I decided to buy a Garmin Nuvi for my trip from SoCal to Colorado and back, mostly for navigation, but also to check speedo accuracy. Not sure how accurate GPS is for speed, but seems pretty spot-on to me. I am running 205/50/16's versus the 195/55/16 stock tires, so there is a difffrence of 1 MPH at 65....65 indicated is actually 64, according to Tirerack.com. Assuming this is correct, and my GPS is accurate, here are my findings, corrected 1 MPH for the tire size difference. And I am sure that the speed difference for tire size variation is a factor at higher speeds. I am not a math major, but here is my personal experience:

75 GPS, 77 indicated.
85 GPS, 88 indicated.
95 GPS, 99 indicated.
105 GPS, 109 indicated.
115 GPS, 119 indicated.
125 GPS, 130 indicated.

That's the furthest I pushed her. In 6th at 125 GPS, she was turning about 5200 RPM's, and the speed was slower to build. I am sure I could have gotten to 140, but sustained high-rpm engine damage was not anything I wanted to experience. I must say that at 125, she felt PLANTED and solid. No wandering, no twitching, nothing scary at all! My wife didn't even freak out. She was amazed at how stable this little car was at 125. I was, as well!

This speed run was done on west-bound Interstate 10 in Arizona, 10/10/15, perfect daytime visibility, ambient temp of 96, unsure of elevation. I made sure to have clear road, and plenty of vision of the road ahead. Speed runs were done in stages, not on one run. The LAST thing anyone needs is someone changing into the fast lane at 70 or so in front of you while you are blasting along at 125! That could lead to a HUGE mess!

I do not condone nor approve of ANY of my driving adventures. BUT....I did it, and thought I would throw some info out there.

 

Your Garmin is more accurate than your Mini. What is strange is that the European models I have rented were a lot more accurate. I recently changed my tires and it improved a little but I am still off 2 at 50, 3 MPH at 65, and 4 at 80.

Go an extra step and reset your trip meter in the Mini and on your Garman. This will tell you how off your odometer is.

As far as how "planted" you Mini felt at 125 a little crosswind gust would have given you a reality check. I have pushed my German rentals into that range but I backed down to 100 where it felt a lot more stable in crosswinds and was not affected quite as much.

 

Boy you aren't kidding, crosswinds can change everything in an instant especially if you're coming around and out of a mountain pass and come out into a crosswind! It's like torque steer at 100 mph or should I say 97-99! Pass go but do not go directly to jail!

 

I had the opposite feeling in my 79 camaro, was out in the middle of the high plains and could see straight for 10 miles or more so I hammered it, at about 130 my steering got so light I thought I was on ICE, way too much body lift. Then step forward to my 95 Transam , I could cruise at 155 all day as it was planted, much better aero design with a functional rear blade.

My clubman feel good at 130-140 but the engine is working so hard it's hard to enjoy the speed vs my TA that had .5 6th gear that worked well at speed.

 

Those winds in the Colorado canyons are no joke. I did the 15-18 hour drive from Denver to OC area and going up through the canyon roads on I-70 was a nightmare as there were high wind advisories. Some guy was towing a motor home with his F250 and the wind picked it up as it was so light. Ended up slamming back onto the ground and turning the truck all the way 180 degrees.

 

Aerodynamics play a huge part in high speed driving. A Mini has a shape similar to a brick with slightly rounded edges and no extra downforce. My 74 Corvette has rather sloppy handling characteristics below 60 mph. It starts to tighten up at 65 and gets better the faster it goes. It tops out at 120 and runs like it was on rails 85 and above.

 

The speedo thing is really well known, and documented on dozens of threads around here.

If you check the speed on the OBD2 port, you may find that this reading actually tracks the GPS a lot closer than the actual gauge does. At least, it does in my car...

 

The speedo thing is really well known, and documented on dozens of threads around here.

yep . . .