Incorrect Speedo

[Ad-mk2Ibiza]

Basically i fitted some alloys which usually knocks the speedo out
these are 205/55/15 id rather change down the 45 but aint got the money atm.
so how much should this be out? i read into it abit and it could be way out
just wanna check up.
is it much to get it recalibrated?

 

[Kai_1600]

Borrow someone's GPS and check your speed with that

 

[andycupra]

what were the orignal wheel tyre measurements? i should be able to calculate the difference.
(but then so would anyone with basic maths).

 

[andycupra]

but on the assumption that 205/45/15 would be spot on, then 205/55/15 will read approximately 9% less. (as one turn of the wheel will travel 9% further).


at an indicated speed of 30 you would be doing about 33.7
at an indicated speed of 40 you would be doing about 44.6
at an indicated speed of 50 you would be doing about 54.5
at an indicated speed of 60 you would be doing about 65.4
at an indicated speed of 70 you would be doing about 76.3

However, as speedos tend to over-read as standard, you may find that you could halve the errors above compared to 'actual speed'.

 

[The stig 2]

If you have a search on google there is a tyre calcualter and you type in what your tyres and are it willl tell you how much off etc you are.

 

[andycupra]

If you have a search on google there is a tyre calcualter and you type in what your tyres and are it willl tell you how much off etc you are.

you could put the link on here...

 

[Fl@pper]

http://www.miata.net/garage/tirecalc.html

is my favourite

 

[ross p]

need username and password

 

[Fl@pper]

need username and password

the new one is java based

try the original http://www.miata.net/garage/tirecalcold.html

 

[Ad-mk2Ibiza]

thankyou
at 60 i will really be doing 63.8

is it much to get it recalibrated

 

[andycupra]

are you not planning on changing to tyres in the future?

no idea what the cost if to recalibrate, perhaps you can call some places and find out.
Bear in mind the car wont accelerate as quickly. however it will of course have a slightly higher speed available in any 1 gear.

 

[Ad-mk2Ibiza]

yeah i want to change to 45s but it wont be for a long time yet these tyres have loads left and i have no money

 

[andycupra]

given that speedos generally start off by reading to0 high, (as if its going to be out is safer to read high) the 'real' error is only going is likely to be 2-3 mpg at 60.

So if i was you id would either:
1 - just remember you travel a little under the indicated speed.
2 - use a gps reading. eg via sat nav.

 

[surcalation]

GPS via sat never will be cheaper then have your speedo re-calibrated.

Mathmatically it seems that you car should be way out but in reality I found mine to not be that far out (couple mph) with 17's on

 

[Ad-mk2Ibiza]

cheers lads
il try with a sat nav see what it says

at 30 its about right i think cos them warning 30signs dont flash at me at 30

my mate insisted he was doing 90 the other night when i was infront of him mine said 80
so looks like il have to try a satnav