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I Need Help! My Speedo Is 15Kph Slower

AlexWaltham
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AlexWaltham Contributor

AlexWaltham

Posted
Posted

1993 R33 Skyline GTS

I Need Help When Im Doing 100kph my speedo reads 85kph What The F*@k do i do to fix it

Someone Please Help me is there a easy fix ( Car had a Manual convertion ) not done by me Previous owner

PLEASE HELP ME Iv Got no idea what to do

ANY HELP WILL BE APPRICIATED

  • 2 weeks later...
MBS206 Grand Master

MBS206

Posted
Posted

im running 225/40r18 and the tyre placard says to run 245/40r18s but the width of the tyre wouldnt make much of a difference

The height of the tyre, is proportional to the width.

That number 40, means 40% of the width.

IE, a 225/40 has a side wall that is 90mm

A 245/40 has a side wall that is 98mm

Once you start to factor everything in, that's a much smaller rolling distance, and it means for your car speedo saying 100KM/H, your car is going quite noticeably slower.

But I would say, you really need to convert the diff from auto to manual so that it'll all read correctly again, this would be the easiest and best way to fix your problem as well as changing tyres to the correct size.

mad082 Grand Master

mad082

Posted
Posted

The profile is actually a percentage if half the width.

So your 225/40 would be half of 225 and then 40% of 112.5 which is 45mm- 90mm sounded a little high for a 225/40!

Sorry but you are wrong. I can provide links to various tyre manufactures websites to verify that it is the percentage of the full width if the tyre if you like. you have to remember that the sidewall starts at the bead of the tyre. 45mm is small. For me it's the width of 2 thumbs, and given that the rim covers about 1 thumb width, I know that the tyres on my skyline were more than a thumb width from the rim

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