I've got a '07 R56 non-S w/ stock Conti 175/65/R15 tires. The speedo reads about 10% high (e.g. 55MPH speedo is about 50MPH actual). I need new tires so now seems like a good time to correct the speedo. Has anyone done it, or gotten close? If so, what tire brand/size worked? Keep in mind that I don't want to change the wheels.

I know this should be easy with a tire calculator, but it isn't. For example, the stock tires turn about 844 revolutions/mile. Since my speedo reads about 10% high, I need tires that are 90% of that or about 760 revolutions/mile, right? The math works out, but it doesn't seem to jibe with reality.

Why no jibe? I'm told that R56s with 205/50/16 tires have speedometer readings that are right on. According to the calculators, the correct tires make about 850 revolutions/mile (+/- 10 revs depending on brand). So here's the rub: 850 revs is about the same, and even a little higher than my stock tires at 844 revs - which is not even close to the target revs of 760. They didn't change the speedo programming for different wheel sizes did they? In discussing this with my local mechanic, he didn't think so.

I know a number of owners have complained about the speedo accuracy on cars like mine. I'm hoping someone has substantially improved the accuracy of their speedometer and doesn't mind sharing - or they can show me the error in my thinking. I'm not trying to get it "perfect" but I'd like to make the accuracy within a few percent.

Thanks
Dave

 

Dave, I feel your pain!

I also have an '07 Justacooper with a substantial speedo error. With the OEM Goodyear runflats the speedo read about 6% faster than ground speed (measured by portable GPS on three different occasions over 100 mile loop). I opted for some better tires (same brand, GS/D3 max summer performance) which had a smaller rolling circumference, hence an even greater error. No speedo shop in the country (that I contacted) would even consider 'fixing' the error, and my local dealer refused my offer of $1,000 (yep) to do it. So I made a speed differential chart. Listed posted speeds on one line, the speed required on the speedo to get it right on the next line. Taped the chart to the dash just left of the multifunction display. Works great as long as it's daytime.

What I found really annoying is that my car has the OEM nav system, a $2,100 option including a gps, which knows exactly how fast I'm going. And I had to buy ANOTHER gps to tell how fast my car was actually moving!

Anyone who has installed a speedo on a bicycle knows that there is a simple mathematical formula that accepts the rolling circumference of the tire to generate ground speed based on rpms. Somebody, somewhere, might be able to tap into the OBC and find the code for that formula. In the meantime, I suggest making the chart until you find a better answer.

And more to your point, I just bought a new set of 17" rims and put GS/D3s sized 215/45x17 on them (I LOVE these tires on the street!), and the speedo is now within about 4% or accurate. And here's a link to the Miata tire size calculator which will help you figure out the effects of different sized tires on your speedo.
Miata Tire Calculator

I sure hope this helps. Please PM me for more info.

Ed

P.S. MINI (and BMW) seem steadfast in refusing to consider speedo replacement unless the error is greater than 10%.

 

Thanks Ed. Your experience is the kind of thing I was looking for - and am hoping others will share.

I used your link to compare the approximate improvement in your tire and wheel arrangement vs. my stock 175/65/15. The results are that you should see a 2.8% improvement (which, when added to 4% would put me off by about 6.8%). I can agree with those numbers at some speeds.

In full disclosure, my speedo is off between 6% and 10%, depending on speed. Although everything should be linear in a gear-driven setup, it is not. I could build a conversion table, but I am hoping to get close enough by changing tires so I don't have to think about it.

Why isn't the Mini speedometer error linear? They probably do a conversion from RPM (engine/driveline/wheel) to MPH in software and use the same parameters for all wheel sizes and transmissions. (Just a guess)

If others don't mind sharing their experiences, please do. Also, feel free to correct me if I'm leading folks astray.

