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        General >> OBD Advice >> VE: VSS accuracy
        
(Message started by: ADroste on Aug 16^th, 2007, 9:22pm)

Title: VE: VSS accuracy
Post by ADroste on Aug 16^th, 2007, 9:22pm

Hi,
here is an academic question, which I hope the experts can answer.
Connecting the "Pepper Cable" and starting VE you can receive VSS data, if the car is moved. You can change display between mph or km/h, but you cannot tell VE, which car you have, neither your tires, nor your gearbox. So, how can VE display the speed halfway correctly, if it does not know the transmission factor? Is it possible that all OBD-2 compliant cars are adjusted to the same fixed VSS scaling factor? But then changing tire diameter is excluded.

Regards
Armin

Title: Re: VE: VSS accuracy
Post by scorpio_man on Aug 16^th, 2007, 9:31pm

hi there

iirc, the gear wheel on the end on the vss is different depending on which engine/gearbox combination you have, hence the ecu (obd2) knows what the 'real' speed is.

hth

Title: Re: VE: VSS accuracy
Post by ADroste on Aug 16^th, 2007, 9:47pm

on 08/16/07 at 21:31:03, scorpio_man wrote:

.. iirc, the gear wheel on the end on the vss is different depending on which engine/gearbox combination you have, hence the ecu (obd2) knows what the 'real' speed is.

So you say: all OBD-2 cars have the same and constant "frequency per speed unit" factor. And that would also mean that you have to change the gear wheel, if you use tires with different diameter.

Regards
Armin

Title: Re: VE: VSS accuracy
Post by scorpio_man on Aug 16^th, 2007, 10:02pm

hi there

if you change the diff on the 24v for the one from the 2.0 16v, which changes the overall gearing, you need to change the vss gear wheel as well.

the same thing applies to changing to bigger wheels. you need to reduce the ratio to keep the speedo correct.

ie, 195x65x15 is the same (just about) as 225x50x16

is this what you mean? :-/

Title: Re: VE: VSS accuracy
Post by ADroste on Aug 16^th, 2007, 10:22pm

on 08/16/07 at 22:02:21, scorpio_man wrote:

the same thing applies to changing to bigger wheels. you need to reduce the ratio to keep the speedo correct. ie, 195x65x15 is the same (just about) as 225x50x16 is this what you mean? :-/

Yes and no. I am interested in your "(just about)". I know that the tire diameter decreases, if the profile gets thin. I guess you loose 1% precision with 225x50x16 loosing 3mm rubber.
The reason for my question is to find out, if it is possible to check the accuracy of the speedometer by comparing it with the value given by VE without beeing aided by GPS.

Regards
Armin

Title: Re: VE: VSS accuracy
Post by scorpio_man on Aug 17^th, 2007, 8:26am

hi there

if the car is standard, then the read out from the vss should be about 5-10% less than the speedo.

you can check this in 2 ways.

1. by doing a dash reset and scrolling to speed (read out in is kph) http://www.fordscorpio.co.uk/instreset.htm

2. if you have the trip computer, reset it as you drive along and for a few moments it will show your 'true' speed. http://www.fordscorpio.co.uk/images/handbook020.gif

hth