Went over the 155mph on a stock used M3. I thought 155 was the limit?

[Longboarder]

Every M3's speedo is different then, since people claim to be doing comparisons against GPS tools and finding wild differences.

I have a VBox (i.e a legit GPS device) and I get the car up to 160MPH+ (my car is de-limited with the ESS tune) legally in controlled conditions where I can actually look at the speedo and not have to worry about traffic or cops.

What I found is that my speedo is not that far off. It's around 1.5 MPH off at 80 MPH and around 3-5 MPH off at 150 MPH.

[larryn]

Tire size and tread life can greatly influence indicated speed. losing 1/2' tread, as you near tire life end, nets an inch in tire height and a lot more than that in circumference, and can easily swing the speedo +10mph from new to bald tires.

[Longboarder]

Originally Posted by larryn

Tire size and tread life can greatly influence indicated speed. losing 1/2' tread, as you near tire life end, nets an inch in tire height and a lot more than that in circumference, and can easily swing the speedo +10mph from new to bald tires.

^ ^ ^ very good point. I should add that I run 275 35 19's and saw a reduction in the delta between the speedo and VBox when I changed the tires out.

[Sapper_M3]

I've posted this before in another thread about the E9X M3's top speed, but here's my 2 cents:

This is purely anecdotal of course, but I reached an indicated 270 kph on a good stretch of autobahn. My pbox recorded me at a little over 161 mph (I had the pbox set to show me mph, and I kept the car's digital display set to kph so I didn't exceed posted speed limits).

[MMM3PWR]

Got up to 174 in my 08 sedan... Shifted to 6 too early but woulda gone further...

[FVM3]

The gps is the most accurate

[Richbot]

Originally Posted by larryn

Tire size and tread life can greatly influence indicated speed. losing 1/2' tread, as you near tire life end, nets an inch in tire height and a lot more than that in circumference, and can easily swing the speedo +10mph from new to bald tires.

The diameter can only change by at most 20/32", or 5/8". A .625" difference is less than 5mph difference at 8000 rpm in 6th gear, and hopefully nobody is going to be running a totally bald tire at 180+, so you're talking about 1/2" or less between spec and actual

As to tire slippage, a stock car isn't putting enough torque to the ground in 5th and 6th for it to be significant, and I have never slipped a tire on a rim on this car, not even hoosier a6's

[CanAutM3]

Originally Posted by Richbot

As to tire slippage, a stock car isn't putting enough torque to the ground in 5th and 6th for it to be significant, and I have never slipped a tire on a rim on this car, not even hoosier a6's

I am not talking about tire slipping on the rim, but rather the tire slip relative to the road. It is an inherent characteristic of a tire, where drive wheels spin slightly faster than the road speed due to the deformation of the tire elements as they contact the road. It has nothing to do with a burnout, the tire still maintains traction. Essentially the same concept as what creates slip angle when cornering. The slip is greatest while accelerating in the lower gears but it is still present at speed to overcome aero drag. It can equate to a few % at top speed.

[Richbot]

I would buy a few tenths of a percent but not whole percentage points. You're not asking much of the tire in terms of forward accel and the normal force is much, much greater at high speed than low speed. M3 is zero lift in the rear as well (slight downforce). GPS and gearing/rpm calculators are not dead on balls on top of eachother on my car, but they're damn close and certainly not a whole 1 % off (using the logged rpm readout of course, the tach might as well be a wild guess using your eyeballs) Maybe if I didn't have a civic M3 it'd spin the tires harder in 5th gear...

[larryn]

Originally Posted by Richbot

The diameter can only change by at most 20/32", or 5/8". A .625" difference is less than 5mph difference at 8000 rpm in 6th gear, and hopefully nobody is going to be running a totally bald tire at 180+, so you're talking about 1/2" or less between spec and actual

As to tire slippage, a stock car isn't putting enough torque to the ground in 5th and 6th for it to be significant, and I have never slipped a tire on a rim on this car, not even hoosier a6's

Hi Rich, long time no see. We did ED on the same day back in 2005!

You might be forgetting that there are very significant circumference variations between tire brands as well.So, you might be starting out with a smaller tire to begin with. Couple that with a near spent tire, and you have more than your variation, and easily

- A 275/35/19 has a variation between 81.3" and 83.6" in circumference during it's life.
- A 265/35/18 has a variation between 77.2" and 79.5" in circumference during it's life.