Dave

 

You're welcome, Dave. Everything I've read and heard indicates that MINI speedometers are in fact electronic, and not gear driven. If they were geared, it would be fairly straightforward to correct the error by swaping out the gears, as we used to do 'back in the day'. I must admit that I'm baffled by your speedo 'not being linear'. Every car I've had (that the speedo was off), it was off by a fairly consistent percentage, all through the range of the speedo (legal speeds, of course ) and higher speeds on the track. I did find that there was occasionally some very significant fluctuation in GPS readings in certain areas, on certain days, which is why I took the averages of three different days over the same 100 mile circuit. Interestingly, even though the speedo was off by a much larger percentage, my odometer was only of by 2 miles in 100, or 2%. Go figure.

I wish all of us luck in getting this resolved, but honestly have little hope that MINI will a) admit that there is a problem or b) correct it. I worked for a BMW motorcycle dealership for 6 years, and observed that BMW doesn't admit that there's even a problem until they have a solution in the works for it. Imagine buying a brand new $20k bike and getting a call from your dealer that you have to bring it back and can't ride it for the best three months of the riding season, since there's a potentially lethal problem with the differential that might cause it to seize up at any speed. Talk about a customer relations nightmare. But that's BMW. Occasionally bringing vehicles to market before they're thoroughly tested and debugged. Something as minor as a speedo off by less than 10% is not even on their radar.

 

My '03 was only off by 2 mph at 70, which I considered pretty good <for a MINI> given that I've read of a lot of MINIs with up to 10% error in their speedo readings. I also do not understand why BMW does this, all my other cars are spot on, including my wife's '04 Audi, our '98 Lexus ES300 and my '08 Tundra. So if all these guys can (or will) make their speedo's accurate, why doesn't BMW, and why can't we hold their feet to the fire till they do?

My '09 is off by 3 mph at 70 with the stock 16's and runflats, and it bugs the heck out of me.

I went to a 215/45 over the stock 205/45 on my '03 and did not notice a measurable difference in the speedo reading, I'm thinking the amount of change in rolling diameter needed to make it accurate is large enough that it would affect the performance of a Justa cooper, like having even taller gears. I'd find a way to live with it, or force MINI to fix their damn speedos!

 

my just a cooper is off 3 mph at 35 mph annoying. dealer says sorry no fix,so i just drive faster !!!!!

 

Mine's off 3-4mph going 65-80, not more. I haven't checked it at slow speeds. I have 205-45-17 tires and heard that if you go with 215-45-17s the speedo is spot on.

 

I also heard that a set of Kuhmo Ecsta tires at 215/45/17s were "right on" by the racer/mechanic at the Mini Dealer. But according to the Miata.net tire calculator, these "right on" tires turn 819 Revs/mile vs stock 175/65/15 tires at 842 Revs/mile - the difference is a mere 3% vs the speedometer difference of almost 10%. This shouldn't be rocket science. Others may be like me and prefer not to spend several hundred dollars buying a new set of tires only to find out they didn't help.

 

The tirerack.com web page shows that ContiProContacts175/65R15 have an overall diameter of 24". That is small for a MINI. My MCS came with 17" tires that had an O.D. of 24.5" and the speedo was optimistic. I'm not surprised that yours is off by 10%.

You heard the wrong info on 205/50-16 tires. They would be similar in O.D. to what you have. They are listed at 24.1" O.D.

I'm now running 205/55-16 tires with an O.D. of 24.9" and the speedo is much closer to reality. I think the digital is within a few % of true speed while the analog speedo around the NAV is a bit more optimistic.

How wide are your wheels? If they are 5.5" or larger, you could manage the wider 195/65-15 tires that are 25" O.D.

 

After much research, I decided to go with a 185/65R15. The reasons: good brand (Kuhmo), good price, recommended diameter (24.4"), 50k mile life. The result was that my speedometer accuracy improved about 2%, no more.

I was hoping that my math was wrong and soothsayers were right - 24.4" diameter was the magical answer to correcting my up to 10% speedo error. I should have trusted my math.

