Cooper S Built-in Speedo ERROR of 4% n OE Tire/Wheels
Cooper S Built-in Speedo ERROR of 4% n OE Tire/Wheels
I have spent about 30 min. searching this Forum but have not yet seen reference to built-in speedo error of up to 4% on OE wheel/tire combos.
According to Tirerack info on OE wheel/tires, (http://www.tirerack.com/tires/Select...ber+Valve+Stem)
OE 195/55R16 diameter is 24.5", and OE 205/45R17 diameter is 23.5 - a difference of about 4% or 2.4 mph.
Do we know which is the sizing is used to set up the speedo (probably the 16"???) - or did they set it right in the middle of the two.
I find it strange that is a car where so much attention is paid to performance detail there is a discrepency which exceeds often cited rule of thumbs concerning acceptable changes in OE diameter.
Especially when you consider the case of someone (using the OE 17" diameter as their reference point) deviating 5% from the 17" 23.5 - they could actualy end up 8-9% away from the 16" 24.5.
Do we have a definitive conclusion on this issue?
tim
According to Tirerack info on OE wheel/tires, (http://www.tirerack.com/tires/Select...ber+Valve+Stem)
OE 195/55R16 diameter is 24.5", and OE 205/45R17 diameter is 23.5 - a difference of about 4% or 2.4 mph.
Do we know which is the sizing is used to set up the speedo (probably the 16"???) - or did they set it right in the middle of the two.
I find it strange that is a car where so much attention is paid to performance detail there is a discrepency which exceeds often cited rule of thumbs concerning acceptable changes in OE diameter.
Especially when you consider the case of someone (using the OE 17" diameter as their reference point) deviating 5% from the 17" 23.5 - they could actualy end up 8-9% away from the 16" 24.5.
Do we have a definitive conclusion on this issue?
tim
5th Gear
Joined: Nov 2003
Posts: 628
Likes: 0
From: Indianapolis
The speedo reads several percent higher than the OBDII reading (which is what the car's ECM thinks the vehicle speed is), so BMW is deliberately giving you a higher speed than you are actually going with the OEM tires. They do this so owners can't use speedometer accuracy as an excuse for speeding.
Scott
90SM
Scott
90SM
Useful Replies
Thanks for the info!
Now - does anyone have a "definite" answer?
Moderator?
As we all know, we can go 82 mph on a fairly busy hwy (Mass Pike./ NY Thruway) without a ticket - but you'll get nabbed at 84 mph.
Wheel/tire diameter affects top speed, gearing, warranty (you might get to 50K sooner
), gas consumption, and car lowering.
Is our MINI speedo calibrated to 24.5 " or 23.5" ?
:impatient
Now - does anyone have a "definite" answer?
Moderator?
As we all know, we can go 82 mph on a fairly busy hwy (Mass Pike./ NY Thruway) without a ticket - but you'll get nabbed at 84 mph.
Wheel/tire diameter affects top speed, gearing, warranty (you might get to 50K sooner
), gas consumption, and car lowering.Is our MINI speedo calibrated to 24.5 " or 23.5" ?
:impatient
Last edited by irishpunk; Oct 6, 2007 at 06:06 PM.
6th Gear
Joined: Jan 2006
Posts: 1,705
Likes: 3
From: Chattanooga, TN
Almost all cars under report speed, that way manufacturers don't have to worry about getting blame for your speeding ticket.
I have 215/40-17 tires and that gives me a 6% speed error, GPS checked.
The best way to find your true error rate is measure with GPS and learn to live with it.
I have 215/40-17 tires and that gives me a 6% speed error, GPS checked.
The best way to find your true error rate is measure with GPS and learn to live with it.
2nd Gear
Joined: Feb 2007
Posts: 54
Likes: 0
I'm going to have to take issue with the calculated tire diameters. I created a spreadsheet a while back to help me understand the effect on tire diameter of changing tire sizes. For the 3 standard OEM sizes I get the following calculated values, which are confirmed by checking the respective sizes on the Tire Rack's web site:
205/45-17 => 24.26" (100.0%)
195/55-16 => 24.44" (100.7%)
175/65-15 => 23.96" (98.7%)
Not so much variation as it turns out.
Alan
205/45-17 => 24.26" (100.0%)
195/55-16 => 24.44" (100.7%)
175/65-15 => 23.96" (98.7%)
Not so much variation as it turns out.
Alan
Good Work
Alan,
Those were the figures I pulled off TireRack today (just follow the link I gave above).
Call me lazy, I thought we could trust them since they moderate.
I used to just do the math with a pencil (simple formula probably Googleable).
I believe you might be right so Alex should get the correct figures posted.
So thanks!
------------------------------------------------------------------------
ps. After .0000673 ms here is what we get on Google:
We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula, where TD is tire diameter, SW is section width, AS is aspect ratio, and RD is rim diameter:
TD = (2 x SW)/25.4 x AS/100 + RD
When you plug in the values from our sample tire, the formula looks like this:
TD = (2 x 235)/25.4 x 60/100 + 15
Now, let’s do the calculations:
TD = 2 x 9.25 x .60 + 15
TD = 2 x 5.55 + 15
TD = 26.1 inches
Those were the figures I pulled off TireRack today (just follow the link I gave above).
