putting bigger wheels on car

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heavyusr

83 posts

Master Geek

#153942 13-Oct-2014 14:09

Current car has 15 inch factory wheels and normal tyres
I want to switch to low profile tyres and get bigger wheels but I want the speedo to stay accurate
What is the formula here? If I go to 16 inch wheels and get low profile tyres will the speedo still read the correct speed?

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#1152864 13-Oct-2014 14:15

http://tire-size-conversion.com/tire-size-calculator/

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tdgeek

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#1152865 13-Oct-2014 14:15

I'd just keep it simple and measure the diagonal, on roughly the same tread wear. You may find there are various levels of low profile that you might need to check to get the closest one

Poll

343 posts

Ultimate Geek

#1152887 13-Oct-2014 14:42

Do be wary going to 16's, I've got factory 16's and they're an odd ball size for nz apparently, could be worth looking at 17's which are much cheaper for rims and tyres.

ubergeeknz

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Vocus

#1152891 13-Oct-2014 14:46

Poll: Do be wary going to 16's, I've got factory 16's and they're an odd ball size for nz apparently, could be worth looking at 17's which are much cheaper for rims and tyres.

Yeah whichever size you are intending on, get some tyre quotes and ask availability. Some sizes are less common and it will end up costing you. I had 16" wheels on a previous car as well, and tyre choices were fairly limited (read expensive).

vexxxboy

4244 posts

Uber Geek

#1152909 13-Oct-2014 15:05

just a warning , if your daily journeys require you to go over judder bars, speed bumps or pitted roads, then dont get low profile tyres. They just make your trip miserable.

Common sense is not as common as you think.

Batman

Mad Scientist
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#1152913 13-Oct-2014 15:10

16 17" and 40 45 profile is ok.

Otherwise life will be hell

scuwp

3885 posts

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#1152935 13-Oct-2014 15:24

Yep, be aware that the tyre is actually part of the vehicles suspension system. Swap the tyre for a lower profile with less flexibility = harder ride.

I agree 16" are an off size now, 17" will give you mush more scope for tyres.

You can get your speedometer re-calibrated, but also agree a simple diameter match should be about right, there are so many variables it is never 100% all the time anyway.

Other thiings to note (although a reputable tyre shop will see you right on this stuff)
http://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/tyres,-wheels-and-hubs/tyres-and-wheels
http://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/tyres,-wheels-and-hubs/tyres-and-wheels#table1011

You shouldn't need certification for 17" mags, unless you are going to lower it as well.

Lazy is such an ugly word, I prefer to call it selective participation

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Batman

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#1152937 13-Oct-2014 15:29

just stay with 45 profile if possible that is the sweet spot

ilovemusic

1439 posts

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#1153092 13-Oct-2014 19:26

op;
what car ?
many older cars are bad partners for modern low profile tyres.

Coil

6614 posts

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#1153093 13-Oct-2014 19:27

Athlonite

1828 posts

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#1153102 13-Oct-2014 19:43

^ ROFL^

Mark

1653 posts

Uber Geek

#1153173 13-Oct-2014 21:54

Hmmm ... this is my dream of what I want my car looking like ...

https://www.youtube.com/watch?v=MDU5BU_qSJU

;-)

ckc

321 posts

Ultimate Geek
Inactive user

#1153193 13-Oct-2014 22:29

joker97: 16 17" and 40 45 profile is ok.

Otherwise life will be hell

Combination of harder shocks, 215/45R17 makes mine handle very well, but you feel every single bump in the road. It depends on what you have to begin with.

But to answer the question, your speedo is probably on the low side on standard wheels anyway. The standards on mine are either 15 or 16, and AFAIK they don't actually bother to adjust the speedo for either of those. With 17s, when I'm doing 100 on the dial I know I'm doing 100 on the radar.

1eStar

1604 posts

Uber Geek

#1153195 13-Oct-2014 22:38

The first number is the width in mm

The number after the / is the profile height as a percentage of the width

Then wheel size is in inches

So say your standard tyre wheel combo is 185/65 14
diameter is (185*0.65*2)+(25.4*14)=596.1mm

Then you might choose to go to a 205/50 15
Diameter is 586mm

Or 205/45 16 = 590.1mm

Note that tread is around 7mm when new. So you can see these would ask be close enough for the speedo to be accurate enough. A tyre shop usually has a big chart on the wall with compatible tire sizes. From memory the diameter needs to be within 10% for a wof.

What is important is rim width, backspacing and offset. A tyre shop can help you here. This is how far out the tyres stick from the car.

Your tyre tread that contacts the ground must be completely obscured by a guard when viewed from above. The tyre must not foul the body through its travel.

Unless you just want to look cool, I'd thoroughly recommend spending your money on high quality tyres on the standard rims. You will be surprised how good quality tyres feel, handle and their wet weather perforance etc. Low profiles are bumpy and prone to damage easier to puncture etc.

ilovemusic

1439 posts

Uber Geek

#1153402 14-Oct-2014 10:28

1eStar:

What is important is rim width, backspacing and offset.

and suspension adjustability.

with many older cars (what car is op going to put the tyres on ?) just plonking on fancy low profile tyres will not work without fine tuning the suspension settings.

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