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1995-2002 Range Rover P38A
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Discussion Starter #1
I noticed the other day that my GPS was 3 miles per hour slower than what the speedo was saying on my P38. Taken at 70 - 80 mph range. I tried this GPS in another car and it was dead on accurate. How can I confirm the speed? And if it is off how do i fix this? Is this just an adjustment at the back of the gauge or is the gear at the trans warn out?

I'm concerned that not only is the Speedo off but that it would be affecting the Mileage count and affecting my extended warrantee. This is obviously making me record more mileage on the truck than i really am. I'm doing an 8 hour total trip this weekend and that will equate to up to 24 miles off.
 

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your tyre diameter over the life of the tyres can cause the speedo to be off that much. There is no adjustment available.
 

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It can out for a number of reasons. From tyre size or how much they are worn. A new tyre for example has a larger diameter than a worn one. SO for a given rpm the distance covered would be further with the new tyres and your gps reading quicker than the speedo. Hope the make some sort of sence.
 

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1995-2002 Range Rover P38A
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Discussion Starter #5
Tires have almost full tread depth. What is the stock tire size that the SE and HSE comes with? And I will see if the tires are the wrong diameter.
 

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PMPN8EZ said:
Tires have almost full tread depth. What is the stock tire size that the SE and HSE comes with? And I will see if the tires are the wrong diameter.
255/55/18 for 18" rims
 

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Discussion Starter #8
Thanks I have the 16" rims. I'll check.
 

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The US allows 5% error either way - so it's within spec (ie nothing wrong - just forget about it!)

The UK is less stringent providing it is overeading, we are allowed 10% + 6.25mph over but the speedo can never read less than the 'real' speed.


Different makes of tyre can make a big difference, for my series LR I have some 235/85/16 tyres, I've got some MT's and some AT's they are both sold as 235/85/16 but the MT's sit about an inch higher than the AT's
 

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I have three vehicles and all of them are off by a different percentage, but when setting my cruise on the interstate I always go by the GPS speed. I want to get the maximum speed allowable `)
 

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1995-2002 Range Rover P38A
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At an indicated 70mph on the speedo, my Tomtom is telling me I'm doing 68mph. So mines 2mph out.
 

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im on 255/55/18 and have noticed that my speedo readings are far more consistent with the GPS speed than in any of the six vehicles I drive, my 2.4 petrol Frontera for example was a full 10mph faster on the speedo than GPS at 70.

If I set the speed warning to 80mph on my P38 the alarm sounds with the GPS reading 77, so my calibration is the same as the original poster.

The speedos can be recalibrated fairly cheaply [by 'Speedy Cables Ltd' here in UK for example] but a couple of mph difference is almost inconsequential in real terms imo. I look at the fuel guage more than the speedo!!! 31.5mpg average over 1000mile trip last week about 50/50 highway and Welsh back roads burning 75% SVO/diesel mix. A little shocked to see £124.9/ltr on M1 on route back home. guess that will be £125.9/ltr tomorrow?? :evil:

Gordon
 

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I don't suppose anybody's thought that the GPS could be having a bad day? :D They are notoriously fickle machines dpendent upon so many outside influences and dependent upon the smoothing characteristics of the installed software can give erroneous speed position readings
 

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The GPS does always seem to be more accurate when you compare it to the fixed roadside speed displays etc, it's not 100% accurate but will usually be more accurate than the speedo.

I had a toyota celica which read about 10mph faster than the GPS, whilst I had the dials out to replace a bulb I managed to recalibrate it simply by removing the needle and putting it back on a bit further round - took 3 or 4 goes to get it right but I ended up where it was just about correct all the way up to 70 where it was 1mph out.

As far as your warranty goes, I don't know how many miles it's for but most dealers would give you a little to make up for any inaccuracies like this, if it was a 30,000 mile warranty then you're only talking about 900 miles. If I had doen 31,000 and something went wrong I'd expect them to take responsibility. If you are really concerned talk to them and see what their policy is regarding how precise they are to the allowed mileage, if they are strict explain about the 1000 miles difference and see what they say. I just don't think it's enough to be worried about!

Think yourself lucky, vauxhall (GM) had a case here where people where complaining because everytime you opened and shut the door it added 7 miles to the odometer - making services etc come round much quicker! They blamed it on some faulty firmware
 

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All car manufacturers are allowed an error with the odometer reading. They all try to set to the upper limits of the error thus showing more miles on the car than it has actually done.

This makes you think you are doing more MPG which makes you feel good.

It also makes the warranty for x Miles come quicker which makes them feel good.

They will never be accurate due to, as mentioned by rrtoadhall, tyre wear etc.

In the UK:-
"Lord Whitty states to the government: "The Road Vehicles (Construction and Use)
Regulations 1986, as amended, allows the use of speedometers that meet the
requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both
the EC Directive and the ECE Regulation lay down accuracy requirements to be
applied at the time of vehicle approval for speedometers. These requirements are that
the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h."
Association of Chief Police Officers guidelines
ACPO guidelines for all types of speed enforcement, advise that, under normal
circumstances speed cameras should be triggered by motorists speeding by a certain
amount, normally 10% + 2 miles per hour. This is to safeguard against any
discrepancies between speedometers and police equipment. Cameras will not
normally be triggered below these thresholds."
 

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I wonder how the standard speedometer variance defense would go over with the nice officer who pulls you over for doing 68 in a 65, when you thought you were doing 65...hmm....it is then end of the month and the local budget is low.... 8)

I should have tried this defense when a MD trooper pegged me doing 88 in a 65 in my X type last year.... :lol:
 

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I've just been knocked 3 times for speeding, twice on the same day and 1 on another road all becuase of 17" rims with larger tyres so I attached my GPS, 60kmph or speedometer is 64 on GPS so if I go a touch over 60 Im speeding.

End of the year here in mmm what suppose to be sunny South Africa they are rolling out the demerit system so fellow Souf Afwicans be warned and its going to be more complicated than the UK if you get busted, just another African scam scandal..
 
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