I just thought I would note that our speedometers might not be very accurate.

I hooked up my portable GPS (Garmin Streetpilot c330) in my 07 G35X. The GPS will give your exact speed over ground.
So at 100km/h (62.14mph) on my speedometer I was really doing 95km/h (59mph) on the Garmin GPS.
It had a 5km/h (3.1mph) error. I am running Blizzak WS-50 snows in exactly the same size as the stock tires and according to the manufacturer's specs they are the same diameter as the stock tires that came with the car.
This might not seem significant to a lot of people but if you are using the cars computer readout for MPG figures, then it is all wrong, or if you are calculating it by the amount of fuel you put in and the odometer reading it will also be inaccurate since your odometer is not accurate.

Just thought I would mention this.

 

That's interesting Wayne. I take it your '05 was accurate. Mine seems to be just about exact. It might read a tiny bit fast in the car.

Is it possible your new tires are a bit smaller and might be making part of the difference?

 

I tested my Garmin in my cars and usually find a 1 -2.5 mph optimistic reading.

 

That's correct Deane, when I did the same checks on my 05 G35X it was very close, almost exact.
There might be a little bit of difference between manufacturers, but when you are using the same p-metric size I would expect your speed to be a bit closer than what I found, I thought 5km/h to be quite a bit.

 

I've noticed my speedo reads a little slower than a couple of the radar speed signs we have all over the place out here too.

 

All cars are designed from the manufacturer to have a 3-5% overestimation to slow drivers down.

 

My experience has been that the speedometer gauges always read a little faster than your actual speed by a few percent, but that the odometer readings and the speeds and distances measured by the trip computers are pretty much dead on.

I always just assumed the car manufacturors didn't want you to be able to complain that you got a ticket when your speedometer said you were going the speed limit.

 

Well guys I have you all beat. I have the 07 sport 5AT with Blizzak WS-50's and appropriate rims that I bought from the Infiniti Dealership and I am always reading 9-11 km/h faster in the car than another car at the same speed or from my portable GPS.... Annoying. I have to get it serviced with the dealership. I also have the same problem with the summer tires...

 

Wow, that is a lot. I thought mine was extreme since it reads 5 km/h fast.
What size are your WS-50's?

 

Well I did another test on my 07 G35X yesterday on a trip. I was checking the speed with my portable Garmin GPS. I did this over a longer run than previously, and set my speed with the CC.
Like I said previously, the car came with 225/55/17, and for the winter I am running the same size Blizzak WS-50.

Here are my results:
Speedometer Reading.................Actual reading(GPS)
80 km/h (49.7 mph).....................75 km/h (46.6 mph)
100 km/h (62.1 mph)................... 94 km/h (58.4 mph)
120 km/h (74.56 mph)..................113 km/h (70.2 mph)

As you can see it is quite an error, the faster you go the greater the error. It will knock your mileage figures all to heck too.
Just thought I would bring this to your attention.

 

I replaced the stock Goodyear’s with a set of Michelin’s the first week I had my car. My old car had the same exact tire size 225/55 17 and I just bought them so I swapped. Lucky for me, or so it seemed. My car seemed to be going slower that my speedo say so I used my Garmin and checked. My speedometer was then off by 1.4 MPH. I now have my new tires and wheel 245/45 18 and the speedometer is accurate again. I've learned that each tire manufacturer has the discretion to make their tires any size they want. My guess is your snow tires are smaller than stock in circumference.

www.tirerack.com has a good page which shows the specifics on each tire, and I think I saw your snow tire listed. You might check it out.

Here an off topic thought. Any reason why a tires uses such an unusual combination of measurements? I wonder the origin for using inches and mm together. 225/55 17 = 225mm / 55% 17 inches.

 

I think I noticed this on my wife's 07 Sedan as well. My Speedo on my 03 shows 60MPH while hers shows about 64mph when we're driving next to each other at the same speed... but I'm not sure. Is there a way for the Dealership to test this?

 

As I stated above, all cars a designed from the manufacturer to have a certain overstatement of speed to slow drivers down. 4mph at ~70mph is not a huge error.

 

Geesh - I'm not sure GPS is what you want to use to measure MPH.
Those local radar machines are +/- 2 MPH and are +/- 2 MPH with my speedo. (not the bathing suit).
A DYNO even at low speeds might answer the question...
Or maybe go 100 MPH and then ask the officer what you were doing !??

 

If his unit uses a Sirf III chipset, you could not be more wrong.
This will give you a far more accurate reading than what you would get from the car dash or the "you are going too fast" radar boards.