Hi all,

A friend ofmine comented about how my car doesn't feel that its going as fast as the speedo says, I just thought it was the car, but we both agreed that I didn't seem to catch up to people even when I was doing 80, 90 etc.
So...we done a little test, took his car out and mine, firstly he was doing 60, me doing 60 and he was pulling away from me. Drove side by side, him 70, me 77/78 to keep level. Him 80, me 89 to keep level.
Now, the original wheels on my MY97 Cataluna were 205x55x15, the previous owner changed these for 205x50x16 and i've now got the same 16" wheels but 215x40x16's. So is this why? I might try to work out the wheel diameter changes but am not very mathematic and already have a headache, so any help much appreciated.

Many thanks...

 

From the OE Cat wheels to the current ones the car:-

Difference in rolling circumference is -88.27 mm or -4.63 %.

This will mean that your actual speed will be 57.22 mph when your Speedo reads 60mph.

Have a look here at this link I found a while back .

 

Many thanks...
We have just being doing an excel spreadsheet, trying to work it out and kinda come up with the same sort of thing, but this link is much better.



Paul

 

Important 2 bear in mind all speedos will over read a bit as at least that way they're legal as yer allowed 10% over read but none under! So bit close on the 77mph thing but all good still as it were U kud always pull needle off speedo hope it dont break & put it on a bit more anticlockwise woodnt bother thou myself as i broke enuff stuff doin white dials lol
Si (just off 2 plug in my new road angel 2 find out how accurate mine is )

 

I agree with Si

Most manufacturer's speedo's read slow, even the electronic ones. Plugging in our Auto-Diagnos into the ECU and going for a drive will normally say the cars doing about 5kph (never mph - damn thing) slower than the speedo reads.

Add to that your difference in rolling circumferance for even greater increase in error.

 

well, at least i know now. Thanks guys.
I've just got one of those Apex boost controller things, and it gives a speed readout (in km) but where does it get the speed from? would it be as inaccurate as the speedo?

 

ARGH BUGGER just typed in essay & got flood controlled as i was replying in another window as well! GRR
Anyway round 2:
i wood guess it works in the same way as the HUD's (heads up displays) in that it's wired in the back of the dash etc & takes reading direct from there & is thus more accurate as no needle misalignment etc but wood still b dodgy c/o strange wheels etc. All presumtions on my part
Anyway just bin out 4 a razz & mine is just under 10% over reading although i wood guesstimate 2% or so was from when i did white dials & put needle on a tad cautiously in terms of over/under reading even thou i marked where it was originally Anyway feminine intuition & guess work over i must say it is nice 2 no how fast u r actually going. I mean it'd b nice 2 reply to the "do u no how fast u were going sir?" with a yes oraphiser well over a tun lmao hmm maybe not so cunning then
Si

 

feminine intuition ?
U r a girlie? Cool. My g/f will be soooo envious, she can't afford the insurance on a scoob...
I've never ever tried to play around with the dials on a car, i had white ones my escort as standard, and would not attempt to take anything apart on the scoob. Not very carry me, likely to break it. I'm more comfortable taking pc's apart, that way when i end up with a few screws left at the end, i don;t run the risk of it falling apart while to 150mph, erm, i mean 70mph

 

LOL no i only wear skirts at the weekend (shhhh )
Si (the man mummy i'm scared )

 

skirts at the weekend, wow you too ?

sh! don't tell my g/f! they're her skirts!

 

my price 4 silence, school girl skirt & fishnets lmao
Si

 

Mmmm, like to see piccies of that!

 

I have similar experiances in my MY01 WRX, I have a Road Angel and it says I'm only doing 70 when the clock says 77. I reckon its a Subaru thing.

Edited to say it has the standard wheels on mine as well!

[Edited by turbo wrx - 3/9/2003 10:07:57 AM]