Quick true or false about alloys and tyres?

daveleebond

DW Regular +
Joined
Jan 31, 2007
Messages
3,462
Reaction score
314
Location
Manchester
if i fit after market low profile rims to my car, does it affect my speedo, if so could you use this to overturn a speeding ticket, with the case you did not know this? i thought wheels was wheels!
 

Dutcho

VIP POOF
VIP Member
Joined
May 27, 2001
Messages
20,880
Reaction score
519
Location
Been to Rat's, have rash to prove it ;)
If the rolling diameter changes then yes I would of thought it would effect your speedo but as to if you could use that to over turn a speeding fine I don't know, depends how fast you were going.
 

daveleebond

DW Regular +
Joined
Jan 31, 2007
Messages
3,462
Reaction score
314
Location
Manchester
whats the margin of a speed camera?
surely if the company who fitted them did not inform you of this then they are liable?
insurance knows they are fitted.
 

drahcirk03

Inactive User
Joined
Jul 31, 2008
Messages
138
Reaction score
1
is the speedo on the outside of your tyre, no, so therefore it makes not the slightest difference,.
 

edcase

Inactive User
Joined
May 2, 2005
Messages
1,161
Reaction score
18
is the speedo on the outside of your tyre, no, so therefore it makes not the slightest difference,.

That is surely not the case. Im no expert but I think modern speedos work out the speed based on the speed of a shaft somewhere between the output of the gearbox and the wheels.

Surely fitting smaller wheels (outside diameter of tyre) would cause the car to travel a smaller distance per revolution of the shaft, so the car would be physically travlling slower than is shown on the speedo.

Im sure somebody else will know better than me though - would be intersted to know exactly how they work myself.

edcase
 

Kris

DW Member ++
Joined
Jul 19, 2004
Messages
1,719
Reaction score
6
That's my understanding of it too. Without the speedo being recalibrated, any significant change in the diameter would produce a mismatch.

Say you had a tyre 19 inches in diameter, that gives a circumference of 59.66". If you change that for an alloy and tyre that gives you a 22 inch diameter, 69.08". That's almost a foot more you're travelling to each revolution.

Therefore, lets just say you're travelling 100mph as it appears on your speedo. You'd actually be travelling at, give or take, ~110mph with the larger diameter.


Depends if you decide you want to stick bigger wheels on the car that just about fit on compared to the originals! But going for alloys with lower profile tyres compensate each other and might not make a significant difference.
 

Moley-UK

Member ++
Joined
Nov 28, 2005
Messages
446
Reaction score
3
You will not get out of a speeding fine by using that as an excuse.
At the end of the day, they give you something like 5% above speed limit plus 2mph or something like that

Plus the majority of speedos read over, like my corsa reads 77mph but is going exactly 70mph on the gps receiver.
 

Munkey

DW Regular
Joined
Jan 27, 2006
Messages
5,849
Reaction score
179
if i fit after market low profile rims to my car, does it affect my speedo, if so could you use this to overturn a speeding ticket, with the case you did not know this? i thought wheels was wheels!

What size rims on what car mate? Just being a little nosey.
 

nozzer

VIP Member
VIP Member
Joined
Jan 25, 2005
Messages
6,662
Reaction score
106
Location
Who knows !
if i fit after market low profile rims to my car, does it affect my speedo, if so could you use this to overturn a speeding ticket, with the case you did not know this? i thought wheels was wheels!

No, ignorance is never deemed to be a valid defence !

Any speed difference caused by this is likely to be quite small anyway.
 
TEST
Top