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      07-11-2014, 11:35 AM   #1
wrsbmw
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The supposed 155mph limiter

Well after making measurements with the Vbox that I bought and having a documented speed comparison at exactly 155mph, I conclude that the speed limiter doesn't limit the car to 155mph.

I posted this at the back of a thread on the mods forum but it's probably worthy of a general discussion.

From what I can tell, the difference in real speed and speedo speed at 160mph is 5mph. I got this from the Vbox data which showed that the car was going exactly 155mph when I saw 160 on the HUD. This is when I slowed the car down on the dead empty completely straight road I accidentally ended up on going home on Wednesday. I made a wrong turn in a small town in west texas and took a 17 mile detour and when I discovered the mistake, the GPS showed me a road to get back on track that didn't involve retracing my steps. It turned out this road was nearly perfect for speed testing except it's not completely flat and it's not a real good road, it's a FM road in west Texas. Anyway, I hooked up the Vbox and did several quarter miles which I posted over in the mods forum. On one of them I just kept going until the road/car got a little to wiggly for me to feel good about testing the top end on the car. I was hoping to get back to 168 or 169 as I did on the way out and see what the Vbox said was the GPS measured speed. The road felt too bouncy/floaty/unstable at 160 so I shut it down. That turned out to be exactly 155 as measured by the Vbox. This along with the RPM data I observed at 168 on the way out leads me to conclude that the limiter isn't exact because my car was going probably 163 on the way out when it showed 168. I confirmed this by doing a RPM to speed calculation based on the wheel circumference of 7.06 feet and 5th gear ratio of 3.15.

I also have spoken to someone who is knowledgeable of the attempts to crack the ECU on the M5 and he says that there are five things the ECU looks at to apply the limiter. So whatever they are, they don't all kick in on my car at 155.

Anyone else had their car over 155 on the speedo?
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      07-11-2014, 11:58 AM   #2
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Speedos has error there is no doubt, but how much?
This has been discussed several times and for no reason I feel it must be 2-3 % (exactly like what you got 155 vs 160); however some members claim UP TO 10% !... different numbers from car to car. I saw a guy posted the dealer document that UP TO 10% + 2-3 mph is normal and they are not gonna correct it if it's in this range. I think I downloaded that letter's pic and will see if I have it at home, will post it here later...
and I feel the limiter is abit more than 155 mph... BMW is generous lol
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      07-11-2014, 12:08 PM   #3
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Can tire wear have an effect on the tire height and thus make it inexact because each car is slightly different?
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      07-11-2014, 12:29 PM   #4
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Quote:
Originally Posted by Tom C View Post
Can tire wear have an effect on the tire height and thus make it inexact because each car is slightly different?
Some, but not a huge difference. I calculated 1/4" of wear would give 164mph at 6500 rpm in 5th vs 165.5 for a new tire which mine are.

The difference of 26.97" diameter vs. 26.72".
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      07-11-2014, 12:34 PM   #5
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My 2014 M6 CP tops out at 159 GPS but the limiter def kicked in at exactly 155 mph GPS it was just accelerating so hard it continued to 159 .
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      07-11-2014, 12:44 PM   #6
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Quote:
Originally Posted by M6-Coupe
Speedos has error there is no doubt, but how much?
This has been discussed several times and for no reason I feel it must be 2-3 % (exactly like what you got 155 vs 160); however some members claim UP TO 10% !... different numbers from car to car. I saw a guy posted the dealer document that UP TO 10% + 2-3 mph is normal and they are not gonna correct it if it's in this range. I think I downloaded that letter's pic and will see if I have it at home, will post it here later...
and I feel the limiter is abit more than 155 mph... BMW is generous lol
The manufacturer is only required to have the speedometer read within 10% accuracy as long as it always reads more than the actual speed . BMW def shoots for around 5% which at 159 mph is 167mph which is exactly what my speedo showed .
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      07-11-2014, 12:54 PM   #7
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      07-11-2014, 12:58 PM   #8
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Quote:
Originally Posted by gmd2003 View Post
The manufacturer is only required to have the speedometer read within 10% accuracy as long as it always reads more than the actual speed . BMW def shoots for around 5% which at 159 mph is 167mph which is exactly what my speedo showed .
Well that is different than mine. I definitely saw 160 and then hit the brakes on the Vbox run. The Vbox shows 155 as the max speed which would be a 3% error. You can see I hit the brakes pretty hard and I did it right at 160 because the car felt like it was sliding around.
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      07-11-2014, 01:00 PM   #9
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Quote:
Originally Posted by wrsbmw
Quote:
Originally Posted by gmd2003 View Post
The manufacturer is only required to have the speedometer read within 10% accuracy as long as it always reads more than the actual speed . BMW def shoots for around 5% which at 159 mph is 167mph which is exactly what my speedo showed .
Well that is different than mine. I definitely saw 160 and then hit the brakes on the Vbox run. The Vbox shows 155 as the max speed which would be a 3% error. You can see I hit the brakes pretty hard and I did it right at 160 because the car felt like it was sliding around.
That's within reason for sure for speedo error but it does seem the soft limiter is at exactly 155 mph GPS .
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      07-11-2014, 01:14 PM   #10
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Quote:
Originally Posted by gmd2003 View Post
That's within reason for sure for speedo error but it does seem the soft limiter is at exactly 155 mph GPS .
Well I guess it makes sense that it starts to dial back the power output at that speed but it can't do it instantaneously. Hence, I should get a higher GPS speed with the Dinan tune installed because the car should have greater acceleration when 155mph is hit. Did you have the BMS box on when you made your measurement ?

I am thinking that the run in this graph will make the perfect before and after comparison for the Dinan tune. The difference will show up in the slopes of the speed graph.
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      07-11-2014, 01:32 PM   #11
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I saw 168 on the HUD when in Germany last month and there was no limiter at that speed.
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      07-11-2014, 03:11 PM   #12
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Quote:
Originally Posted by Butty45
I saw 168 on the HUD when in Germany last month and there was no limiter at that speed.
An indicated 168mph is within the allowable tolerance of error.
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