540i speedometer off

'02 540I sport. I bought it CPO in August '04 with 29,000 miles. It had front end damage, repaired, but the speedometer reads 5 mph high in the 50-80 mph range. It's not a real problem except it's tacking up more miles
on the odometer than is really on the car. (and I always have to figure it in when I set the cruise control).
Any suggestions? It's still under the CPO warranty but I don't know if they'd replace? or even consider it. Is there an adjustment they could do?
thanx
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It's actually quite normal. You could get larger tires the next time you need them, and this will shave a few mph from the speedo so that it reads closer to reality.
The idea is, if the speedo READS 70 when doing 65, then you will not be exposed to speeding tickets that come when the reading is 65 when the real speed is 70.
If your car has 235/50 tires on it, then you might consider 245/50 the next time you need tires. The 235 or 245 number represents the WIDTH of the tire, the 50 number is the sidewall as a percentage of the width. Multiply the first number by the second number to find the height of the sidewall. If you increase the width, and keep the aspect ratio the same, the result is that the sidewall gets taller. This "taller" size will adjust your speedo.
PS I pulled the numbers out of my ass to illustrate the point. You can easily insert accurate numbers to see what really happens to the tire size. You need to make the overall diameter (or circumfrence) larger by a few percentage points over what you have now to slow the speedometer.
The formula for overall diameter is width (235) X aspect ratio (45, 50, etc.) X 2 (to get the entire sidewall height) / 25.4 (to convert from mm to inches) + rim size.
You can see that a 235/40x17 tire has an overall diameter of 24.4 inches because 235 x .40 = 94 x 2 = 188 / 25.4 = 7.40 + 17 = 24.40.
Mix the numbers a bit to find a size that both fits the car, and is the proper percentage larger to slow the speedo read out. My '94 3 Series reads about 1.5 mph faster than reality when I'm doing 80ish.
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Correct except for the reason. German law requires speedos to never read slower than actual speed. BMW's interpretation of that law results in a highly optimistic speedometer. Check your OBC speed, it may reflect true MPH (or not, my 01 did, my 03 is identical to the needle). Go to OBC average speed and reset it while cruising at a constant speed. If it's about 5mph below indicated, you're good to go.
OBC SHOULD record accurate mileage (and undoubtedly will if the OBC speed is accurate) regardless of the speedometer's optimism.
R / John

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I thought I said that, the speedo is designed to read faster than reality.
German law requires the speedo to read faster, BTW, not slower. When the car is going down the road at 47 mph, one wants the speedo to read 50, not 45.
Both of my 3 Series cars, '94s, are about 1.5 mph off at 80ish. 80 comes back on the OBC as 78.5

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5% fairly normal. If your tyres are worn add 2%, if someone has fitted the wrong rims and tyres it could be worse.
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dwilli wrote:

I have a Garmin GPS in my X5. The speedometer does read faster but about 4-5 mph at 80 mph. The odometer follows the GPS almost exactly but the two might not be connected. It's tough to say for sure since the GPS covers distance "as the crow flys" so hills aren't accounted for in its numbers. We have a very flat terrain here so I still think the odometer is more accurate than the speedometer.
Dan
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I have a Lowrance GPS and have clocked it at 5 MPH fast but never thought of checking the ODOMETER reading to see if that's accurate. I've always assumed the MPH and odometer are connected. (actual calculations for the mph problem confirmed that) Will try...thanks. BTW, I had a 98 328i that was much closer to accurate reading, probably 1/1-1/2 off. My 02 is on stock tires and rims so that factor is moot(sp?).

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You will still read up to 2% faster when they are nearly worn out.
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FWIW: Tires new. I just thought maybe they put a wrong speedometer gear in when they repaired the front end.
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Most BMWs have the speedo sensor driven off the rear wheels.
--
*Could it be that "I do " is the longest sentence? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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dwilli wrote:

Just as an FYI... the speedo input is from the drivetrain, not the front end.
--
-Fred W

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That didn't happen, the speedo is driven off of the rear axle.
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