Speedo Vs Wheels Vs Gps

P38Aaron · March 12, 2006

Hey people.

Ok i got a road angel few months ago, incase you didn't know it tells your speed with GPS (satelites) apparently really accurate.

Ok my IS200 Sport has got the standard 17" alloys on.

If im doing 70mph with my road angel my speedo in my dash reads about 76mph.

In turn this must surely affect the overal milage of the car on the tacometer as are both run through the throttle cable???

My girlfriends got a Polo (poor little thing) :) thats got 14" wheels and when the road angel sits in there at 70mph, its about 72mph on her dash...so obviously vw have done a better job calibrating the wheels to the car, or something?

Am i right thinking the entry IS200 has 16" wheels? if thats the case speedo should be fairly spot on?

All you guys with 18's are probably really racking up the miles and thinking your going faster than what you really are!

That is probably just gobledegook but i think i can understand it. not a good sign...anyways can the speedo get re-calibrated? i realise there is a tollerence for tyre pressures and depth.

Bazza · March 12, 2006

Hey people.
Ok i got a road angel few months ago, incase you didn't know it tells your speed with GPS (satelites) apparently really accurate.

Ok my IS200 Sport has got the standard 17" alloys on.

If im doing 70mph with my road angel my speedo in my dash reads about 76mph.

In turn this must surely affect the overal milage of the car on the tacometer as are both run through the throttle cable???

My girlfriends got a Polo (poor little thing) :) thats got 14" wheels and when the road angel sits in there at 70mph, its about 72mph on her dash...so obviously vw have done a better job calibrating the wheels to the car, or something?

Am i right thinking the entry IS200 has 16" wheels? if thats the case speedo should be fairly spot on?

All you guys with 18's are probably really racking up the miles and thinking your going faster than what you really are!

That is probably just gobledegook but i think i can understand it. not a good sign...anyways can the speedo get re-calibrated? i realise there is a tollerence for tyre pressures and depth.

the rolling radius of the 16", 17" and 18" are very much the same, so doubt if that would make any difference

they are designed to be out and allways reading above true mileage, reputed to be 10% , which makes the lexus more accurate than a VW

what does the VW read at 30 mph ??

ColinBarber · March 12, 2006

the speedo is designed to read over. If it was reading 72mph at a real 70mph then that would probably be out of spec and need replacing, it should read 73-79mph.

Neimad · March 12, 2006

The law states that speedometers should have a tolerence of between -0.0% and +10.0% I believe, so that means that no speedo should ever under-read, but they can over-read by up to ten percent. That doesn't mean they are designed to over-read (it's simply very expensive and requires constant re-calibration to ensure that a speedo is 100% accurate), nor does it mean that one reading 72mph at a true 70mph is any more or less fit for purpose than one reading 76mph -- quite the opposite, in fact.

MacRS200 · March 12, 2006

If you have a speed limiter it is bang on 112 (confirmed by GPS on a track).

So why can a speedo not be that same, and bear in mind that if your speedo is reading +10% so is your mileage. So a 60k mile car is actually a 54k mile car. Good for the manufacturers as you they get more services in.

ColinBarber · March 13, 2006

The law states that speedometers should have a tolerence of between -0.0% and +10.0% I believe, so that means that no speedo should ever under-read, but they can over-read by up to ten percent. That doesn't mean they are designed to over-read (it's simply very expensive and requires constant re-calibration to ensure that a speedo is 100% accurate), nor does it mean that one reading 72mph at a true 70mph is any more or less fit for purpose than one reading 76mph -- quite the opposite, in fact.

if you look at the workshop manual you will find the tolerances for the speedo, it is designed to read over. The tolerances are different for different countries, for example in Aus the tolerance is much tighter.

P38Aaron · March 13, 2006

the rolling radius of the 16", 17" and 18" are very much the same, so doubt if that would make any difference
they are designed to be out and allways reading above true mileage, reputed to be 10% , which makes the lexus more accurate than a VW

what does the VW read at 30 mph ??

polo reads around 31, unlike IS that reads 33/34...

is quite good just means that if you keep on schedule then most IS's are serviced very well(as they get in before hand)

thanks for everybodys reply.

Jiberjaber · March 13, 2006

Dont forget your GPS will only reflect true speed when you are on a flat surface!

Mr Morse · March 13, 2006

Dont forget your GPS will only reflect true speed when you are on a flat surface!

Only if less than 4 birds have been acquired....4 or more and the positioning is 3d

Jiberjaber · March 13, 2006

Consumer GPS generaly only provides the horizontal component (2D) of speed, irrespective of the number of locks.

P38Aaron · March 13, 2006

So what should you take notice off when going past a speed camera to be safe?

Speedo or GPS???

Jiberjaber · March 13, 2006

So what should you take notice off when going past a speed camera to be safe?
Speedo or GPS???

