will bigger wheels change my speedo

[VXII_S_smokin]

i'm in the market for a set of 18's to put on my VX, and a mate of mine (who knows ****all about cars but is a real know it all) says to me

"if ur whells become bigger wont that make ur speedo in-accurate"

will bigger wheels and tyers make my speedo read slower that the car is accually going?

does my speedo need to be recalibrated?

help soon (cause to treed on my back tyers is really bad and i need new tyres soon)

 

[EvoVIIIJDM]

the laws of wheel revolution obviously if you increase the circumference or wheel diameter, it takes longer to do one full revolution of the wheel, which will be different from your factory wheels whichyour speedo is set to. I guess its going to make much more of a difference from say a 15" wheel to an 18" wheel then say from 17 to 18s. There definately will be an increase in wheel size so your speedo will be out, but as to how far its out im not sure. Hope this at least a bit helpful.

 

[sixshooter]

Bigger diametre tyres = greater distance travelled per rotation of the axle per time frame .: speedo will read slower than what your really doing

Smaller diametre tyres = less distance travelled per rotation of the axle per time frame .: speedo will read faster than what your really doing

And as for changing diff ratio...say 3:1 ratio is standard

so you have 3:1 ratio = 3 rotations of tailshaft for 1 complete wheel rotation

.: 1/3 rotation of tyre = 1 rotation of tailshaft

compare to 4:1 ratio = 4 rotations of tailshaft for 1 complete wheel rotation

.: 1/4 rotation of tyre = 1 rotation of tailshaft

speed = (distance travelled / time)

.: in the time taken for one revolution of the tailshaft the 4:1 ratio has turned the wheel less per time frame .: the distance travelled is less .: the instantaneous speed will be slower than what the speedo says as the speedo is driven off the gearbox which thinks its turning a 3:1 ratio but its not...

 

[AirStrike]

Speedo wont change as long as you change to an appropriate tyre. If on a 18" you use a 235/40/18 you will be fine, the change is like 0.6% out.
AirStrike :air:

 

[jNINE]

Airstrike is right - changing the wheel size won't affect the rolling diameter if you decrease the tyre profile.

If you are still not certain use this calculator I found on the net;

http://www.1010tires.com/tiresizecalculator.asp

 

[AirStrike]

Airstrike is right - changing the wheel size won't affect the rolling diameter if you decrease the tyre profile.

If you are still not certain use this calculator I found on the net;

http://www.1010tires.com/tiresizecalculator.asp

Using that program, comparing a 205/65/15 to a 235/40/18 there is a Speedometer Difference of 0.343% too fast. So there speedo will be showing faster then what you are doing, so if it says 100 on the speedo you would be doing like 99.7km/h.
AirStrike :air:

 

[VXII_S_smokin]

thanks for the advice

is it at all possible to have the speedo re-calibrated so that it reads the speed accuratly?

 

[deserthead]

I put 18s on my VX not long ago. Cant tell u the tyre profile off the top of my head.
But I haven't noticed any speedo change at all. I still seem to go faster than most people. . . . . .

Went down the M4, cruise was set at 115 went past a cop with radar and nuthin. So it cant b that much out.

 

[Boonz]

your also forgetting the 7% leway that cops give on radars.....so 7% of 110 is about 8ish so they would of flashed you if you were doing 118km/h

 

[deserthead]

Yeh I no they got leway, but my point was, it didn't throw the speedo out enough to get me booked wen i was doin ova the speed limit.

I got told it was 10% + 1 in NSW. . . .?? Correct me if im wrong though.