speedo slow

I have just purchased a 1999 Discovery II GS, with 105000 miles on the clock. generally it is in good "nick", and has a service history. But I have a problem with the speedo. It reads 8mph too slow throughout the range according to my sat nav (TomTom). When doing 30 on the speedo it shows 22 on the sat nav, same at 40 on the speedo - 32 on the sat nav. The tyres are 255/65 R16 generall Grabber AT2 on alloys.

Any Ideas please?

thanks

Rob

Size tyres you got on,? mine also reads about the same about 10-15 mph up top end , i tried to have a look at mine but decided to leave it to the sat nav for speedo as didnt want to loose it altogether.

if the tires are different size then speedo will be out. also speedo's are slightly slower than normal anyway, cant remember why tho. My speedo's are about 4mph out

Hold on, I'll get back to ya on this one....

need to txt me sis and ask her to find a thread she showed me on a photography forum she showed me a while back.....something to do with the way sat navs are calibrated and the curvuture of the earth means sat navs will always under read the speed.

Somewhere in here.....

http://www.amateurphotographer.co.u...Main=659463&Words=+Maui888&topic=&Search=true

Thanks everyone, guess I'll have to live with it. Thread on AP forum interesting but a bit over my head in places.

Rob

By law speedos are not allowed to indicate a speed slower than you are actually traveling. This means that manufactures ensure the speedo reads slower and the faster you go the bigger the differance (usually).

don't you mean read faster

yes

robsea said:

Thanks everyone, guess I'll have to live with it. Thread on AP forum interesting but a bit over my head in places.

Rob

Click to expand...

Yeah, it gave me a major headache the first time round.....

Almost on thread, if I use the 'quick GPS' setting on my HTC Kaiser it no longer is accurate, not only to speed but also to actual location.

If I put in an address when the quick GPS has expired it is accurate to about a metre from the front door. if I subscribe to quick GPS it can 50 or 60 metres out and reads slow about 10 kph, the only adavntage it gives is the map comes on line in about 30 seconds as oppossed (is that spelt right?) to about a minute without.

regards
Dave

My speedo also reads a bit slower than the satnav...

...but the distance is similar! I regularly do a 134 mile journey, and the satnav/odometer are almost identical (certainly nowhere near the 10% discrepancy error of the speedo)....

Must be that speedos read somewhat faster than they are, because the odometer's about correct (on mine).

Anyone similar/different?

I would imagine it depends on the gearing of the speedo compared to the gearing of the numbers.

MartinK said:

My speedo also reads a bit slower than the satnav...

...but the distance is similar! I regularly do a 134 mile journey, and the satnav/odometer are almost identical (certainly nowhere near the 10% discrepancy error of the speedo)....

Must be that speedos read somewhat faster than they are, because the odometer's about correct (on mine).

Anyone similar/different?

Click to expand...

That's interesting. Anyone know?

Tachographs are more accurate than speedos, and technically you need one if you pull a big trailer, so some of you should (hem hem) have one fitted. Anyone tried a comparison there?

Agreed. But the thread was heading towards tyre diameter as a reason for speedo error. If the odometer's correct (ish) then the tyre diameter can't be ther problem...must be the speedo innacurracy rather than related to tyres (or some sort of rolling diameter)...

4pmh slower than the indicated speed does not sound like a problem with the speedo, just the manufacture maing sure that there is no comeback on them when you are cought speeding.