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Discussion Starter #1
My 159 speedo reads exactly 10% faster than my sat nav, with similar differences in mileage recordings. I'm on a 20,000 mile a year contract, and I'm going to be at around 23,000 this year, so this could be an expensive problem too. I know some of you guys have found switching to to bigger diameter wheels has solved this problem, but is there a way to reprogramme/recalibrate the speedo to make it accurate?
alex
 

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10% is loads. Mine is always within 1 or 2 mph, even up to V max;)
 

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I often use the Sat Nav when monitoring my speed - the speedo generally over-reads by 1-3mph. I think it's an inbuilt error, so that if you're doing an indicated 30mph, your actual speed may be 28mph. Thus you will always be under a designated speed limit if you sit with the needle exactly on that speed (20, 30, 40mp etc..)

The Sat Nav is pretty accurate, as its whole way of working out where you are and where you're going is based on speed = distance/time calculations, using accurate clocks on the satellites. Granted, there will still be tolerances within that, but still more accurate than your speedo IMHO.
 

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Discussion Starter #6
Does that mean that our MPG is also 10% out too then???? :cry:
Absolutely it does. My 41 mpg average is more like 36. 'Disappointing' to say the least given the pitiful performance of this car, particularly at low revs (is this really a diesel??). Its 8-10 mpg down on my old 2.4 20v 156!
 

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I often use the Sat Nav when monitoring my speed - the speedo generally over-reads by 1-3mph. I think it's an inbuilt error, so that if you're doing an indicated 30mph, your actual speed may be 28mph. Thus you will always be under a designated speed limit if you sit with the needle exactly on that speed (20, 30, 40mp etc..)

The Sat Nav is pretty accurate, as its whole way of working out where you are and where you're going is based on speed = distance/time calculations, using accurate clocks on the satellites. Granted, there will still be tolerances within that, but still more accurate than your speedo IMHO.
Agreed - and you can rely on SatNav to give you an accurate reading.
 

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Mine is within 2 MPH of the satnav at any speed. As the speedo reads the ABS wheel rings (average of all 4 in case one wheel is spinning) it's possible for the speedo to be spot on. So this 2 MPH over is clearly a design standard to stay within the law (up to 10% over must never read under).
 

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On the Ti with the 19's its there or therabouts, within 2mph

On 16 inch wheels, you will find an over-read of about 10%
 

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Discussion Starter #10
ok interesting chat guys, so it sounds like the clocks are designed to run with the larger diameter wheels, but alfa dont bother recalibrating the clocks for use with the smaller (lower spec) wheeled cars, because they are still (just about) legal... I dont think this is going to work though as an argument as to why I went over my contract mileage... At 7p a mile, this could get expensive...
 

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You could use 'bigger' tyres. 215 60 R16 is only 1% smaller than 235 40 R19 whereas 205 55 R16 is 7% smaller.
 

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think all cars are designed to show higher reading than you are. Apparently the BMW's and sat nav are spot on. this german engineering is too accurate!! :lol:
 
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