gps V dash Speed

Miffy

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Yesterday I managed to stick to a constant speed around the m25 so decided to compare the two speeds.

Once I got to 70, I stuck the cruise on and let it take over. It seems that the GPS reads slower by around 3mph on avg

@ 70 (dash needle and digital) GPS was showing 67
@ 80 (dash needle and digital) GPS was showing 78

each reading was based on a constant speed over at least a mile

anyone else compared the two speeds before?
 

st4

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Aye, speedo's will always under read. Not a major issue as they are designed to in order to protect the manufacturer from getting claims from owners against them saying the speedo said 70mph but I got a ticket for 73mph or something.

Modern cars are capable of displaying true speeds (e90 3 series can be re-coded to do this) but don't for the legal reasons.

2mph at an indicated 80mph isn't too bad an error. My old BMW was 5mph by this time.
 

mercmadjim

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Yes, mine is about the same, there was a question about speedometers on Chris Evans show last week, the question was, "why do speedo's show a much higher speed than the car can manage" part of the answer involved a cars speedo must read a certain % higher, ie at 70 mph on the speedo, the car is only doing 67mph, so I am guessing the GPS is right, and the car is slightly wrong!!
 

television

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I think that every car made indicates a higher speed than you are actually doing, this has to be the case to stop people claiming that their car was wrong when caught for speeding
 

SQ_W211

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I agree with Malc, Manufacturers have to cover their back. Plus if a you change the diameter of the wheel/tyre the speed can change which needs to be taken in account. For instance if you buy SL500 with standard 16" wheels and then upgrade to 19" with lower profile you are likely to have variance in speeds.
 

television

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I agree with Malc, Manufacturers have to cover their back. Plus if a you change the diameter of the wheel/tyre the speed can change which needs to be taken in account. For instance if you buy SL500 with standard 16" wheels and then upgrade to 19" with lower profile you are likely to have variance in speeds.

I did change my wheels on my SL and the profile and the effect is now that my speedo is exact. So I have to allow for that
 

SQ_W211

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True but surely if you are increasing the total diameter with the new wheels then it would make an impact on the speedometer.
 

television

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True but surely if you are increasing the total diameter with the new wheels then it would make an impact on the speedometer.

Yes it has,,the fuel consumption now shows less as the car goes further than it thinks it has
 

100%Bitch

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True but surely if you are increasing the total diameter with the new wheels then it would make an impact on the speedometer.

Doesn't that depend on the profile of the tyres?

The bigger the wheel (19" against 16") then the lower the profile (30 against 45). If you get it right there should be no difference in the rolling circumference and therefore no impact on the reading on the speedo.
 

*Thumper

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My car is 5 mph out across the board .......... so was my last car !!

No real issue, so long as you remember as you pass a speed camera .......it gives you a safety buffer ............. all manufacturers do it !
 

television

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Doesn't that depend on the profile of the tyres?

The bigger the wheel (19" against 16") then the lower the profile (30 against 45). If you get it right there should be no difference in the rolling circumference and therefore no impact on the reading on the speedo.

I did not want to damage my new 19" wheels, or get a worse ride, so I have upped to 23mm larger in diameter, by changing the width and profile, I also saved £300 in cost, as on AMG rims, the tyres are very expensive
 

izy_wizy

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this has always intreeged me. I will be sitting on the motorway doing erm 70 and my road angel will say i'm doing 67 and my tom tom before my merc and command of cause yet i do sometimes use it as well cos u cant beat tom tom and tom tom will say 67 as well. No matter what sped i go up or down lol to its always been around 3-4mph difference. I have been told that this is to allow for the transfer of speed to the tyres and through the tread to the road. and i have always belived it!!
 

television

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this has always intreeged me. I will be sitting on the motorway doing erm 70 and my road angel will say i'm doing 67 and my tom tom before my merc and command of cause yet i do sometimes use it as well cos u cant beat tom tom and tom tom will say 67 as well. No matter what sped i go up or down lol to its always been around 3-4mph difference. I have been told that this is to allow for the transfer of speed to the tyres and through the tread to the road. and i have always belived it!!

There is 6.7mm of tread difference here between new and old tyres, in my Comand I can set new or used tyres
 
A

alindsay81

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the max speed on my garmin is 501mph - i turned it on while flying from belfast to mallorca last month :)
 

television

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the max speed on my garmin is 501mph - i turned it on while flying from belfast to mallorca last month :)

Did the post code work while doing that;):D
 
A

alindsay81

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No but the girl in the satnav said re-calculating about a million times as we flew over france
 

Dosco

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No but the girl in the satnav said re-calculating about a million times as we flew over france

I bet she was hacked of then:lol::lol::lol:

and to confirm my sat nav also shows 3mph slower than the speedo at whatever speed I am doing....
 


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