Cruise Control Actual Speed

[BMWRX]

I recently set my cruise control for 100km/h and switched on my trusty TomTom to verify the speed and found that I was actually doing 95km/h. Then worked my way down the speed range (90, 80, 70, etc) and found that at each point, the GPS was showing 5km/h below what the cruise control display was showing. I would imagine my TomTom is pretty darn accurate as all it is measuring is distance and time and calculating a speed from that.

Is this a typically BMW thing? Has anyone else noticed this?

[MickyD]

GPS is about as accurate as you can get, assuming you've got a decent signal. In all the cars I've tried it in, I've NEVER found a car that was reading less than about 4km/h fast compared to the GPS

[cozza]

Most cars' speedometers are slightly out. Mine was 7kmph out (107 in the car was actually 100), and now that I've changed the wheels and tyres it's out by 5kmph.

[38 Special]

Mine does the same with my TomTom : cruise control set at 130 km/h will result in an actual speed of 123 km/h as per the GPS device. I'm running factory 207M rims fited with properly sized tires (at the moment, Michelin Pilot Alpin PA3 205/50 R17 front 225/45 R17 rear)

But my father's Mercedes B Class is VERY accurate and will never show more than 2 km/h difference between the dashboard (both analog & digital) and the GPS. Most of the time the diff is only 1 km/h.

I wonder if this can be re-calibrated by a dealer ?

[SimonB]

IIRC, Australian design rules specify accuracy of within 10%, on the under-read. I guess some cars are less, but I concur - 115 km/h by GPS equals 110 km/h speedo in my 135 too.

[Lester]

All speedos read high. They are designed and built that way on purpose. It is to protect the auto manufacturers against liability in case of an accident.

[Taymaishu]

They read high, definitely.

My Subaru was 8% out - clocked it against GPS and the police speed checks at 100km/h, and it always said 92km/h, whereas my dial said 100.

[MadMan77]

Originally Posted by Lester

All speedos read high. They are designed and built that way on purpose. It is to protect the auto manufacturers against liability in case of an accident.

Exactly.

[ennislaw]

Originally Posted by Lester

All speedos read high. They are designed and built that way on purpose. It is to protect the auto manufacturers against liability in case of an accident.

LOL. I rather suspect it is because calibrated speedometers cost more money and the consumer isn't demanding an extremely accurate speedometer, and wouldn't want to pay the extra costs.

Besides, the Highway Patrol vehicles in our state are only certified + or - 5 mph, so most speedometers aren't that inaccurate overall.

pge

[takahashi]

Mine 118 for 113

107 for 100 as Cozza said. I am more likely to scratch my limits on the 110 zone as there are not suppose to have speed camera on these road (except I have seen a few mobile ones from time to time)

[d3syd]

My Volvo is doing 105km/h when the speedo is showing 100km/h

I'm always wary of this when I drive it.....

[MickyD]

Originally Posted by takahashi

107 for 100 as Cozza said. I am more likely to scratch my limits on the 110 zone as there are not suppose to have speed camera on these road (except I have seen a few mobile ones from time to time)

ermm... there's speed camera's all the way up the Hume, and they're point to point as well as instantaneous speed....

[x5mad]

Originally Posted by Lester

All speedos read high. They are designed and built that way on purpose. It is to protect the auto manufacturers against liability in case of an accident.

+1.

Originally Posted by ennislaw

LOL. I rather suspect it is because calibrated speedometers cost more money and the consumer isn't demanding an extremely accurate speedometer, and wouldn't want to pay the extra costs.

Besides, the Highway Patrol vehicles in our state are only certified + or - 5 mph, so most speedometers aren't that inaccurate overall.

pge

Not true. There is actually a hidden function whereby you can display the actual speed in the instrument cluster. Search ''Easter Egg Menu''.

So if they wanted to they can make 100km/h = 100km/h for same cost.

[RadiuM]

after reading this i did some tests with my phone and car today.

100, phone says 94.
110, phone says 103.
120, phone says 113.
800, phone said "O Rly?"

[larryn]

Originally Posted by Lester

All speedos read high. They are designed and built that way on purpose. It is to protect the auto manufacturers against liability in case of an accident.

... AND has side benefits of less actual miles on leased cars. You are paying for 15k miles a year, and only getting 14250 actual miles. You can bet your ass that if you dropped the car off with 15790 per year, they would charge you the overage.

FYI, years ago I've also heard BMWNA state that due to rotating circumference differences between a new and spent tires, they build in tolerances. This argument makes more sense, but these "tolerances" can be seen with new tires as 5%, and only would become more of a discrepancy as tires get worn.

A set of worn tires, with 3/8" less tread depth, is (nearly 3) inches shorter in circumference, meaning you are likely getting closer to 10% speedometer/odometer difference. Just a thought.