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Tyres Size => Speedo Error

tjandriesen
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tjandriesen Proficient

tjandriesen

Posted
Posted

I know that changing the diameter of my tyres will result in an error in my speedo reading. I'm not sure what the "original" GTR tyre size was but I'm going to a 255/40/17. Anyone know how much "adjustment" i need to figure into the speedo?

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https://www.sau.com.au/forums/topic/166324-tyres-size-speedo-error/
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Beer Baron Grand Master

Beer Baron

Posted
Posted

i'm pretty sure the standard 33 GTR size is 235/45/17, but either way it's bugger all difference. the speedo is only accurate to about 10% to begin with so i wouldn't worry too much.

Lunatikk Rising Star

Lunatikk

Posted
Posted

Just beg borrow or steal a SatNav or similar & you'll then know exactly what you're car's doing. Anything else is guesswork

For reference, my 32 GTR is on 255/40/17's up from the factory 16's & reads roughly 7k's down at 100kph but as BB has already said that's within the factory specs

Taleb Tyres Proficient

Taleb Tyres

Posted
Posted

Mate, you need to work out the size based on whatever your tyre placard states, it should be whatever the stock R32 GTR size is, however sometimes it may be a bit diferent(mistakes at compliance), you're allowed to go over or under the stated tyre size rolling diameter by no more than 15mm according to rta, so keep that in mind when changing sizes. And if you like you can get your speedo recalibrated for the difference.

wonkynutz Experienced

wonkynutz

Posted
Posted

This is simple mate. Like T187 said the RTA will only let you go 15mm bigger in overall diameter without getting an engineer's cert.

If you want to do the calcs on any wheel/tyre combo it goes like this:

1/ Start by converting the wheel diameter to millimetres. I'll use my car as an example (R32 GTS4).

Stock size is 16 inches. Multiply by 25.4 to get 406.4 mm (yes I know the .4 is stupid but bear with me).

2/ Work out tyre height. Stock width ( 205 ) multiplied by profile ratio ( 0.55 ) multiplied by 2 = 225.5 mm

In this example 55% profile is the same as 55/100 which is 0.55. Times 2 coz tyre height is added twice.

3/ Add the wheel diameter to the tyre heights to get stock overall diameter of 631.9 mm. Cool?

I'm now running 235/40/18's. The numbers for this combo are:

Wheel: 18 x 25.4 = 457.2 mm. Tyre: 235 x 0.4 x 2 = 188 mm. Overall diameter: 457.2 + 188 = 645.2 mm.

645.2 - 631.9 = 13.3 mm more diameter than stock = no worries.

Speedo error = 631.9 divided by 645.2 = 0.9794, so at 100 k's the speedo will read 98, assuming it was accurate to start with, which it probably wasn't.

Bugger all difference though.

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