Speedometer reads 10% too high
 

Hi. Since I bought my used 370Z several years ago, it's been bugging me that my speedo is way off from any readings on my GPS. I have checked it with Google Maps on three separate mobiles. It's around 10% (maybe a bit less) too high. Speedo shows 100km/h and my GPS shows 92-93km/h. Do these readings happen to other Z owners? What makes gives me a bit of a clue is that the previous owner equipped Z with aftermarket rims (Work). But the tyre profile and rim size is like stock : 275/35ZR/19 on rear and 235/40ZR/19 on front. Could my rims differ (although too 19") so much from stock 19" that it has different diameter, thus false reading?

The wheels don't matter it's the overall height of the tire that determines speedo error. Keep in mind not all brands size is exactly the same. A 275/35/19 could be 27.4" high for Firestone but 27.9" for Michelin.

I think that you’ll find all the 370s do this.
I was told that Nissan were advised to set the speedometer 10% more than what it should be to reduce speeding.
So when your Speedo reads 55 you’re really only doing 50, 80 is 72, 100 is 90 and so on.

Normal for high speedo readings
 

I agree and most cars read 2-4 km high on average. After that it's most likely tire height that is responsible. My 2016 is 2 kph high with a new set of Yokohama AS and was the same with the original. Yours seems a tad high. Hope that helps

Fun Fact:
If your 370 has navigation, the speedometer is *exactly* the same as a GPS app.
The navigations system uses GPS position updates to correct the "wheel speed" to true vehicle speed, and this is what the speedometer shows.

When I compare wheel speed to vehicle speed there is a 3-5% difference, depending on summer or winter tires.

My speedo reads dead on with GPS running 305/35/19's in the rear.

Mine also reads dead on with stock 245/45/18s.

I forgot to thank you all for replying. This thread was unanswered for a few days so I stopped looking. But I see there are a lot of contradictionary statements (it'ok / it's not) :)
My main concern is if it messes with traction control/abs? Also, is it possible that two 19" rims could have different height with the same tire? I've noticed I didn't mention. My tyres are Michelin 4S with the same profile as touring edition.

IF you're abs/trac. light isn't coming on during normal driving your size combo is fine.

Generally, the incorrect speed reading will not affect the abs or traction control. They are both read off the axle splines near the differential. Abs or traction control will kick in if there's a mismatch between the wheels or axes. It doesn't care if all wheels are spinning faster or slower due to tyre size (let's say all wheels 255/40/19 instead of 275/35/19) as there is no absolute reference, like a gps to compare it with

I agree. 2-4kph on the high side is normal. Frankly 8-10 is too much in MHO. See if speedo can be recalibrated proding that you are sure tire height is per factory specs. Let know what happens.