Taking the two extremes, both are within the variation of the M3's speedo calibration. That means comparing 77.2" to 83.6", which is a 8.4 percent variation, and at 155 indicated, you are at 13mph variation.

This is taking into account your assumption that people change their tires before the wear bars, which we all know doesn't always happen.

[CanAutM3]

Originally Posted by Richbot

I would buy a few tenths of a percent but not whole percentage points. You're not asking much of the tire in terms of forward accel and the normal force is much, much greater at high speed than low speed. M3 is zero lift in the rear as well (slight downforce). GPS and gearing/rpm calculators are not dead on balls on top of eachother on my car, but they're damn close and certainly not a whole 1 % off (using the logged rpm readout of course, the tach might as well be a wild guess using your eyeballs) Maybe if I didn't have a civic M3 it'd spin the tires harder in 5th gear...

You are correct, I overestimated slip. Going back to my literature, tire slip at top speed on the M3 would be around 0.5%.

[Richbot]

Originally Posted by larryn

Hi Rich, long time no see. We did ED on the same day back in 2005!

You might be forgetting that there are very significant circumference variations between tire brands as well.So, you might be starting out with a smaller tire to begin with. Couple that with a near spent tire, and you have more than your variation, and easily

- A 275/35/19 has a variation between 81.3" and 83.6" in circumference during it's life.
- A 265/35/18 has a variation between 77.2" and 79.5" in circumference during it's life.

Taking the two extremes, both are within the variation of the M3's speedo calibration. That means comparing 77.2" to 83.6", which is a 8.4 percent variation, and at 155 indicated, you are at 13mph variation.

This is taking into account your assumption that people change their tires before the wear bars, which we all know doesn't always happen.

No we didnt, I must have a doppelgänger

I do see what you were saying, but I'm saying tire diameter is easily determined for verification purposes (and it does not change by an inch over the life of the tire, circumference yes and then some, but even truck tires are only molded with 15/32" or so of tread)

Of course a different pr worn tire can and does affect when the stock limiter kicks in, but will not affect the calculation, tire to tire variaton is irrelevant, you use the diameter of the tire you have to do the calculation, which can be measured by rolling it across the floor. unless you are trusting the internet more than direct measurement to provide your numbers (not something I do with any regularity, it's the internet after all)

[Richbot]

Here's a dumb question, where does the ecu get its speed signal for limiter purposes? Iirc there is no single vehicle speed sensor and it's a calculated value from some or all of the wheel speed sensors? If it's based on the 25.7" tire on the fromt of the M3, that could explain the 26.3" rear tire carrying the car past the 155 limiter by about 6mph to the 161 many have observed in spite of the limiter setpoint in the ecu? If it comes from the rear wheel speed sensors, maybe their outputs are calibrated or otherwise manipulated based on the shorter front tire, or even the oem winter tire fitment at around 25.3"?

[Sleeper519]

I'm just gonna get some 13" rims so I can go 200 mph...

[armyav8tor]

[k-lo]

Originally Posted by armyav8tor

sweet!

[happos2]

[QUOTE=Richbot;14188564]No we didnt, I must have a doppelgänger

I do see what you were saying, but I'm saying tire diameter is easily determined for verification purposes (and it does not change by an inch over the life of the tire, circumference yes and then some, but even truck tires are only molded with 15/32" or so of tread)

[QUOTE]

I don't know enough about this stuff to be too helpful...but 15/32" my thoughts:

15/32 of tread contacting the ground (radius), but also have to account what is on top when talking diameters. That is close to 1" total on the Diameter . Granted there is a few 32nds left at the wear bar and as stated, hopefully nobody is doing 150+ on tires beow wear bars.

[JJ_01]

273 kmh on speedo = 258 kmh on GPS

I was really happy the first time I saw this since my old 330i E90 was doing 250 kmh on speedo and only 338 kmh on GPS (man I was angry when I saw this).

Other measurements for M3 (got only so far, need to do them all just to be sure if it really is percentage):
Speedo/GPS (kmh)
90/87
100/97
110/106
120/115
130/124
140/134
150/144
160/155