Tyres Size => Speedo Error

[tjandriesen]

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?

[neil_se]

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

[n15m0]

Or here:

http://www.1010tires.com/TireSizeCalculator.asp

But first you need to find out what your factory tyre profile is and then compare it to what you are getting..+/- 15mm is nothing to worry about.

[evil_weevil]

another...

http://www.carbibles.com/tyre_bible.html

half way to 3/4's down page

[tjandriesen]

Using the frist calculator going from 245/45/17's (stock size) to 255/40/17 is a 2.5% difference.

[Beer Baron]

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.

[gun32gtr]

i wouldnt worry. unlike on my landcruiser having 33muds on it changes it a fair bit the bloke at shop said

[Lunatikk]

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

[evil_weevil]

my speedo is accurate till bout 60-70 - on freeway, sit on 110 on dash - powerFC shows under 105

[Taleb Tyres]

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.

[Lunatikk]

my speedo is accurate till bout 60-70 - on freeway, sit on 110 on dash - powerFC shows under 105

And where does the PFC get it's speed signal from????

[wonkynutz]

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.

[mattymagoo22]

power fc gets its reading from the speedo head