aharper

Having 20" rims put on this week and called the dealer to schedule installing an aux port next week. While on the phone with the SA I asked about re-programming my speedo to match the new wheels (I upsized by 1). He said that there is no need. According to him, when traveling at approx. 55-60mph with the stock rims (speedo shows 55-60mph) I'm actually traveling around 5mph faster. He said that this is normal because there are different sizes of tires that are offered and that it's the only way that they can allow for tolerances.

This sounds insane to me to come from BMW off by 10% at 50mph. I'll test with my GPS later today but found the info interesting. They also raised the price of the AUX port install to $323!! Yikes. I wish there were other BMW dealers that I could price against in the Indy area.

ETNav382

I've semi tested this, partly by getting pulled over, lol, and driving by the radar detectors stands on base. I have the stock 124s and mine was faster by 3 by the radar. I tested this by going just fast enough to turn on cruise control and it was 3 below the speedo readout. I even increased the speed to make sure and it stayed the same. The other times the cop said I was going 71 in a 60 and my cruise was at 73/74. So this is very plausible.

aharper

Interesting now that you mention a ticket. I bought the car in Dallas and drove it back to Indy. On my way I was pulled over for speeding in Arkansas. I thought I was well over 90mph but he said he clocked me at 86 in a 65. Anyway, that's opposite of what they are saying with regard to the direction of being off but interesting none-the-less. I'm going to test against my GPS and will also ask my brother to clock me in his cruiser just to see how far off it is with the new rims/tires.

jhnnyfiv

My speedo is 3 mph faster than my GPS with stock wheels. It's 4 - 5 mph faster with summer wheels.

jhnnyfiv

My speedo is 3 mph faster than my GPS with stock wheels. It's 4 - 5 mph faster with summer wheels.

Percy_991

Interesting......

KAF

Speedo will always be faster than real speed.

In Europe it is illegal for a speedo to read slow, but they can be up to 10% fast, so they are all set up to read around 5-7% too fast.

When I am doing 70, speedo reads 74.

Changing wheel sizes will alter the reading, as has been suggested, check real speed with GPS on a FLAT road for a more accurate speed.

Excession

Interesting; however the bigger question is about the accuracy of the odometer.

ETNav382

This is what has been bugging me since I found out the speed was off. Hopefully somebody can answer this for us.

usmabmass

I can confirm this. The speedometer is precise but not accurate. Meaning that the speedo reads 3-5 mph faster than you are actually going. So, when the speedo reads 75, you are actually doing 71-72. It's for the reasons KAF wrote in post 7.

This surprised me when i first got to Germany. My 06 Impala SS always read 100% accurate. I bought my first BMW in Germany and noticed it read about 8 km/h faster than my TomTom did. I've tested it in my 98 540iT, 08 550i, and a bunch of rental cards (Euro Fords, MB, Audi, and Fiat), and all the German cars had too-fast speedos. Most of the American cars (Dodge, GM, etc) read just about dead on.