20" 9J alloys. 35 or 40 aspect ratio?

Days out · Aug 11, 2016

Hope some one can clear up a question on tyre aspect ratio for me.

I'm looking to buy some 20" 9J alloys with an offset (ET) of 45. I know that some people are running 275/35/20 tyres and some running 275/40/20 tyres.

I have phoned several wheel companies and been told slightly different information.

1) 275/35/20 tyres is exactly same diameter as standard tyres fitted and thus keeps speedo readout correct. And 275/40/20 tyre is slightly bigger thus the speedo will read slightly slower.

This makes sense. Do you agree?

2) I was told that 275/35/20 tyre is the correct tyre for a 9J rim. Although the 40 will fit and most agree looks good because it has a little more rubber it can rub on the full lock and I was told that this might not be acceptable to insurance companies. The 35 tyre is the right tyre for this size of rim

Q. Unsure about this! Any one got any experience of this?

3) 275/35/20 tyre is a bit more expensive than the 275/40/20. Probably less used size and thus can take longer to get hold of if one needs a replacement.

Q Is this true?

My California is lowered by 30mm.
Will prob go for Pirelli PZero

I'd like to get some advice if to go for the 35 or 40 tyre from those out there running these sizes.
Also does ET of 45 sound right?

Thanks in advance to all those Cali experts out there!!

Geezso · Aug 11, 2016

Firstly I wouldn't be going bigger than 19 for the ride. Your speedo will vary on all non standard wheels. Plus those sound like you will be needing yearly wheel bearings, they are awful wide.

Days out · Aug 11, 2016

Can I ask why I'd need new wheel bearings if the offset is the same as the standard rims. Surely the load would be spread more evenly or am I missing something? I spoke to quite a few owners running 20" and even some on 22" and no one mentioned wheel bearing issues.

Geezso · Aug 11, 2016

The offset is the distance from the hubface to the centre of the alloy. So a 245 wide wheel can have the same offset as a 295 wheel. But there is just 25mm extra on the inside and 25 mm on the outside.
The wider wheel is harder to turn as more surface area is in contact with the ground. This added strain doesn't help your suspension components.

Days out · Aug 11, 2016

Thank you for the information Geezso.

Days out · Aug 11, 2016

Still looking for answers to my original questions.

Loz · Aug 12, 2016

Days out said:
Still looking for answers to my original questions.

1. A. the 275/35 is not the same as standard it is 11mm larger diameter equal to a 2% increase, will change your speedo to probably close to exact gps speed as the standard speedo setting are deliberately underestimated.
1. B. the 275/40 is 38mm or 5.5% larger so well out of the advisable 2.5%

Also remember that your braking efficiency will be reduced if you increase the overall diameter.

2. If it goes on the rim it goes on the rim, a fatter tyre can be good to protect the rims especially if your rims will be lower than the kerbs. If it did rub you just need a smaller offset to move the wheel out a bit, but agree with Geezo regarding loads on bearings etc.

3. True for all non common tyres

Re, insurance, ANY deviation from a standard option will be an insurance issue if they want it to be...

.

Days out · Aug 12, 2016

Thank you Loz. I worked out that the diameter of my standard 215/60R17" tyres is 27.15" and the 275/35/R20 is 27.57". So you are right it's a little bigger but below 2%.

I will go for the 275/35/20 tyre.

Does anyone know if you use same tyre pressures as on the sticker inside the door or are they different for wider tyres?

WelshGas · Aug 12, 2016

Days out said:
Thank you Loz. I worked out that the diameter of my standard 215/60R17" tyres is 27.15" and the 275/35/R20 is 27.57". So you are right it's a little bigger but below 2%.

I will go for the 275/35/20 tyre.

Does anyone know if you use same tyre pressures as on the sticker inside the door or are they different for wider tyres?

Tyre pressures will be different, not just because of the wider tyres but the narrower profile to fit the larger wheels. Probably higher to prevent too much flexing of the side walls.
Higher pressures may affect braking performance. All this information should be made available to the insurance company and get their approval because they will look into it in great detail if a claim is made.
In the past, with a different vehicle, I used a tuning company with a rolling road to provide all this information leading to a reduction in the initial premium, because in this case braking efficiency was improved due to other modifications.