Hello everyone
I decided to swap my wife's
​​​​2004 4.2 Na
from the 17 inch to a set of 18 inch i had sitting in the garage
So now running 245 40 18 on a 18x8 et 60
I
now I have the problem with speedo
At 50 km ph fine
At 80 km ph car is doing 76 km ph
At 110 km ph car is doing 100 km ph
These were tested on a rolling road
Any help would be greatly appreciated
Thanking you
Darrol

 

When I worked at the dealer and a customer needed a new INSTRUMENT PACK the setup was required.
A few questions needed to be answered such as WHEEL SIZE and steering column info etc.

You can possibly change the wheels with the INSTPK setup?

WDS, IDS or similar will be required.

 

Hi thanks for the reply
I tried my JLR IDS SDD
found in system where it said
16 inch rim
17 inch rim
18 inch rim
But that made no difference
Beginning to think it might have something to do with the Differential final drive ratio
as the wifes came factory with 17 inch rims
With a final drive ratio of 2.867
and the 18 inch rims came from my wrecked str
which I think might be
Final drive ratio of 3.31:1
i could possibly put my spear str differential in
not sure how complicated this would be
Or even if it would fit would have to take some measurements to see if the str one is Narrower
Thanking you
Darrol

 

What were the 17s? That is, what was the rolling radius & what now?

It's only out 10% at 110kph, and in the safe (not speeding) direction, so not a disaster.

 

Hi jag v8 thanks for the reply
Ok
The 17s were jaguar factory on 235 /55/17
The 18s are 245 40 18 front and rear on 18x8 rims
I also have 2x 18x9.5 rims
That are identical from my wrecked str
Not sure what police are like where you live
But in new Zealand if you are 4 km ph over the speed limit
You will get a ticket for speeding
at the moment we just go off the GPS on my uniden r8 radar detector and set Cruise control
Thanking you

 

You look LESS likely to get a ticket because the speedo will show X but you'll be going slower than X.

Here, speedos over-read anyway (and are by law not allowed to under read).

 

The 245/40/18 tyres are the correct size for an S-TYPE on 18" rims (as you mention the STR is the only version that runs wider rears)

The 235/55/17 inch tyres that were on the car seem to be the wrong ones for an S-TYPE ?

I have never seen a 'facelift' S-TYPE on a 55 profile tyre, 235/50/17 seems to ring a bell from when I had my first S-TYPE.

The 18" wheels with the correct tyre size have a significantly smaller rolling radius than the size of 17"'s that were on your car, I would therefore expect a big difference in speedo readings.

 

Hi yes you are right
Just looked at the old 17s they have 235/50/17 on them
I got mixed up
As I had just ordered some new tires for my daughter's
Vw
Sorry about that

 

235/50/17 is much less of a difference.

This online tyre size calculator has a comparison for overall size / speedo differences.

https://tire-calc.com/comparison/235...nd-245-40-r18/

 

Interesting . . .

Coz mine has worm 245/45 on Mercury 18" since new and, according to my GPS vs speedo is either spot on up to 100Kph, then only 1-2Kph off at 110Kph.
I have checked it again today,

As JagV8 says . . . check your rolling radius . . . or draw comfort from knowing you have a margin of headroom (favorable error) that will avoid that speeding ticket!

Cheers

 

I swap between the 16" winter and the 18"summer tyres on my S-type and the speedo readings stay about the same.- but less than the GPS reading. The standard sizes all have very similar diameters and don't seem to need any alternative calibrations. You have the correct size of tyre for the car, so I wouldn't worry about it.