 65. And about 2 - 3 mph slow below 45.
I have tried slightly different revs per mile in my SCT. That made no difference.
BTW: My '05 F350 speedo is 3 to 4 mph slow too.

 

For me, my speedo needle was off. Even with the engine off the needle would sit below zero. It wasn't until I installed my Pioneer AVIC-D1 that I could tell the discrepancy was true. To fix, I pulled out the gauge cluster, removed the needle, then replaced it in the correct position.

 

The comment above got me thinking, and I wonder if it's meant to allow the mileage to more accurately represent engine life. If you think about it, the number of miles on the car doesn't take into account all the time you sit at idle, but all that while the engine is still running, burning gas, and getting "older." Perhaps it's the manufacturer's built-in way of accounting for that. Just a thought.

 

My speedo is EXACTLY spot on with my Garmin 76CSx. It actually reads 99.6km/h when I'm doing 100km/h.

Now when I first got my tune from Doug I asked to have a future mod of 4.10 gears added well I didn't know that was the default flash! So I was going 114km/h when my speedo said 100km/h! That was a bit scary, all sorted out now

 

Hmmm- good point.

Im defnitely going to figure out the correct rev/mile adjustment and tweak this correctly with my Xcal2.

On the other hand- someone could also swing this far the other way and WAY underreport their mileage on the odometer....hmmm.