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        General >> Give/Need advice to/from others >> MPH Discrepancy
        
(Message started by: SCDCarter on Oct 10^th, 2007, 11:32am)

Title: MPH Discrepancy
Post by SCDCarter on Oct 10^th, 2007, 11:32am

Hi,

I have a early 1995 Scorpio Executive 2.5 TD.

My TomTom is set up to display my driving speed.

I have noticed that at beyond 60MPH on the car speedo the TomTom is registering a speed of 10MPH slower. (This does not happen in any of my other cars). One has to assume that the satellite is the more accurate measuring system unless of course it is used for guiding a missile.

The Scorpio is fitted with 195/60/15 tyres and wheels.

Does anyone know if these are the correct size? or, should I be looking elsewhere for a solution to this problem.

Steve

Title: Re: MPH Discrepancy
Post by Alastair on Oct 10^th, 2007, 1:37pm

when I checked with Ford prior to my MOT I was told that my car (2.3 16V 1998) came with either: -

195 65R 15V or 205 60R 15V. I'm not sure what difference having 195 60 15 tyres would make.

Alastair

Title: Re: MPH Discrepancy
Post by SCDCarter on Oct 10^th, 2007, 3:19pm

Thanks for the reply,

Not being a mechanic, I am also unaware of the differences made by all these different size wheels and tyres. I understand though that the middle number of 195/60/15 refers to the size of the tyre wall so, if the car is supposed to have 65 rather than 60 it appear logical that the wheel is rotating more often than it should. However, I doubt that it is turning that fast that the reading would be 10mph faster after reaching about 50/60 MPH.

Any of you technical wizards want to point out the flaws in my thinking.

Steve

Title: Re: MPH Discrepancy
Post by taliban aka Cheekyboy2 on Oct 10^th, 2007, 3:23pm

no technical wizzard needed; i think you'll find it common to have a speed discrepancy in a slightly older ford...... ::)

Title: Re: MPH Discrepancy
Post by SCDCarter on Oct 10^th, 2007, 3:35pm

Thanks for that, I will relax then. It was not so much the speed discrepancy as the thought that I might have the wrong wheels and tyres fitted.

steve

Title: Re: MPH Discrepancy
Post by Matt on Oct 10^th, 2007, 6:43pm

the wheel size will play a small factor but it will still be out

rule of thumb for any speedo is about 10% out, which at slower speeds you wont notice but up to 70mph (and beyond) you will see the difference

Title: Re: MPH Discrepancy
Post by dan_the_man on Oct 10^th, 2007, 7:06pm

press and hold the trip clock untill the little lcd display goes funny then press it in a stabbing motion to check the speed in km/hour. this is much more accurate than the ford speedo. 62 mph = 100 km/ph. ( you won't loose your distance on the trip clock either )

Title: Re: MPH Discrepancy
Post by dan_the_man on Oct 10^th, 2007, 7:17pm

ps your outside diamiter of the wheel is 615mm ( 195 x 0.6+195 x 0.6) 234mm + (15x25.4) 381 615mm outside diamiter. My scorpio came with an outside dimension of 627 ' I understand this is standard '. I changed wheels and juggled the tyre sizes to 624. anywhere within 5% is cool and you'll have better acceleration , which in my opinion is where the Scorpio is slightly lagging !

Title: Re: MPH Discrepancy
Post by SCDCarter on Oct 10^th, 2007, 7:48pm

Thanks, I think, whilst your math is understandable, what all those numbers actually mean is beyond the comprehension of my brain, limited as it is by years of living in the agricultural belt.

195 I assume is the Tyre width.
0.6 is the tyre wall height (60) expressed as a fraction
15 the wheel size?

what is the 25.4?

Steve

Title: Re: MPH Discrepancy
Post by dan_the_man on Oct 10^th, 2007, 7:56pm

the wheel size is in inches. the other measurements in mm. it is more simple for me to work in mm. One ipmerial inch = 25.4 mm the '60' size is known as the aspect ratio, 60 % of . it varys with different makes and models.

Title: Re: MPH Discrepancy
Post by SCDCarter on Oct 10^th, 2007, 11:32pm

Sorry mate,

I was not being sarcastic. I genuinely do not understand what your computations actually mean. I do understand the math but translating the numbers in physical meaning is not within my capabilities, at the moment.

Steve

Title: Re: MPH Discrepancy
Post by Snoopy on Oct 11^th, 2007, 8:11am

THIS (http://www.chris-longhurst.com/carbibles/tyre_bible.html) is a good source of information about tyre sizes and the comparisons that is worth having a look at . May explain one or two things for you better that me telling you .

Title: Re: MPH Discrepancy
Post by harry.m1byt on Oct 11^th, 2007, 11:30pm

Your speedo legally can read upto 10% high, but not low. High readings impress customers, so many do read at the top end of 10%.

Your sat nav can under some circumstances read a little low, such as when you are going along a winding road or to a lesser extent up and down hills. They also only average over a 1 second period and lag current speed by 1 second.

On the flat, on a straight road and doing a constant speed - they are usually extremely accurate assuming a good sat fix. With a poor fix I can sometimes see 2,000mph ;D

Title: Re: MPH Discrepancy
Post by peteHull on Oct 12^th, 2007, 6:29am

on 10/11/07 at 23:30:18, harry.m1byt wrote:

With a poor fix I can sometimes see 2,000mph ;D

Isn't this the norm for a 12v 2.9? ;D

Title: Re: MPH Discrepancy
Post by Robiko on Oct 12^th, 2007, 9:04am

on 10/11/07 at 08:11:34, Snoopy wrote:

Scroll down to: Minimum legal tread depth does not mean "safe".

Sacrilege, smashing up a Scorpio just for a TV demo, hanging's too good for 'em... ;o)

Title: Re: MPH Discrepancy
Post by ChrisF on Oct 12^th, 2007, 5:34pm

With a Sat Nav the speed will read about 5mph low it is all due to the inbuilt 'error' because we mere mortals are not military personel and only have TomTom's and the like. Were we to have military equipment we would see accuracy to with a few feet and not 15yards and speed that was near nigh perfect.

Title: Re: MPH Discrepancy
Post by harry.m1byt on Oct 14^th, 2007, 10:13pm

on 10/12/07 at 09:04:08, Robiko wrote:

Scroll down to: Minimum legal tread depth does not mean "safe".

Sacrilege, smashing up a Scorpio just for a TV demo, hanging's too good for 'em... ;o)

..and a Granada!

Bet they had to disable the ABS to get the steering to twitch like that.

Title: Re: MPH Discrepancy
Post by Highlander on Oct 15^th, 2007, 7:43am

we had a thread about that fifth gear demonstration before. it was a complete waste of time, there was no way even with NEW tyres the Scorpio could have stopped in time!
And yes things were disabled! ABS and the airbag did not operate.. ;)

A much better test would have been to show them stopping side by side but then thats not good telly ;)