Changing tyre size

Wheels, Tyres, Suspension, Chassis, Faults and Fixes
Doctordeano
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Changing tyre size

Post by Doctordeano » Tue Nov 06, 2018 2:38 pm

Hi, I wonder if you can help with a tyre question.

I have a Discovery Sport HSE Lux with 245/45/20 standard tyres fitted. The tyres are due for replacement very soon and I was wondering is there is any benefit in changing the tyre size to 255/55/20.

1. Is there any benefit changing the size with the slightly wider size and taller profile?
2. Will the change affect the drive of the car?
3. Should I stick with the same size?

Many thanks
Dean
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Dashnine
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Re: Changing tyre size

Post by Dashnine » Tue Nov 06, 2018 5:05 pm

Doctordeano wrote:
Tue Nov 06, 2018 2:38 pm
Hi, I wonder if you can help with a tyre question.

I have a Discovery Sport HSE Lux with 245/45/20 standard tyres fitted. The tyres are due for replacement very soon and I was wondering is there is any benefit in changing the tyre size to255/55/20.

1. Is there any benefit changing the size with the slightly wider size and taller profile?
2. Will the change affect the drive of the car?
3. Should I stick with the same size?

Many thanks
Dean
The rule to keep the same rolling radius (for speedo accuracy) is a 5% drop/rise in profile per 20mm width increase/decrease. You're increasing both width and profile.

Going from 245/45 to 255/55 means your sidewall is 55% of 255mm (140mm) instead of 45% of 245mm (110mm) so:

1. I guess you'll get a more compliant ride with marginally more grip. You might get clearance issues with the taller sidewall.
2. Not much, but your speedo will be out by around 8%
3. Prob not worth changing for marginal gains - depends if 255s are way cheaper.

Check it out at https://tiresize.com/comparison/.
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Doctordeano
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Re: Changing tyre size

Post by Doctordeano » Tue Nov 06, 2018 5:26 pm

Thanks for the response.. Much appreciated.
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Mav71
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Re: Changing tyre size

Post by Mav71 » Sun Jan 06, 2019 11:17 am

One question, Why?


DiscoSteve
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Re: Changing tyre size

Post by DiscoSteve » Fri Mar 01, 2019 12:13 pm

Mav71 wrote:
Sun Jan 06, 2019 11:17 am
One question, Why?
I joined the forum specifically to find out about putting bigger tyres on mine. The ride quality is pretty crap on the 245/45s - more sidewall coupled with less chance of curbing it would be very welcome... I've been in a base 18" wheeled Sport before and it was so much more comfortable.

Did the OP end up changing anything? I've been looking at fitting 255/50's, and while I know the circumference must surely be fine on a 4x4 like this, I'd love to know if the width is worth worrying about - ie, I'm assuming they'd fit on the rim ok and wouldn't rub on full lock...?


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Re: Changing tyre size

Post by Mav71 » Fri Mar 01, 2019 12:52 pm

The cars are set up for the profile of tyre supplied in the factory.

If you fit a bigger profile and wider tyre, you will effect the handling, ride quality, gearing and speedometer.

Translated, you will ruin the car.


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Re: Changing tyre size

Post by Dashnine » Fri Mar 01, 2019 1:06 pm

If you wanted 55 profile tyres you need to drop to 225 width to keep (almost) the same rolling radius, and that’s with 19” wheels too.

Keeping the same wheels with 225/55 would mean a speedo reading of 70mph at 72.4mph actual. Not sure you can get 225/55R20 however.

https://tiresize.com/comparison/
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Woolmeister
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Re: Changing tyre size

Post by Woolmeister » Sat Mar 02, 2019 8:18 am

Also worth checking what your current insurer’s attitude is to using a different tyre size to what the manufacturer recommends.

Last thing you want is to find out that you’ve inadvertently made your insurance invalid by doing this.


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DiscoSteve
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Re: Changing tyre size

Post by DiscoSteve » Sat Mar 02, 2019 12:57 pm

Mav71 wrote:
Fri Mar 01, 2019 12:52 pm
The cars are set up for the profile of tyre supplied in the factory.

If you fit a bigger profile and wider tyre, you will effect the handling, ride quality, gearing and speedometer.

Translated, you will ruin the car.
It's never going to ruin it! The whole point is to affect the handling and ride quality, because the standard car is very poor on the 20" alloys. The sidewall size is still less than that of the 19" wheel so it's hardly going to become a wobbly mess. The speedo difference will be negligible. I just wanted to hear from anyone who had made the change, to see if it had improved things significantly.


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Dashnine
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Re: Changing tyre size

Post by Dashnine » Sat Mar 02, 2019 1:19 pm

DiscoSteve wrote:
Sat Mar 02, 2019 12:57 pm
Mav71 wrote:
Fri Mar 01, 2019 12:52 pm
The cars are set up for the profile of tyre supplied in the factory.

If you fit a bigger profile and wider tyre, you will effect the handling, ride quality, gearing and speedometer.

Translated, you will ruin the car.
It's never going to ruin it! The whole point is to affect the handling and ride quality, because the standard car is very poor on the 20" alloys. The sidewall size is still less than that of the 19" wheel so it's hardly going to become a wobbly mess. The speedo difference will be negligible. I just wanted to hear from anyone who had made the change, to see if it had improved things significantly.
Very poor on 20”? Mmm, it’s subjective really, depends what you want from the car, each to their own and so on. 18” will give a more compliant ride, handling a little less sharp if you like chucking down the lanes, and 19” is in the middle.

But even if you put 255/50R20 on, you’re only increasing the sidewall from 45% of 245mm to 50% of 255mm, which is 17mm so a marginal improvement for the various issues (clearance, speedo, etc.) you might face plus the (brand name) tyres don’t look cheap.

Wouldn’t you be better off switching the wheels to 19” or 18”, you might find someone or a dealer prepared to swap at at least keep the vehicle within the manufacturers specs?
MY19 P240 HSE, Indus Silver, Black Pack, Privacy Glass, 12 Way Memory Seats, Roof Rails, Spacesaver, Cold Climate Pack, 2 x USB in 1st & 2nd rows.
Gone: MY16 D180 HSE Black, Santorini Black


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