how does changing wheel size affect speedo reading?

Post Mon May 16, 2005 11:30 am

Hi, i have tried thinking about this and it hurt

, so i shall ask you guys. One e30 i have has 15" alloys on it, but tyres are 195/50R15 or somethin so diameter isnt much different. so speedo should be fine yes?

My brother however went from 155/65/R13 to 185/65R14 which was fine, but then he went up to 205/40/R17 as he needed a couple of inches clearance to get a body kit on.... this would severley muck up the speedo wud it not? (it aint a BM im just wondering)

Hannah

Post Mon May 16, 2005 11:34 am

AT MOST UR SPEEDO COULD BE OUT BY 5MPH BUT IF U BUY A ROAD ANGEL U GET EXACT READING AND NO SPEEDING TICKETS :lol:

Post Mon May 16, 2005 11:35 am

try looking at calculator and see what it comes up with...

beemers tend to take a signal from the diff somewhere and so speedos are not affected by tyre changes but this is not true for all cars.

paul

Post Mon May 16, 2005 11:36 am

ah ok, do you know which way it would be out by? cos 35 in a 30 would be seen as excessive speed... 25 in a 30 would just p**s people off

as for a road angel.. my bro drives 5 miles a day... occasionally 10... not exactly worth it!

thanks

Hannah

Post Mon May 16, 2005 11:42 am

ITS HARD TO SAY 1 OF ME M8 SHEDS HAS 18" ON AND IS 3MPH OUT (33 IN 30) AND ANOTHER IS ON 19" AND IS 7MPH OUT (23 IN 30)

Post Mon May 16, 2005 11:45 am

Hmmm ok, thanks anyway.. will have to get him to drive thru motorway roadworks... the ones that have digital readouts on the side telling you your speed to make you feel guilty for speeding!

Hannah

Post Mon May 16, 2005 11:50 am

155/65/R13 => 185/65R14 = 12% difference (speedo under-reads)

205/40/R17 = 185/65R14 : no change whatsoever!

205/55/15 => 195/50R15 = 5.4% difference (speedo over-reads)

A 12% difference from the original wheels means he's gonna be doing nearly 80 when the speedo says 70

But then most speedos over-read as standard so it's probably not that far out. I've got GPS satnav in my car so I know it over-reads a bit on standard tyres.

Post Mon May 16, 2005 11:54 am

placey wrote:
beemers tend to take a signal from the diff somewhere and so speedos are not affected by tyre changes but this is not true for all cars.

paul

Are you sure mate? Surely if it works out how fast you're going from how many times the gears in the diff turn or whatever, it's still gonna be put out by different tyres? Ie all it knows is how many times the wheels turn, not how far it goes each time they turn?

Post Mon May 16, 2005 1:49 pm

if you put on wheels with a larger cicumference, one full turn will take longer, so in affect would that not put speedo out.

Post Mon May 16, 2005 4:20 pm

nav786 wrote:if you put on wheels with a larger cicumference, one full turn will take longer, so in affect would that not put speedo out.

Exactly! One turn as measured at the diff, will actually be further on the road...

Post Mon May 16, 2005 6:57 pm

oh yeah jon you're right. i just accepted what i was told and repeated it like a fool...

paul

Post Mon May 16, 2005 7:14 pm

placey wrote:oh yeah jon you're right. i just accepted what i was told and repeated it like a fool...

paul

I was about to do the same thing, then I thought 'wait a minute...'!

Post Wed May 18, 2005 4:43 pm

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

Try that, should help.

HTH, Mark.

[EDIT] Ok, it's already on the thread, ooops!!

Post Thu May 19, 2005 7:13 pm

I have a little spreadsheet that does the job. It would be nice to be able to lodge it on the web site for all to use. In the meantime, if you are mathmatically minded its fairly simple to calculate as shown below.

Multiply Tread Width by By Profile Divided by 1270. Add to this the diameter of the wheel. The result is the total diameter of the wheel/tyre combination.

So if we use my Alpina as an example. Original Tyres were 225/45x16 which have been replaced by 215/40X17 F, 245/35X17 R

(225 X 45/1270) + 16 = 23.97244094 inches (608.9mm)
(215 X 40/1270) + 17 = 23.77165354 inches (603.8mm)
(245 X 35/1270) + 17 = 23.75196850 inches (603.3mm)

So the new tyres are about 5mm in diameter smaller, with the rear being half a miilimeter smaller than the front (Bu**er all in other words)

The percentage error can be derived by as 1 minus (New Combo Diameter/Old Combo Diameter) x 100 to give the percentage difference. If the answer is negative, the speedo is under reading.

In the example above (the rears affecting the speedo), the percentage error is calculated as

(1 - (23.75196850/23.97244094)) * 100 = 100 x (1 - 0.991666) = 0.83%.

If the new wheel tyre conbination is bigger than the old set, then the speedo will under read. In legal terms, the speedo can overread by up to 10% (i.e. read 77 when its really doing 70), but must not under read at all. In reality most overread some, so a slight increase in diameter is probably ok.

In my example the Alpina over reads by 0.83%, or in other words a tad over 1mph at 150mph.

[Edit] PS, Shouldn't this be in the Tech Help Section?

Post Sat May 21, 2005 9:43 am

I guess it could have gone in the tech help, but thought i would stick it here as Its about alloy wheels, and just thought i would ask the kinda people who stick huge wheels on their cars!

Hannah