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Quick Question About Bigger Tyres / Gauge Readings.

Bennis
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Bennis Rising Star

Bennis

Posted
Posted

Ok so I know that if you put a bigger tyre on a car, then it will travel faster than indicated.

Eg. with standard tyre at 30km on the speedo you will do 30.

Bigger tyre at 30km on the speedo you might be doing for example 35.

But I just need to know if the relationship between the tacho and speed changes?

Eg. with standard tyre at 100km on the speedo you might be doing 3000rpm in 5th gear.

With a bigger tyre, I know that the speedo will be out, but will the tacho be out too? Will 3000rpm in 5th gear with a bigger tyre still equate to 100km/h (ignoring the speedo) ?

Hope this made sense

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

No for the same engine speed you will be going faster but the difference will be slight. Its only your speedo that will be different - the tacho will still tell you how fast your crank is spinning. You can calculate it if you measure the outside circumference of the old and new tyres and get the difference as a percentage - your speed will pretty much increase by that same percentage.

Duncan Grand Master

Duncan

Posted
Posted

BTW don't assume your speedo is correct with factory tyre sizes. ADRs say a speedo has to read 0-10% more than actual speed.

You should check your speedo against a gps speed, eg on your phone.

Sinista32 Grand Master

Sinista32

Posted
Posted

http://www.willtheyfit.com/

Input new and old sizes, it'll tell you the speedo difference at 60mph/100kmph.

Some cars have a potentiometre in the dash you can change the speedo calibration.

That's a great site - thanks

89CAL Grand Master

89CAL

Posted
Posted

I usually find that speedos shows 5km/h faster than actual (standard tyre sizes, compared to GPS based speed).

Yeah mine is around 10% out for whatever reason. Tacho is out a touch too. Probably dodgy cluster lol

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