If speedometers are supposed to over-read
18 September 2006, 04:14 PM
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If speedometers are supposed to over-read
Does that mean that the tachometer is calibrated to over-read too?
Only reason i ask is that for a given engine rpm, i can work out what the speed of the car should be, and within a really small margin of error, the two do match up, well past 100mph when i'm on a private airfiled.
So what, if anything, is over-reading? I don't get it!
Only reason i ask is that for a given engine rpm, i can work out what the speed of the car should be, and within a really small margin of error, the two do match up, well past 100mph when i'm on a private airfiled.
So what, if anything, is over-reading? I don't get it!
18 September 2006, 04:20 PM
#2
Never know what to make of these 'over reading' things.
I have a Road Angel in my car, and 101mph indicated on the clock is 100mph on the Road Angel, so i would assume my speedo is almost exact.
In my friends Cavalier 41mph on his clocks was 35mph on the Road Angel.
I have a Road Angel in my car, and 101mph indicated on the clock is 100mph on the Road Angel, so i would assume my speedo is almost exact.
In my friends Cavalier 41mph on his clocks was 35mph on the Road Angel.
18 September 2006, 04:22 PM
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The tacho overreads too, according to the PFC (ECU logs). About 200rpm over at the top end IME
18 September 2006, 04:24 PM
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What's the point of doing that though? Isn't it easier to manufacture them so they display what's actually happening in the engine?
Does IME mean in your estimation? Is that what it is, an estimation?
Does IME mean in your estimation? Is that what it is, an estimation?
18 September 2006, 04:33 PM
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In my experience 
Probably something to do with offset tolorences, as in -0 and +1% so that you don't hit the limiter before the tacho says, IMG (thats guestimation
)
Probably something to do with offset tolorences, as in -0 and +1% so that you don't hit the limiter before the tacho says, IMG (thats guestimation
)
18 September 2006, 05:02 PM
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Speedometers cannot UNDERREAD by law. That is why the majority overread (the 41 really being 35 in your cav example). How far they are from the truth depends on how well they are designed in the first place. part of which includes, for example, how many different wheel/tyre diameters the same unit has to cope with. (The same model car can have wheel diameters that span several inches, e.g. ...).
As for tachos - no reason for them to over or underread. Again, depends on how they were designed and built (to a cost or to an accuracy figure). One way to check the tacho is to get a meter that takes the ignition signal (petrol engine) and converts it to rpm.
Dave
As for tachos - no reason for them to over or underread. Again, depends on how they were designed and built (to a cost or to an accuracy figure). One way to check the tacho is to get a meter that takes the ignition signal (petrol engine) and converts it to rpm.
Dave
18 September 2006, 05:17 PM
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Originally Posted by Moley_WRX
Never know what to make of these 'over reading' things.
I have a Road Angel in my car, and 101mph indicated on the clock is 100mph on the Road Angel, so i would assume my speedo is almost exact.
In my friends Cavalier 41mph on his clocks was 35mph on the Road Angel.
I have a Road Angel in my car, and 101mph indicated on the clock is 100mph on the Road Angel, so i would assume my speedo is almost exact.
In my friends Cavalier 41mph on his clocks was 35mph on the Road Angel.
Are you running standard size wheels?
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18 September 2006, 09:21 PM
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My rev counter is pointing midway between 7300 and 7400 at the rev limiter, which a PC connected to the diagnostics socket and the official set point says is 7000rpm. This corresponds with a speedo that over-reads by best part of 10% at the motorway limit, and seems to be consistent with most other people's speedos when tested with road angel etc.
19 September 2006, 09:30 AM
#10
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Speedo takes its drive from the gearbox, and the tacho comes from the coil pulses, so not sure how the tacho can over read as they are not related at all.
ECU's use the tacho for rev limiting etc, so they need to be spot on (ish)
ECU's use the tacho for rev limiting etc, so they need to be spot on (ish)
19 September 2006, 09:50 AM
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You've also got to factor in tyre wear and tyre pressure, both of which have an effect on the rolling diameter of the tyre and therefore actual vehicle speed.
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