Should I trust my new sat nav ?

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I have got used to driving at 55-60. speedo speed.

Irish chav nag sounds like our last president mary mcaleese.
Got to traffic lights earlier she said turn left now ,I nearly did and the light was red. Almost.
 
What size are your wheels/tyres?

I used to have the original size 205/85r16. With those the speedo read 70 when i was actually doing about 60 according to the tom tom. Changed now to 235/85r16 and it reads 70 when the tom tom says im doing 71.

I'd trust the tom tom more over long distances on the motorway. It will be less accurate at very low speeds though. At best they only have an accuracy to within 6meters, and are affected by large buildings etc.
 
mundungus said:
What size are your wheels/tyres?

I used to have the original size 205/85r16. With those the speedo read 70 when i was actually doing about 60 according to the tom tom. Changed now to 235/85r16 and it reads 70 when the tom tom says im doing 71.

I'd trust the tom tom more over long distances on the motorway. It will be less accurate at very low speeds though. At best they only have an accuracy to within 6meters, and are affected by large buildings etc.
Click to expand...
My experience mirrors yours nearly exactly I calculated the increase would be 9%. Factor into that new tyres against old and you can very easily have a 10mph apparent change. I would be more inclined to trust your sat nav than speedo, however to do a proper comparison you have to travel at a constant speed to let the sat nav reach a steady reading.
 
waldershelf said:
My experience mirrors yours nearly exactly I calculated the increase would be 9%. Factor into that new tyres against old and you can very easily have a 10mph apparent change. I would be more inclined to trust your sat nav than speedo, however to do a proper comparison you have to travel at a constant speed to let the sat nav reach a steady reading.
Click to expand...

Yeah, when I was looking at what size tyres to buy, i found that etyres.co.uk website has a great tyre size comparison calculator.

The defender manual gives two tyre sizes as standard, the 205/85r16 and 265/(something)r16. When i did the comparison on that site, it said about 10% difference between the two. So I figure the landy speedos must be tuned to the higher of the two tyre circumferences. This gives a 10% over reading if you're using the smaller of the two. I guess LR have erred on the side a caution, so that it never reads under, so you wont break the speed limit unaware.

Thats why your tom tom is probably more accurate.
 
Just got a replacement TT and its very accurate on the TD5 110 TT reads faster though by a couple of miles an hour. Older series I have had are inaccurate though.
The GPS unit depending on how many satellites it supposed to pick up (the more the better) then this will depend on the accuracy. I have a GPS system for work and used this to check the speedo in previous landrovers, they have been up to 10 MPH slower or higher than GPS.
Hope this helps.
 
Car speedo's are not usually all that accurate. They are usually out by approx 10%,which is why the police have the unofficial 10% +3 mph tolerance for speed limits. Whereas sat nav is usually accurate to less than 1 mph.
 
mundungus said:
Yeah, when I was looking at what size tyres to buy, i found that etyres.co.uk website has a great tyre size comparison calculator.

The defender manual gives two tyre sizes as standard, the 205/85r16 and 265/(something)r16. When i did the comparison on that site, it said about 10% difference between the two. So I figure the landy speedos must be tuned to the higher of the two tyre circumferences. This gives a 10% over reading if you're using the smaller of the two. I guess LR have erred on the side a caution, so that it never reads under, so you wont break the speed limit unaware.

Thats why your tom tom is probably more accurate.
Click to expand...

The two sizes common on defenders is 205/80/R16 or on newer models 235/85/R16 the difference is about 9% I don't know if Landrover did/do anything about the difference ex factory.
 
martin1512 said:
Car speedo's are not usually all that accurate. They are usually out by approx 10%,which is why the police have the unofficial 10% +3 mph tolerance for speed limits. Whereas sat nav is usually accurate to less than 1 mph.
Click to expand...

I think its part of the construction and use regulations speedo's have to be within the plus 10% minus 0% so everyone has a statutory defence that if they are driving up to 10% over the speed limit.
 
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