Just picked up my 2007 MCS two days ago and while on my way home with my wife following me noticed that the speedometer is about 5 mph off, too fast, per her speedometer and gps speed. The car came from the factory with 17" wheels and has 205/45-17" on it. Is there something in the OBD that will check or diagnose correct speed or is there something I can do to calibrate the speedometer.

 

Mine reads about 3 high. It's a feature that lets you drive fast without getting a ticket.

 

Interesting, my speedo is 3 miles slower then my gps... I tested the gps in 2 other cars and it was spot on.
I could never understand why everyone was always passing me.

 

The speed difference, actual vs. indicated, is programmed into the system. It is there on purpose.

 

My speedometer is 2 mph faster than the GPS.

 

So your saying the speedometer is intentionally off?

 

Yes, a slight MPH over-read is "built in", thus intentional. Most reported over-reads seem to fall into the 2 to 3 mph range, however (our 4 cars all fall into that range), so a 5 mph over-read might be slightly higher than average. I gather that most can live with the 2 to 3 mph over-read, but I might also seek to reduce a 5 mph over-read.

What one might do to reduce the over-read include speedometer re-calibration (is this even still done?), and mounting slightly taller (larger diameter) than OE tires.

Even with the OE sized tires, new tires with full tread depth will show a bit less over-read than worn down tires. Those who have mounted tires smaller than OE diameter experience even greater over-read.

 

That is so bizarre, Is this because the realize we are spirited drivers and they are trying to help us avoid getting speeding tickets?

 

Nope. It's to prevent the manufacturers from getting fined. In the People's Republic of Europe, auto manufacturers can be fined for producing speedometers that read slower than actual speed. So they intentionally design in an error that makes the speedometers read a little faster than actual speed.

They also tend to design the speedometers for the largest tires offered as an option on the model. So you could wind up with a combination of two errors -- a percentage error caused by smaller tires, and the built in fixed error.

My Cooper still has a 3 MPH error, but it had an additional 2 - 3% error until I got the bigger wheels and tires.

 

My ScanGauge has a reading for MPH that it gets from the engine management's computer. That tracks pretty much exactly with the GPS (this is with brand-new as-delivered stock tires). The speedometer shows 2-3 MPH faster than the SG and GPS do. The difference is 2 MPH up to 53 MPH, then 3 MPH above that. Not sure where it goes down to a 1 MPH or 0 difference--it probably does, but I haven't paid attention that low.

More evidence that the difference is very deliberate.

 

Thank you, I learn something new every day.

 

Euro regs allow for a speedo to be 0% low and 10% high. So the mfgs generally build to be about 5% high. Interestingly the specs for odometers are typically more like -2%/+2%, so they're pretty close to accurate. These two combine to give you an odd system where two instruments that work off exactly the same data (wheel speed) are deliberately inconsistent with one another.

By reg, the mfgs are not allowed to provide any way to adjust these instruments to correct these errors. With tampering such a problem, odometers/speedometers are considered pretty sacred beasts.

- Mark

 

On my Honda Gold Wing, I installed a speedohealer. You can buy them on Amazon and other places.

I do not know if you can buy one or something like it for a car

http://www.amazon.com/YAMAHA-YZF-R6-.../dp/B0064NPVGK (This is for a Yahama)

There are also other speedo correction devices for motorcycles.

 

Yep. My odometer was spot on, but is now reading 3% low since I got the larger wheels and tires.

 

Thanks guys. I hooked my gps up today to see what the real difference was and just like everybody seems to agree it was 3 mph at 60,65, 70, and 75. So what size tire can I go to and get very close to real speed? 215/45-17 or 215/40-17? Any suggestions?

 

The speed difference, actual vs. indicated, is programmed into the system.

 

Three mph over is fairly standard; not enough to change tire sizes over, in my mind anyway. Just drive 3 mph faster, like most of us do.

The 215/40 size you mentioned would be shorter than your current 205/45 tire, and the 215/45 size would only be .3" taller, so wouldn't offer much in the way of correction. The 205/50 size would be about .8" taller, but I'd be hesitant to give up that much in handling prowess; unless I really wanted a softer ride.

 

Here is a link to a tire size calculator. On my r53 I'm putting on 215 45 18 (1" larger diameter than stock and will raise the car 1/2") and it will correct my factory error of 2.5 mph. Tire rack also has a similar calculator.

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

 

compared to my GPS. The GPS does agree with my other vehicles. I have heard this is common to BMW and MINI vehicles.

 

If your speedometer is consistently 3 MPH off throughout the speed range, then changing tire size really won't help. Changing tire size makes a percentage change in speedometer readings, not a fixed change. You could calculate a tire size that will make your speedometer spot on at a certain speed, but not one that will make it accurate at all speeds.

If you want the speedo to be accurate at 70 MPH:
3 MPH ÷ 70 MPH = 0.0429
You need a tire that is 4.3% taller.

I you want it accurate at 55 MPH:
3 ÷ 55 = 0.055
You need a tire that is 5.5% taller.

Your 205/45-17s are 24.26" tall.
235/45s are about 4.3% taller.
245/45s are about 5.5% taller.

DIY tire height calculation:
Tread width (mm) x Aspect ratio ÷ 1270 + Wheel diameter (in.) = Tire height (in.)

205 x 45 ÷1270 + 17 = 24.26"