confused speedo

[dwaynes]

Okay i have a 89 gq patrol that when traveling on the freeway sitting on a hundred my gps say 91 but the speedo dectectors they have down here in vic say im doing 101 now this is with my 32's on if i go to the 35" the gps and speedo match.

So what speedo reading would be the most accurate?

Im going the GPS but still confused about the freeway detector being so far different.

[dogbreath_48]

Okay i have a 89 gq patrol that when traveling on the freeway sitting on a hundred my gps say 91 but the speedo dectectors they have down here in vic say im doing 101 now this is with my 32's on if i go to the 35" the gps and speedo match.

So what speedo reading would be the most accurate?

Im going the GPS but still confused about the freeway detector being so far different.

I've heard plenty of stories of those overhead speedo's being inaccurate.

I had a similar problem - on my 32's my speedo was spot on according to my GPS. Now since going to 34's (speedo read ~10% low) and back to 32's my speedo reads ~10% high, comparing to GPS.

I can only assume my speedo is stuffed. GPS is always pretty spot on isn't it?

[Apocalypse]

The speed detectors in the highways are messed up. Theres one when coming from Geelong to Melbourne, that one says my speedo is 10kmh slower, then there the one on the hume going out of Melbourne, that one says my speedo is 3kmh slower.

[shakes]

The speed detectors in the highways are messed up

makes you feel confident in there speed camera's then doesnt it?

[dwaynes]

thanks guys will stick to what the gps is saying and hope for the best.

[Shadow]

isnt a GPS onlyu accurate to 10%?

[CairnsZook]

Remember the Earth is not flat. GPS are accurate for a ship doing 12 knots on a "relatively" flat (round) ocean. I don't know how accurate it would be for a vehicle on land. It would all depend on how close the land there is in relation to the Datum.

[dwaynes]

okay now you all have confused me even more.
As i thought the gps would be the most accurate.

["CANADA"]

Unless the GPS takes into account altitude then it is going to be out for sure

[RN]

Ok...

I have a GQ with 16" GU rims and standard 275/70 tyres

I have a GPS built into my UHF.

When I travel north along the Hume the over head speed detectors gantries reads 1 kmh slower than my speedo.

GPS indicates that I am travelling 1 kmh slower than my speedo.

Overall I think it is so close it does not matter.

[awill4x4]

Use your GPSr to measure a distance of 1Km or more (preferably 5Km to get a good average) and travel at a speed of 100 kmh on your car speedo. If your speedo is accurate it should take you 36 seconds per kilometer, for 5 kms that's 180 seconds or 3 minutes.
If it takes longer you're going slower if it takes shorter you're going faster. The amount of time difference can then be taken as a percentage either slower or faster than 100 kmh to work out exactly what your real speed is.
Regards Andrew.

[turps]

MY GPS is accurate to +/- 1km/h and I trust it. Only need 3 sats to get a good reading. Need more for real good altitude (preferabley right above).
As for the ones on the Hume heading out of Melb. They read 1km/h less than the GPS. I used to drive along that section a couple of times a week. Did it for over 12mths and when it was working it read the same.

[-Scott-]

In my experience GPS speed readings are only accurate when you're travelling in a straight line - hit a bend and it's hopeless.

While travelling in a straight line, it's accurate enough for speedo checking, and more accurate than any road-side sign I've ever seen. I've also seen those roadside signs completely confused by light rain. A few years back I used to pass some regularly, and they were accurate to with 3-5km/h. In light rain, they were a joke - anything up to 20km/h out, in either direction.

Scott