Simply put, if your speedo is off 8%, the new tire diameter should be 8% larger than your old diameter. Since the original JustaCooper tires are 24", in this example, the new tires should be 25.92". If so, your speedo will be "right on". Tires this big will probably rub, your mileage will sufferer due to the rotating weight, and the agility of the car will be compromised in favor of a smoother ride.

Heads up. You will read posts saying use the Miata tire calculator. Warning - those numbers are calculated and don't tie with manufacturers dimensions shown on Tire Rack's site. Sometimes the numbers are very different. You will read that GPS units are not accurate. Wrong - the absolute positional accuracy may be off a little, but the relative motion (MPH number) is quite accurate at a steady speed. Others will tell you that 24.4" diameter is the answer. At least for us JustaCooper owners, it aint so. Expect the speedo to read 3-4 MPH higher than actual around 50MPH with 24.4" tires. You will read posts that our stock wheel size is 6.5", but the inside of the rim is marked 5.5". I also was advised that swapping wheels was more difficult on my '07 because I'd need tire pressure monitoring on the new wheels. Again, not true, at least for my '07. I looked at every rim, no TPMS hardware.

So I spent a little under $300 to find out 24.4" didn't work. But if I had it to do all over again, I'd probably buy the same tire. These Gokarts are fun and the gas mileage is great, so for me I'd rather live with the speedo. I'll just have to do more math.

 

How do you guys measure the true speeds?

 

I'm using a Garmin Nuvi.

 

I used the measured-mile-markers on the highway and a stopwatch ... do it three times and get the same answer, and you know it's good enough.

Update: original 195/55-16s on my R56 MCS showed the speedo 3% fast; I went to 205/55-16s, which brought it to almost-dead-on.

 

For only $90 I'm tempted to go with the yellow box thing, I'm going to contact them to see if/how it would work with my '09 Clubster. I want to make sure it will change both the analog and the digital readouts......

I was tempted to just remove the speedo glass and reset the pointer, but then it and the digital readout would not be in sync and I figured that would confuse me even more!

 

Getting rid of the RFTs and going to 205/55-16 as well but they will slow the speedometer by only 1.7%.

 

MINIdave & KrautHammer - please post your results so others can zero in on a fix.

 

Removed 195/55X16 GY RFTs and installed Michelin PE2 in 205/55ZR16.

Speedo at indicated 70 was 2.4 mph too fast and now is 1.2 mph fast.

 

The GY Runflats weigh 22 lbs and are 24.4" dia. The new Michelins weigh 21 lbs and are 24.9" dia. (according to Tire Rack). The ratio of diameters predicts a couple of percent improvement in speedo accuracy, which is what you got, AND these tires, being lighter, should feel more agile. It's all good.

 

Don't believe everything you read at Tire Rack. The GoodYears were 22.6 lbs on my scale while the Michelins were 20.8 lbs.

I just posted the speeds before and after because you had asked for them.

At 32 psi the Michelins are worth every penny. Waiting for some rain to assess their ability in the wet which I expect to be good.

 

Thanks for the correction and heads up on Tire Rack rounding error. You saved almost 2 lbs/tire which is significant. I wish I'd weighed my tires before installing. Next time I take a wheel off, I'd weigh it then subtract the wheel weight.

Also, it looks like your speedo is quite close to being right on. Doing the math, the ideal tire diameter for speedo accuracy should be 25.3". This assumes your current diameter is 24.9" and speedo is reading 1.2 MPH fast.

 

Our tire weights are taken from maufacture's spec sheets. We weigh wheels ourselves, but tires are from the spec sheets.

Alex

 

Isn't that a little like asking Wall Street to self-regulate? The incentive is messed up. That could be another value-added service that Tire Rack provides.

Don't get me wrong, I love Tire Rack and have have your tires on 3 of 4 cars (the others haven't been changed yet). Given the importance of wheel/tire rotating weights on Minis, and that we have a smart and helpful guy in Alex, maybe it makes sense to keep track of the options, maybe in a table format.

If it doesn't work for you, no problem. From my interaction with TR, and a little with Alex, you guys really do a good job.