[Misc] Car wheel sizes and speedometer.

larus · Apr 18, 2018

This may seem a dumb question, but how does a speedometer actually work?

If the speed of the car is calibrated by the speed which the axle turns, then do manufacturers adjust the setting of the car if you swap from say 18” to 20” wheels.

For example, 18” wheels will be (in simple terms) moving you forward by 54” for one complete revolution.
20” wheels by 60”. Therefore, the % difference is not insignificant, especially if going past speed cameras. (Not been flashed, just curious for those mechanically minded).

WATFORD zero · Apr 18, 2018

Is it PotG? · Apr 18, 2018

Algernon · Apr 18, 2018

Your speedo will read incorrectly if you go from 18 to 20 inch wheels.
However the aspect ratio of the tyre fitted to the larger wheels can be selected to try and match the rolling circumference of the original tyre/wheel setup.
https://www.oponeo.co.uk/advice-and-tips/tyre-size-calculator

studio150 · Apr 18, 2018

Manufacturers will adjust, if you change tyre size yourself, your speedometer will not be accurate

LowKarate · Apr 18, 2018

Yes exactly yes.

larus · Apr 18, 2018

Algernon said:
Your speedo will read incorrectly if you go from 18 to 20 inch wheels.
However the aspect ratio of the tyre fitted to the larger wheels can be selected to try and match the rolling circumference of the original tyre/wheel setup.
https://www.oponeo.co.uk/advice-and-tips/tyre-size-calculator

OK, curiosity has been satisfied then. Thanks,

maffew · Apr 18, 2018

Never thought about it. Good question. Are they not linked to your sat nav via blue tooth? And if you have tiny wheels I guess you can blag your mates your are doing 200MPH, coool!

larus · Apr 18, 2018

maffew said:
Never thought about it. Good question. Are they not linked to your sat nav via blue tooth? And if you have tiny wheels I guess you can blag your mates your are doing 200MPH, coool!

Not all cars have sat nav. And sat nav doesn’t work in tunnels !!!

PILTDOWN MAN · Apr 18, 2018

maffew said:
Never thought about it. Good question. Are they not linked to your sat nav via blue tooth? And if you have tiny wheels I guess you can blag your mates your are doing 200MPH, coool!

Reading some of your posts recently I really wonder if you're taking the piss or are in fact thick.

Either way just to let you know it's against the law not to have a fully functioning speedo including its accuracy (limits of discrepancy apply)

Notters · Apr 18, 2018

Yes. By a quick calculation tyre wear alone could make a couple of % difference.

RexCathedra · Apr 18, 2018

Small differences are the reason why you really shouldn't get summonsed for being 5%-10% or so mph over.
At that point, you're a revenue source, and not a menace to public safety.

Calculator here....

European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.

Blame Brussels.

StonehamPark · Apr 18, 2018

larus said:
...do manufacturers adjust the setting of the car if you swap from say 18” to 20” wheels.

For example, 18” wheels will be (in simple terms) moving you forward by 54” for one complete revolution.
20” wheels by 60”. Therefore, the % difference is not insignificant, especially if going past speed cameras. (Not been flashed, just curious for those mechanically minded).

maffew said:
Never thought about it. Good question. Are they not linked to your sat nav via blue tooth?

PILTDOWN MAN said:
Reading some of your posts recently I really wonder if you're taking the piss or are in fact thick.

B

maffew · Apr 18, 2018

larus said:
Not all cars have sat nav. And sat nav doesn’t work in tunnels !!!

Ah good point, that probably explains why no one ever seems to get pulled speeding in a tunnel

maffew · Apr 18, 2018

PILTDOWN MAN said:
Reading some of your posts recently I really wonder if you're taking the piss or are in fact thick.

upthealbion1970 · Apr 18, 2018

All depends on size of tyre, general rule go up an inch wheel size go down a profile ie 235/45/18 is roughly same rolling radius as 235/40/19

happypig · Apr 18, 2018

PILTDOWN MAN said:
Reading some of your posts recently I really wonder if you're taking the piss or are in fact thick.

Either way just to let you know it's against the law not to have a fully functioning speedo including its accuracy (limits of discrepancy apply)

I had a faulty speedo in my (almost new) Maestro van, had an indicated 125mph past Roedean school but still got overtaken.

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[Misc] Car wheel sizes and speedometer.

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