Speedo speedo accuracy (merged) - Civinfo
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post #1 of 174 (permalink) Old 23rd April 2006, 16:36 Thread Starter
 
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speedo accuracy (merged)

Slight irritation with my civic - dont know if others are like this or not. Basically, the speedometer is showing speeds approx 10% more than those displayed on handheld GPS sat navs (ive tried 3 different systems - all the same) at a steady speed, but getting less inaccurate as speeds rise eg GPS=30mph, speedo=33mph and when GPS=60mph, speedo=65mph. The odemeter is only approx 1% out, and the MPG indicater is also about 10% out compared to when I work out the fuel consumption manually ie displaying 44mpg but really only getting 40mpg. As the legal requirements are that speedometers must read between 0% and 10% over, my local Honda dealer says there is nothing they can do about it. OK, but I would have expected a more accurate showing from a honda, especially as my girlfriends Honda Jazz is spot on with regard to speed, mpg and the odemeter readings....
For the record my car is the 1.8 SE with std 16 inch alloys

Also, if you hear rattling from the boot area, get your dealer to check out the rear shockers - mine have already had to be replaced and it is proving to be a common problem with the early models.
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post #2 of 174 (permalink) Old 23rd April 2006, 16:51
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Yep. It's actually a common issue with all cars.

You notice it more with a digital read out more than a dial for obvious reasons.

The satnav speeds are definitely more accurate, but do suffer from a little bit of lag (around a second usually).

It's not a constant overreading either, as you point out. It's generally done deliberately so to lower the risk of the user speeding. If you ask people who have "got done" for speeding, they almost always say they thought they were going faster than the official reading.

My view on this is that it's a good thing - most people don't know about their cars overreading (and yes, I do realise that some speedos don't overread at all or very much) and it probably saves a few people some points.

I'm a big believer in not speeding, but even I will do 32 in a 30, 43 in a 40, 54 in a 50, 65 in a 60, and 76 in a 70 because I know I'm only doing the actual speed limit at these boundaries.

Of course, you have also remember that you're hardly ever going to "get done" for 10% over the limit, anyway, so there's usually an extra couple of mph grace on top of the overreading.

I think, overall this gives you the flexibility to do a bit more than the "speed limit" without breaking the law. We all know how easy it is (especially in the petrol) to go a bit over a specific speed as you pull away or whatever.
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post #3 of 174 (permalink) Old 24th April 2006, 08:43
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I too find it excessive. All the German cars I've had (VW, Audi, BMW, Porsche) have all overread by 2-3%. My Civic (2.2EX) overreads by 9% - doubly annoying when the car has GPS fitted as standard! It's all part of a misguided attempt to make a car "better" - it will look faster and more economical.

Not the worst in the world though, in 1985 I bought an ex-police Rover V8, which came with a huge, calibrated speedo in the centre of the dash. Flat out was 127 on the police speedo, 154 on the car speed speedo!
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post #4 of 174 (permalink) Old 24th April 2006, 08:58
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Quote:
Originally Posted by Pottsy
I too find it excessive. All the German cars I've had (VW, Audi, BMW, Porsche) have all overread by 2-3%. My Civic (2.2EX) overreads by 9% - doubly annoying when the car has GPS fitted as standard! It's all part of a misguided attempt to make a car "better" - it will look faster and more economical.

Not the worst in the world though, in 1985 I bought an ex-police Rover V8, which came with a huge, calibrated speedo in the centre of the dash. Flat out was 127 on the police speedo, 154 on the car speed speedo!

It doesnt always overread by 9% though, it is not a linear scale if you look at it, it's worse at higher speeds.

I am pretty sure it's no worse than a dial, but the discrete values of a digital speedo make it more noticable.
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post #5 of 174 (permalink) Old 30th April 2006, 17:10
 
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As everyone has said - all my cars have over read the speed! My Old Focus used to come out about 5-6% over, more at higher speeds - it was a life saver when going past speed cameras a couple of MPH over the speed limit!!

It also explains why some people will argue till they are blue in the face that their 0-60 is faster than it says in the spec as they have 'seen and timed it'!!!

J
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post #6 of 174 (permalink) Old 3rd May 2006, 13:41
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I dont know if anyone can shead any light on this one , yesterday I was travelling back from Birmingham were my girlfriend went to pick up yet another motorbike she had bought. On the way back we had arranged that where it was safe she would ride up along side me and then I would then slow down /speed up to 70 mph. We met up at a service station for a toilet stop, to my suprise she told me that the speed on her readout was the same as mine (her new bike has a digital readout). Any ideas ?
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post #7 of 174 (permalink) Old 3rd May 2006, 13:47
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Yep - all speedos over-read, normally 5-6% (bikes included). Just pop a decent quality GPS (not a speed trap GPS) on the dash for a real speed read out.
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post #8 of 174 (permalink) Old 3rd May 2006, 13:47
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Yes, they both overread!
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post #9 of 174 (permalink) Old 3rd May 2006, 14:02
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So my Satnav that gets its info from space is more accurate than the car which has its tyres on the road....
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post #10 of 174 (permalink) Old 3rd May 2006, 14:13
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Yes, as long as you are running at a constant speed on the flat.
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post #11 of 174 (permalink) Old 3rd May 2006, 14:24
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And there's a lag, too.

The car "knows" the actual speed, but the speedo is programmed to add a bit
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post #12 of 174 (permalink) Old 3rd May 2006, 21:19
 
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Quote:
Originally Posted by gissing
And there's a lag, too.

The car "knows" the actual speed, but the speedo is programmed to add a bit
Very true that - in my old focus there was a little trick you could do with the digital odometer screen readout. It put it in to diag mode, one of which was a digital 'real' speed reading which was always that little bit ahead of the anologue dial and the only way you could get your accurate speed, 0-60 and top speed when you where on the..er...track.

J
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post #13 of 174 (permalink) Old 14th May 2006, 14:47
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This graph was made on an EX with std 17" wheels, using an aviation grade GPS:

speedo.gif

It's a fairly good linear fit, showing a 6% over-read.
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post #14 of 174 (permalink) Old 14th May 2006, 18:43
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So your saying that the speedo is not that far out then...lol
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post #15 of 174 (permalink) Old 14th May 2006, 19:20
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Quote:
Originally Posted by kevin_lufc
So your saying that the speedo is not that far out then...lol
Don't be confused by the points and the red line..
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post #16 of 174 (permalink) Old 14th May 2006, 19:31
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bg....lol...get it...lol
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post #17 of 174 (permalink) Old 14th May 2006, 20:03
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post #18 of 174 (permalink) Old 15th May 2006, 17:02
 
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seems pretty accurate at 80, thats a relief !!
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post #19 of 174 (permalink) Old 15th May 2006, 17:16
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Yep, at high speed it over reads by about 5 mph, which is about normal.
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post #20 of 174 (permalink) Old 16th May 2006, 07:49
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I think it varies between different cars of the same make and model. So that some of the new Civics are more accurate than others. It's not that big of a deal as long as you are aware of the difference between the speedometer and your actual speed.

I read somewhere that you can switch between mph and km/h. Is this easy to do? It's more useful for you who live in the UK obviously, but was wondering if it could be done.
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