On a recent trip, I use my PDA GPS to help me navigate to my destination. My GPS also measures speed. When I compared it to my speedometer, it was off. I've used this GPS on my other two cars are they match each other.
I have an 2005 6MT sedan, in 6th gear aand 3000 RPM, the speedometer reads 80 MPH, but my GPS reads 76 MPH. The question now is which is more accurate, my GPS or the G35 speedometer. I tend to believe the GPS is accurate, for starters it matches my other cars speedometer and secondly, the real time acquistition of data from the satellites to determine position should be quite accurate.
My setup is OEM, 18 inch wheels on Goodyear NCT with tire pressure at 33 PSI.
This 4 MPH discrepancy is quite a bit and I'll be inquiring with the dealer about re-calibrating the speedometer.

 

My car shows a similar discrepancy with the stock wheel/tire setup, 215/55/17. However, when I put on the summer setup 245/45/18, the speedometer and GPS virtually match. I think Infiniti purposely under calibrates the speedometer to keep us from speeding. My GPS is a Garmin 2620. BTW, my wife's car, a 2004 Saab 9-3 cabrio, shows approximately the same discrepancy as my car. My son was still able to get a ticket for doing 85 in a 65 zone!

 

Your GPS would be correct. It's rare for car speedos to be right on, usually the faster you go the greater the error becomes.

Oh, and Esfoad, I just love the pup in your Avatar .

 

Car speedos are almost never accurate. I'd say the GPS speed is correct.
Do a check on some measured miles on major highways to get a fairly close measurement.
C.

 

I noticed something similar at a much lower speed recently. In an attempt to slow drivers down on a 35 mph road on my normal route to/from work, they put up one of those displays that tells you how fast you are going. Knowing it was there I was trying to nail the speed, but when I was doing an indicated 35 mph the sign showed 33mph. So I accelerated to 37 on the speedometer and sure enough the sign showed 35mph.

 

He's 7 lbs. (3.2 Kgs) of PURE POWER. He doesn't know he's little and we're not telling him. Thanks for the nice words. I'll pass them along.

 

Do you think the dog will understand ?

 

I honestly believe some animals understand things better than some humanoids do .

 

My GPS agrees to within 1/2 mpg on my G35. It also agrees with my pickup.

Now, my previous car was up to 7 mpg off at 75.

I've compared it to those radar signs, the GPS, and mile markers/watch.

 

Oh yeah, he (and all animals, I believe) understands way more than we think.