My buddy clued me into a 3 MPH underage on the Speedometer v. a GPS app on his Iphone. He has a 2008 convertible S. I checked my new to me 2011 R60 All 4S against 2 different android apps and found it to run 3 mph slow. i.e. GPS says 78, speedo reads 81. Dealer says they get 5mph tolerance. Has anyone else noticed this? Because at 81 I feel like I am flying, by people and don't want to risk a ticket.

Any way to calibrate this? What would going to 18's do?

 

that is already a known thing, nowadays many auto manufacturers do this by setting speedo 3-5 mph higher...

 

Here's a nifty calculator that will tell you what new tyres will do to your speedo.

http://www.miata.net/garage/tirecalc.html

 

Nothing, overall diameter of wheel/tire unit are almost exactly the same.

My car's speedo reads 2 mph high at 60 mph actual (using a Garmin GPS).

 

Not sure of a calibration option.

You can assess the effect of different tire sizes with a tool like this one:

http://www.miata.net/garage/tirecalc.html

that lets you enter current and planned sizes. Some minor differences in tire brand / size combinations can change the calculations slightly.

 

Mine is off by 2-3 mph. No big deal, all 4 of our cars are off a little.

 

yeah I've noticed this too. The CM is about 3-4 mpg off at 70mph. This difference can save you a speeding ticket!

 

This is a common "feature" of BMWs and MINIs (perhaps other Euro cars as well). The 2 mph over reading is a software setting for the speedometer reading in the tach display and is less accessible for the center stack speedo and cruise setting. The over-estimation is done, ostensibly, for liability issues. My BMW R1200RT motorcycle also reads 2 mph high!

You can turn off the over-estimation for the tach display if you you know how to use the NCS Expert software. I have done this and it now reads true speed, matching my Garmin Nuvi 550 GPS. However, no one has had luck finding the correct parameters to adjust the center stack display or the cruise control. That means that when my tach speedo shows I am going 60 mph, my center stack speedo reads 62 and my cruise control reports itself being set at 62 as well.

For more info on using NCS Expert, go to the R60 Electrical thread. Please read all the cautions and warnings before attempting to use NCS Expert. You can also go to the Gen 2 forum's Electrical thread and look for the NCS Directory post. You might find someone in your area that can make the change for you.

 

Interesting! I have a Scangauge installed into the OBDII port and it reports the same speed as the tach digital display. However, it can be adjusted easily, so I set it to the reported GPS speed at 60 mph (i.e a true 58 mph)

For reference the speedo on my 09 VW TDI is opposite in error; it is almost 1 mph slow at 60 mph on the GPS (speedo 59 mph = true 60 mph on GPS).