Your speedo, that is what it is there for! :)

If wish to use the GPS speed for guadance, then that is up to you. I have never personally had any problems with speed cameras when driving off the GPS speed, but that is a risk I take, YMMV :)

bondms · March 13, 2006

So why can a speedo not be that same, and bear in mind that if your speedo is reading +10% so is your mileage. So a 60k mile car is actually a 54k mile car. Good for the manufacturers as you they get more services in.

It doesn't work like that. If you drive at an indicated 60 mph you would expect your mileage to click up at 1 mile every minute. It doesn't.

With my car, I need to drive at approximately an indicated 65 mph in order for the mileage to go up at one mile per minute so I expect the speedometer is reading high but the mileage is much closer to true.

mally · March 16, 2006

Hello Matey, my car is a IS200 Sport, with 17in wheels. I own a PDA with built in GPS and I use a programme called Vito Navigator, that tells me that when I'm doing 70mph my Speedometer shows 75/76mph. And when i'm doing *** it shows ***, sensored incase the Old Bill are watching :winky: There is always a built in error on Speedometers. Hope this helps. All the best Wayne

oldcro · March 16, 2006

How on earth do some people expect a car's speedo to be accurate? It electronically or mechanically calculates the speed from the number of revolutions a wheel does. Now consider the difference in circumference between a brand new tyre and one that is well worn. Then the difference between an under inflated and over inflated tyre on its rolling circumference.

It is obvious that the indicated speed will vary compared to the actual speed. Which is why manufactures invariably set thir speeedos to read slightly over so a speedo will never read under if the tyres have the minimum amount of tread and pressures are at least the minimum reccommended for them.

Just be thankfull that at 30mph indicated you are doing slightly under 30mph, because doing 31mph in a 30mph is breaking the law and however unlikely you can be prosecuted for the offence.

So endith my feeble attempt to bring some sanity to this discussion.

oldun :shutit:

IanB · March 16, 2006

My Tom Tom One shows the same speed difference as Mally's.

IanB :D

MacRS200 · March 16, 2006

How on earth do some people expect a car's speedo to be accurate? It electronically or mechanically calculates the speed from the number of revolutions a wheel does. Now consider the difference in circumference between a brand new tyre and one that is well worn. Then the difference between an under inflated and over inflated tyre on its rolling circumference.

Fair point but............Say the difference in tread depth between a new and knackered tyre is 5mm that calculates out at less than 1% difference on the rolling radius/circumference. The under/over inflation bit makes no difference to the number of revolutions it is just that you have a straight bit at the bottom the "length" of rubber round the outside of the tyre is the same.

oldcro · March 16, 2006

How on earth do some people expect a car's speedo to be accurate? It electronically or mechanically calculates the speed from the number of revolutions a wheel does. Now consider the difference in circumference between a brand new tyre and one that is well worn. Then the difference between an under inflated and over inflated tyre on its rolling circumference.

Fair point but............Say the difference in tread depth between a new and knackered tyre is 5mm that calculates out at less than 1% difference on the rolling radius/circumference. The under/over inflation bit makes no difference to the number of revolutions it is just that you have a straight bit at the bottom the "length" of rubber round the outside of the tyre is the same.

I get your point, but if the tread depth is 5mm less then the diameter of the tyre is 10mm less which puts the difference at nearer 2%. Not sure I agree with your second point either. If the contact area of the tyre was flat (which only the middle area is) then your point is relevant, but over inflation tends to swell the centre of the contact area thus increasing its circumference. Of course the reverse applies for under inflation. So the circumference does change depending on tyre pressure. Well thats my story anyway.

oldun

oldcro · March 17, 2006

How on earth do some people expect a car's speedo to be accurate? It electronically or mechanically calculates the speed from the number of revolutions a wheel does. Now consider the difference in circumference between a brand new tyre and one that is well worn. Then the difference between an under inflated and over inflated tyre on its rolling circumference.

Fair point but............Say the difference in tread depth between a new and knackered tyre is 5mm that calculates out at less than 1% difference on the rolling radius/circumference. The under/over inflation bit makes no difference to the number of revolutions it is just that you have a straight bit at the bottom the "length" of rubber round the outside of the tyre is the same.

Under inflation makes a big difference, the "length of rubber round the outside of the tyre in immaterial. The tyre is no longer round at the point where it is in contact with the road. In the under inflated tyre the axle is nearer the road so the wheel has a smaller radius and needs more revolutions to cover a given distance.

Say the flat bit is250mm long, mark the tyre at the first contact point then rotate the tyre say 10 degrees until the mark is just leaving the road. The wheel will have moved 250mm. Now fully inflate the tyre and mark where it first touches the road. now rotate the wheel 10 degrees and the wheel will have travelled a greater distance. The mark in the first example have travelled in a straight line, while in the second example the mark moved in an arc. A straight line is the shortest distance between two points, so the mark which travelled in an arc must have travelled further.

oldun