Call me lazy, I thought we could trust them since they moderate.
I used to just do the math with a pencil (simple formula probably Googleable).
I believe you might be right so Alex should get the correct figures posted.
So thanks!
------------------------------------------------------------------------
ps. After .0000673 ms here is what we get on Google:
We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula, where TD is tire diameter, SW is section width, AS is aspect ratio, and RD is rim diameter:
TD = (2 x SW)/25.4 x AS/100 + RD
When you plug in the values from our sample tire, the formula looks like this:
TD = (2 x 235)/25.4 x 60/100 + 15
Now, let’s do the calculations:
TD = 2 x 9.25 x .60 + 15
TD = 2 x 5.55 + 15
TD = 26.1 inches
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OVERDRIVE - Racing Champion
Joined: Feb 2003
Posts: 15,262
Likes: 72
From: Mililani, Hawaii
http://en.wikipedia.org/wiki/Accurate
Accuracy is the degree of veracity while precision is the degree of reproducibility.
Last edited by minihune; Oct 6, 2007 at 08:56 PM.
OVERDRIVE
Joined: May 2005
Posts: 8,233
Likes: 127
From: San Francisco
According to Tirerack info on OE wheel/tires, (http://www.tirerack.com/tires/Select...ber+Valve+Stem)
OE 195/55R16 diameter is 24.5", and OE 205/45R17 diameter is 23.5
OE 195/55R16 diameter is 24.5", and OE 205/45R17 diameter is 23.5
[FONT=Arial] [/FONT]
[SIZE=-1]No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated.[/SIZE]
[SIZE=-1]Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That’s because larger tires cover more ground in one complete revolution. Consider the example below. [/SIZE]
[SIZE=-1]Your car comes with factory-installed tires that are 21.8 inches in diameter. That means the circumference of each tire is 68.5 inches. Now let’s say you want to replace the stock tires with new tires that are 24.6 inches in diameter. Each new tire has a circumference of 77.3 inches, which means it travels almost 10 inches farther with each complete revolution. This has a tremendous affect on your speedometer, which will now indicate a speed that is too slow by almost 13 percent. When your speedometer reads 60 miles per hour, your car will actually be traveling 67.7 miles per hour! [/SIZE]
[SIZE=-1]No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated.[/SIZE]
[SIZE=-1]Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That’s because larger tires cover more ground in one complete revolution. Consider the example below. [/SIZE]
[SIZE=-1]Your car comes with factory-installed tires that are 21.8 inches in diameter. That means the circumference of each tire is 68.5 inches. Now let’s say you want to replace the stock tires with new tires that are 24.6 inches in diameter. Each new tire has a circumference of 77.3 inches, which means it travels almost 10 inches farther with each complete revolution. This has a tremendous affect on your speedometer, which will now indicate a speed that is too slow by almost 13 percent. When your speedometer reads 60 miles per hour, your car will actually be traveling 67.7 miles per hour! [/SIZE]
http://auto.howstuffworks.com/speedometer3.htm
From a less known source....with unknown value of trust worthyness....
My m coupe speedo is almost exactly 10% fast. My x5 speedo is about 5% fast. The x5 computer is almost dead on, so it's electronic odometer will also be dead on. I've checked this against mile markers and a stop watch and also GPS. I don't know about my m coupe's mechanical odometer, but I suspect that it will also be very nearly dead on. The mechanical odometer is counting how many times the wheel goes around, and has no excuse for inaccuracy. The electromagnetic dial is using current to push against a spring and can very easily be inaccurate. And, BMW would rather have you thinking you're going faster than you are. there is almost no downside for them to exaggerate on speedo speed as long as they keep the odo accurate.
6th Gear
Joined: Sep 2005
Posts: 1,188
Likes: 0
From: Chico, CA
Accurate.
http://en.wikipedia.org/wiki/Accurate
Accuracy is the degree of veracity while precision is the degree of reproducibility.
http://en.wikipedia.org/wiki/Accurate
Accuracy is the degree of veracity while precision is the degree of reproducibility.
But if can read what he said he was precise if you understood it he was accurate.
6th Gear
Joined: Sep 2002
Posts: 3,343
Likes: 4
From: South Bend Indiana
Call me lazy, I thought we could trust them since they moderate.
I believe you might be right so Alex should get the correct figures posted.
I believe you might be right so Alex should get the correct figures posted.
Alex
OVERDRIVE
Joined: May 2003
Posts: 7,926
Likes: 40
From: Greensboro, NC
Huge errors in the speedo...
I've been monitoring the speedo's readings since experimenting with various tire sizes over the years ranging from 205/45-17, 215-40-17, 215/45-17 and 225/45-17...not one matches up to the NAV's speed reading, which is accurate to the best of my knowledge...I know this because the OBD's reading matches up my G2X's GPS reading...
