# Speedometer Calibration 2000 BMW 540 Wagon



## ZUGSTAR (Mar 13, 2005)

I just put on low profile rims and tires tires that are a little smaller OD than the original tires. Now my speedometer reads too fast. At a true speed of 65 MPH my speedometer reads 73 MPH.
Does anyone know if the dealer is able to change the calibration factor in the car's software to correct the speedometer reading?


----------



## sph17 (Oct 5, 2003)

ZUGSTAR said:


> I just put on low profile rims and tires tires that are a little smaller OD than the original tires. Now my speedometer reads too fast. At a true speed of 65 MPH my speedometer reads 73 MPH.
> Does anyone know if the dealer is able to change the calibration factor in the car's software to correct the speedometer reading?


Dave Z pointed out this web-page previously for people to use in reference to considering alternate tire and rim sizes:

http://www.powerdog.com/tiresize.cgi


----------



## ZUGSTAR (Mar 13, 2005)

Interestingly, the new rims I bought, 17" x 8" are from the BMW alloy wheel catalog, and the catalog calls out 235/45/17 tires for the 5 series car. The old wheels were 16" x 7" and had 225/55/16 tires. I did not bother investigating the overall OD of the tire considering the BMW catalog, as well as Tirerack.com, calls out the 235/45/17 for this car using 17" x 8" rims.


----------



## sph17 (Oct 5, 2003)

ZUGSTAR said:


> Interestingly, the new rims I bought, 17" x 8" are from the BMW alloy wheel catalog, and the catalog calls out 235/45/17 tires for the 5 series car. The old wheels were 16" x 7" and had 225/55/16 tires. I did not bother investigating the overall OD of the tire considering the BMW catalog, as well as Tirerack.com, calls out the 235/45/17 for this car using 17" x 8" rims.


 The tire size calculator indicates a 5.5% difference in speed, 3 mph at 60 mph. Your difference is more than twice that. I know that similar sized tires are not alwayse exactly the same size, but that seems a mighty big difference, from what the tire calulator indictes.

Now here is the real question. Are you absolutely sure your spedometer was "accurate" before you changed your tires? US law dictates a car's spedometer absolutely can not read a speed lower than what the car actually is taveling. Many auto makes adjust their spedometers to read a bit fast. They engeneer a "cushion" in to their spedometers, so as not to ever break federal law. Many people, when they "calibrate" their spedometers find their spedometers read "fast".

Is it possible that your spedometer read a bit fast with your old wheels/tires, and that you only noticed it and checked with your new tires/wheels now that they are reading an additional 5.5% faster?

FYI. Spedometers in police cars are "certified" accurate by the auto manufactures, and a cretificate is issued with each police vehicle. The certificate is actually only certifying the spedometer unit in the dash is accurate, and is imputed to only be valid with the original tires, wheels and gear in the transmission of the police car.

If you still have your old tires & wheels it would be interesting to see how accurate your spedometer is with them.


----------



## PropellerHead (Jan 3, 2002)

ZUGSTAR said:


> Does anyone know if the dealer is able to change the calibration factor in the car's software to correct the speedometer reading?


They are not. In fact a Loooooong time ago, a guy who was just as frustrated as you are did something about it.

He removed his cluster and recalibrated the spring on the speedo himself! He admitted that it took him a long time. He;d lift and move the speedo needle inards, reconnect the cluster and then go for a drive. He'd figure how much off he was by resetting the avg speed on the OBC. As he got closer, he would adjust less. He finally got the thing within a few .10 seconds of accurate and he declared himself happy. Who he was and how he did it, I wouldnt remember under a 300w bulb.

But he did, and you can too. If it bugs you enough.


----------



## ZUGSTAR (Mar 13, 2005)

How I checked my speed: I used a Navman GPS which has a speed reading thast appears accurate down to 1 MPH. [I very much recommend the purchase of one of the new portable GPS units....a whole lot of fun. There is a new Navman for about $325 that is the size of a typical PDA. At the very least, borrow one from a friend and you can check the speedometer accuracy in your car.] With my old wheels and tires, I found that the speedometer read about 3 MPH too fast @65 MPH, so now with my new tires, that calculate to spin even faster by 5.5%, I now get the error ratio of 73 MPH indicated (speedometer) vs. 65 MPH true (Navman GPS). Is for a particular tire class, I did notice that the OD does vary by manufacturer and even tire model for the same manufacturer as defined by the "revs per mile" spec on Tirerack.com.
Anyway, I got a good deal on used rims from Craigslist, the BMW type 5 that retail for $600 each from the dealer. I paid far less. The tires I fitted are the Goodyear Eagle F1 with a really aggressive looking tread pattern that looks like stacked "V's". When I saw my car for the first time as the tire shop lowered it down from the lift, I could hardly contain a smile: It was a totally different looking car, and I notice that other drivers in sporty cars now do double glances at my wheels at the stoplight. particular tires are in fact smaller in diameter that its peers, so this explains my speedometer error as described. I wonder if Goodyear did this to give cars a little more zip in accelerating away from the stoplight? I do notice the increase in accelleration potential with these smaller OD tires. The penalty will be an odometer that racks up the miles faster, and i guess a small drop in gas milage. Still, overall, givin that it's a BMW, perfomance and eye appeal are more important for me, and I so am happy with the new wheels and tires. I never look at the speedometer anyway unless there is a cop nearby, and in this case I can simply mentally calculate the true speed compared to the speedometer reading.


----------



## sph17 (Oct 5, 2003)

Thanks for the up-date.

I am thinking about a touring, as I miss the ability to go to Home Depot or other stores and purchase things bigger than a bread box.

I am keeping my eyes open for the right one.

Good luck with yours.


----------



## PropellerHead (Jan 3, 2002)

ZUGSTAR said:


> The penalty will be an odometer that racks up the miles faster, and i guess a small drop in gas milage.


You shouldn't see as big a difference in actual mileage. The reading on the speedometer is not the only origin of the reading for odometer. In fact, there are something like three places that the car gets it's actual speed and actual mileage. ABS computer, driveshaft, and engine RPM's are involved if I remember correctly. The display on the speedometer of the car's speed is allowed to be off the car's actual speed by the factory to within what they say are 'acceptable limits" (or error or something like that).

To confirm this, you can use your OBC. Set the car on cruise on a nice even number- like 60MPH. Also handy if you want to use the ol'e 1 mile in a minute method, as well. Find the avg speed setting on the OBC. Reset it. It's initial reading will be your actual speed as computed by the car and as used for the mileage computation.


----------



## ZUGSTAR (Mar 13, 2005)

*The wagon is cool.*



sph17 said:


> Thanks for the up-date.
> 
> I am thinking about a touring, as I miss the ability to go to Home Depot or other stores and purchase things bigger than a bread box.
> 
> ...


I like my wagon. I got it because I have a spoiled Labrador Retreiver, need space for Home Depot, and I even prefer its styling over the sedan becase sedans (to me) all look alike whereas the wagon looks unique....not so wagon-like, and pretty rare in my area. Plus, (eeeevil grin) it is devistatingly more crushing to be trounced by a "station wagon" than by a "regular" BMW. The unwary expect a BMW sedan to be fast, but not a "wagon", which, unkown to them, happens to perform very closely behind the 540 coupe as it is only about 250 punds heavier but with a BMW standard fitted higher ratio rear drive to help compensate for it. This, plus my new smaller OD tires, makes for a stunning performer 

Mine came from an auction/broker in California. Carfax says my car grew up in NY. Are there many over there?


----------

