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Speedo Reading

2K views 14 replies 8 participants last post by  Futura97 
#1 ·
Watched ACA the other night about the speedo readings and how people are being done because there speedo is "lying" to them. In one case there was this persons whos speedo said he was doing 70-75 but in actual fact he was doing 140-150. What a dick i say surely you can feel the speed???

Anyway to my question, My car, XF S, was originally a 2.92 dif with the standard 14" rims(not sure what profile it was). It now has a 2.77 dif and 215/60/15. And it also has the XF Ghia instreument cluster in it. Would my speedo be out? If it is does anyone know which way it would be out - eg 60=70 or 70=60??

Cheers Pal's
Matt
 
#3 ·
yeah your speedo would definately be out! if you change any sort of ratios ie diff, tyres your speedo will change...

but what i dont know that someone else might is how to reset it so its accurate again??

but yeah yours is definately out
 
#5 ·
I think its blue that you need and pink that you have.

But for some weird reason, when I changed my car from auto to man, the gearbox had the pink gear cause the diff in the donor car was 2.92:1 so at first my speedo was way out, so I changed the gear with the one out of the auto and it still read high, so then I put the gear back on the one from the auto and put the whole lot in (transducer and gear), and it was fine after that.

This was surprising to me as I thought you ONLY had to change the gear, but it wasnt the case for me.
 
#6 ·
MADXF said:
I think its blue that you need and pink that you have.

But for some weird reason, when I changed my car from auto to man, the gearbox had the pink gear cause the diff in the donor car was 2.92:1 so at first my speedo was way out, so I changed the gear with the one out of the auto and it still read high, so then I put the gear back on the one from the auto and put the whole lot in (transducer and gear), and it was fine after that.

This was surprising to me as I thought you ONLY had to change the gear, but it wasnt the case for me.
Paul,

the tranducer you got with you gearbox might have been out of an earlier model. Check the part numbers on it. The one you got with the box was probably and earlier one...
 
#8 ·
Some info:
Tyre circumferences
215-65-14 = 199cm
215-60-14 = 192cm
215-60-15 = 200cm
225-50-15 = 190cm
205-60-15 = 196cm
215-55-15 = 193cm

Ratio from 2.92 to 2.77 = 5% reduction in revolutions on input for same output.

In your case
Change of diff = speedo = 100 road speed = 105
Tyres
If you started with 215-65-14 there is no change
If you started with 215-60-14 there is a 4% reduction
ie drive at 100 doing 104 actually (on top of diff change)

You can "correct" the diff change by reducing the rolling circumference of the tyres by the same amount.
If you went to a 190 - 193 cm circumference (profiles above) you counter the diff change.

This is not exact!! +/- 1% or so...
 
#9 ·
ps
I think from XD on the speedo has a dashpot in the back so you can recalibrate it. There is a small sticker on the back with which reveals a screwdriver slot adjuster at the rear of the speedo.
I havent done it myself but heard it somewhere.
A Speedo shop should know for sure.
 
#10 ·
You can get an electronic speedo adjustor.

Its a small device the size of a pack of cigarettes. It has dip switches on it, so that you can adjust the speedo correct to withing 2km/hr.

The best way to measure how much your speedo is out, is by going on a dyno.
 
#11 ·
Another way of checking speedo reading is to time the car over a measured distance at a CONSTANT speed ie. between distance checking markers on the express way, using a stop watch and another person doing the timing(not the driver).
Then calculate the number of seconds per kilometer to the best decimal place.
NOW use the formula "3600 divided by the time in seconds"and the result is the speed in kilometers per hour
 
#13 ·
Ive got a website page, where you can calculate all this.
(Makes things a bit easy).
Also has Transmission gears included in the calculations, so you can play around with it, to get the percentage error down to the minimum amount.

I did this cause I was thinking of changing Diff ratios, and Tyre Profiles, and it was quicker to write some Javascript, than to F@#$ around with numbers all the time

The Link to the page is

http://www4.tpgi.com.au/users/mij/Automotive/SpeedoCalc/SpeedoCalc.html

I dunno what Speedo Sender Gears you currently have, and also there are bound to be more than the ones that I have listed, so If there are any additions, or changes you would like made to the page, then let me know

Cheers
Matt
 
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