Hey guys question got new set of tyres on the Nav had my gps handy to check how much if any variation in speed reading would be showing apparently I've read putting 265/75/16 would correct the speedo. . . . It did overly reading 10k under actual speed although as time goes on it seems to slowly correcting itself I think it's the tyres settling does that sound right?
Speedo change
- Thread starter Boo Boo
- Start date
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The problem is that you can't correct the speedo like that.
The speedo is just a spring-mounted needle sitting on a coil that gets a voltage that SHOULD position the needle roughly in the right position for the voltage given.
On standard 255/70 tyres your car's computer knows almost precisely how fasts it's going and how far it's travelled, because the computer knows that one revolution of the wheel is 2398mm (that's the circumference of the standard tyre). It applies a voltage to the speedo needle that SHOULD position it roughly at the place where your current speed - which is averaged from 3 out of 4 of your ABS sensors - is close to. However, your ODOMETER - and trip meters - will be nearly exact (no needle involved there!).
If you change your tyre size, the computer still works things out on a 2398mm circumference and calculates based on that. Your 265/75 tyres are 2525mm around, or about 5% larger, so the actual distance travelled is 5% more than indicated by your odometer/tripmeter and your speedo needle is STILL just a vague representation of an approximate speed.
You have a GPS - and my recommendation is to follow it, rather than your vehicle's speedo, if you want an accurate measure of speed.
Also, when you are working out your fuel economy figures, add 5% to your "distance travelled" before going any further.
An example fomula for calculating your LPHK would then be:
(litres filled) divided by (km travelled * 1.05 divided by 100)
So if you filled 70.39 litres having travelled an indicated 599.4km (I am using figures from my own fuel records) then the calculation would be:
70.39 / (599.4 * 1.05 / 100) = 11.18LPHK
The speedo is just a spring-mounted needle sitting on a coil that gets a voltage that SHOULD position the needle roughly in the right position for the voltage given.
On standard 255/70 tyres your car's computer knows almost precisely how fasts it's going and how far it's travelled, because the computer knows that one revolution of the wheel is 2398mm (that's the circumference of the standard tyre). It applies a voltage to the speedo needle that SHOULD position it roughly at the place where your current speed - which is averaged from 3 out of 4 of your ABS sensors - is close to. However, your ODOMETER - and trip meters - will be nearly exact (no needle involved there!).
If you change your tyre size, the computer still works things out on a 2398mm circumference and calculates based on that. Your 265/75 tyres are 2525mm around, or about 5% larger, so the actual distance travelled is 5% more than indicated by your odometer/tripmeter and your speedo needle is STILL just a vague representation of an approximate speed.
You have a GPS - and my recommendation is to follow it, rather than your vehicle's speedo, if you want an accurate measure of speed.
Also, when you are working out your fuel economy figures, add 5% to your "distance travelled" before going any further.
An example fomula for calculating your LPHK would then be:
(litres filled) divided by (km travelled * 1.05 divided by 100)
So if you filled 70.39 litres having travelled an indicated 599.4km (I am using figures from my own fuel records) then the calculation would be:
70.39 / (599.4 * 1.05 / 100) = 11.18LPHK
Hey old.Tony thanks for your reply that information was very helpful to me, problem is as you could well imagine I have a mrs who also drives the nav and explaining this to her would require way to much time effort and probably therapy! Ha ha ha
So keep it simple: tell her the speedo isn't accurate, the GPS is, use the GPS and problem solved.
car is only 2 weeks old ... doing 100kph actually only doing 90
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KraftyPg
Guest
Given the tolerance is 10% they may consider fixing it for you but I very much doubt they will bother because the law says they don't have too. Try and make it 11% and they will have to do something though.
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KraftyPg
Guest
My odo is also out but many here are reporting that it's only their speedo that has the problem.
i went offroading 2 weeks ago i reset my odometer to zero
and check my distance uphill rocky climb at 32km but when i went downhill the same route my odometer reads 80km
my GPD tells me the distance is just 18km
how the hell does this speedometer and odometer matches.. shouldnt my downhillc climb be near 64-66km?
and check my distance uphill rocky climb at 32km but when i went downhill the same route my odometer reads 80km
my GPD tells me the distance is just 18km
how the hell does this speedometer and odometer matches.. shouldnt my downhillc climb be near 64-66km?
I think Mr Pythagorus is going to be of some assistance here.
When you're climbing a hill out on a highway (which is what we do 99.9% of the time) the distance travelled in terms of degrees of latitude around the earth is almost (but not exactly) equal to the distance actually travelled that you could get out and measure with a yardstick.
If you're climbing a steep incline, your vehicle moves only a small amount across the face of the earth, but moves quite a distance UP. The satellite doesn't measure up or down travel, only across.
For those about to wonder how some satellites give you the altitude ... it doesn't. In the Sensis map data is information about the elevation of each point on the map, and the GPS knows what point you're at - so it knows what elevation you're at. Roughly.
Anyway, you will also get errors if all of your wheels are spinning. The speed & distance calculations are an aggregate of the input from the ABS sensors. If ALL of your wheels are spinning, your GPS is going to measure no travel yet your car is going to think it's going somewhere.
As for your specific discrepancy - it's got to be a combination of all of that.
Then there's also the HTFDIK factor, which is built into any computational device. It's roughly equivalent to a shrug, or a puzzled look coupled with a spoken "How the ..." and explains how all of the unexplainable things happen.
When you're climbing a hill out on a highway (which is what we do 99.9% of the time) the distance travelled in terms of degrees of latitude around the earth is almost (but not exactly) equal to the distance actually travelled that you could get out and measure with a yardstick.
If you're climbing a steep incline, your vehicle moves only a small amount across the face of the earth, but moves quite a distance UP. The satellite doesn't measure up or down travel, only across.
For those about to wonder how some satellites give you the altitude ... it doesn't. In the Sensis map data is information about the elevation of each point on the map, and the GPS knows what point you're at - so it knows what elevation you're at. Roughly.
Anyway, you will also get errors if all of your wheels are spinning. The speed & distance calculations are an aggregate of the input from the ABS sensors. If ALL of your wheels are spinning, your GPS is going to measure no travel yet your car is going to think it's going somewhere.
As for your specific discrepancy - it's got to be a combination of all of that.
Then there's also the HTFDIK factor, which is built into any computational device. It's roughly equivalent to a shrug, or a puzzled look coupled with a spoken "How the ..." and explains how all of the unexplainable things happen.
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Krankin
Guest
ADR 18/03 allows the manufacturer to choose to tune their speedo's in the range of 0-10% plus 4km/h of true vehicle speed and that the speedo must not indicate a speed lower than the actual speed.
.
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KraftyPg
Guest
So the ADR means it's actually allowable to read 14ks out at 100 not 10ks?
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Krankin
Guest
yes, unfortunatetly, though the ADR does not mandate the the car makers to use this upper limit....bloody fn stupid isn't it.
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KraftyPg
Guest
Well there ya go I never realised they had the extra 4ks up their sleeve, that means my 10ks out is not as bad as it could be.
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Krankin
Guest
On the flip side i have been made aware that its the consumers right to request the manufacturer make your speedo read true speed (re the ADR)
but I think well have to have a massive picket line outside Nissan for that to come about.
No, I'm not angry.......
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Krankin
Guest
Interesting stuff!
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KraftyPg
Guest
Interesting, I'm still not worried about my speedo being out due rarely doing the exact speed limit and because I have a scanguage but it does make for interesting reading.
