So, I have 225/45/18, but I can save a third on the cost of new tyres by going 225/40/18. Also, I’ve been told I can change the tyre width either way to 215 or 245 which also has a positive cost effect.

So, can, what’s the safe limit of the change I and what effect does changing either measurement have?

Think about your bike. Wider tyres and narrower will fit up to a point. Narrower tyres will roll better, wider tyres will have a bigger footprint so more grip but more rolling resistance.

Will They fit should help you.

Some tyres are more popular than others so I guess it’s economies of scale.

If you go from 45 profile to 40 profile you’ll have re-calibrate your speedo I reckon.

Might be insurance implications?

Popular sized tyres are definitely cheaper than rare sizes.

225 refers to the width in mm, 45 refers to the profile/height of the tyre in mm from the edge of the rim bead to the outer tread and 18 obviously refers to the wheel size in inches, if you go lower profile as in down to 225/40 then you will most likely notice a decrease in comfort, depending on how you drive a lower sidewall can give more positive steering but at the detriment to ride quality. If you fit a narrower tyre such as 215mm you may have a tiny reduction in dry weather grip but may have a tiny increase in wet weather grip due to less chance of aquaplaning (all dependant on tyre compound/tread depth etc) , a wider tyre may aquaplane easier but may be more sure footed in dry conditions.

If you fit the 245mm make sure when on full lock side to side that you have ample clearance on suspension components and ensure it does not rub inside the wheel arch.

What about looking for 225/50/18?, they may be cheaper and give more comfort on bad roads

Think about your bike. Wider tyres and narrower will fit up to a point. Narrower tyres will roll better, wider tyres will have a bigger footprint so more grip but more rolling resistance.

Wider bike tyres having inherently more rolling resistance is a myth – they are only “slower” due to air resistance & weight (& differences in tread). In fact in certain situations (MTB) wider tyres at lower pressures will actually roll faster than narrow tyres as they can deform more easily to uneven terrain.

No idea about car tyres, sorry.

45 refers to the profile/height of the tyre in mm from the edge of the rim bead to the outer tread

Eh, percentage not height.

225/45/18 – height of the tyre is 45% of 225mm = 101.25mm

225/40/18 – height of the tyre is 40% of 225mm = 90mm

Not a lot from a radius/diameter perspective, but 10mm closer the ground 🙂

tbh If you wanted to save money, buying a car that runs those kinda tyres ain’t the most sensible. Although I’ve 235/35v19’s on mine. 🙂

My advice on it is stick with the size that is fitted on the car from new…….they specced that size etc because if was the optimum for performance of the sum of the the parts…….it’s only £20 a tyre extra and all forms part of pre purchase questions and checks……….that you forgot to ask about on here 😆

My advice on it is stick with the size that is fitted on the car from new…….they specced that size etc because if was the optimum for performance of the sum of the the parts

And what if its one of those cars that is availible with many tire options from new ?

Insurance sometimes aren’t happy if you fit the ‘wrong’ tyre size.

Very often those tyre options relate to the actual size, width type of the road wheel fitted itsel, same as limitations with Bike wheels.
On most moden cars the relevant tyre size Is on the tyre pressure indication sticker on B posts or inside fuel filler caps of what was fitted on the vehicle.

zilog6128 – Member

Think about your bike. Wider tyres and narrower will fit up to a point. Narrower tyres will roll better, wider tyres will have a bigger footprint so more grip but more rolling resistance.

Wider bike tyres having inherently more rolling resistance is a myth – they are only “slower” due to air resistance & weight (& differences in tread). In fact in certain situations (MTB) wider tyres at lower pressures will actually roll faster than narrow tyres as they can deform more easily to uneven terrain.

Did you really need to post that? Do you really think I need educating on MTB tyres? I thought it was obvious that I was making a broad analogy for the op, and I was thinking about road bikes when I posted it, anyway.

If the car was homologated with those tyre sizes, then you’ll be ok. If not then you are essentially modifying the car and will need to declare the change to insurance. Being alfa, it’s unlikely they’d bother testing many variations on the same 18″ rim size. On 16″s I’d expect to see different options available.

IF you want to go down the modified route, you ideally need to stay within 2% of the original rolling radius. As said above, by making the change you’ll affect: ride (40-section tyres are lower profile ~= worse ride), handling (it may grip more, it may well tramline more), fuel economy (higher rev=worse, but lower to the ground=better), acceleration (better, ish), braking (who can say, ask bosch – they programmed the ABS ECU), wet weather performance.

Part of owning a particular car is being able to afford the running costs, these include the obvious – fuel, insurance, ved, also – servicing, repairs, parts replacement – including tyres.
Just saying. 😉

Did you really need to post that? Do you really think I need educating on MTB tyres? I thought it was obvious that I was making a broad analogy for the op, and I was thinking about road bikes when I posted it, anyway.

LOL, defensive much? 🙂

Aren’t all the roadies going for 25 (and even 28)mm tyres these days?

All good, thanks.

However, although they are 18″ Alfa alloys they are NOT shod with the 235/45/18 that appear in the manual as the retrofit option but 225’s. I can only assume that I have the appropriate alloy size to hand – these:

http://www.ebay.de/itm/NEW-GENUINE-ALFA-ROMEO-159-BRERA-SPIDER-18-MULTI-SPOKE-ALLOY-WHEEL-50508157/141517186649?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D28813%26meid%3Dff8c348e4a54455c98d4595b82b8a78f%26pid%3D100011%26rk%3D1%26rkt%3D2%26sd%3D291275221485

So should I be fitting them with the Alfa specced 235’s – which are even MORE expensive that the 225’s/45/18.

Aren’t all the roadies going for 25 (and even 28)mm tyres these days?

Yes. Someone did the research (think I saw it on RCUK) and found that up to a point wider tyres are quicker on the road, for the same reasons as on an MTB.

Hmm, what you wanted to fit (225/48r18) is nigh on 5% smaller or 32mm in actual height. That’s outside what I’d consider acceptable.

How much are you thinking you’d be willing to spend? For a car like your Alfa, I’d consider about £150 a corner, fitted, to be in the ball park.

http://www.miata.net/garage/tirecalc.html

How much are you thinking you’d be willing to spend? For a car like your Alfa, I’d consider about £150 a corner, fitted, to be in the ball park.

And you are probably right, that’d get me some Michi Pilot sports. I guess I’m smarting from mistakenly allowing the car dealer to put some cheapo’s on it pre sale to me (I negootiated new tyres all round in the deal), that are new, don’t need replacing but are putting a £600 seed of doubt re safety in the current conditions especially in my mind.

Having said that I’ve don 800 miles on them now an not a slip, although I have been driving sensibly.

Hmm, what you wanted to fit (225/48r18) is nigh on 5% smaller or 32mm in actual height. That’s outside what I’d consider acceptable.

And being 225 instead of the manufacturer stated 235 is not an issue becuase…?

Kryton57 don’t need replacing but are putting a £600 seed of doubt re safety in the current conditions especially in my mind.

Get some smaller steel wheel with winter tyres on them for now sure?

its only 1.3% smaller, or 9mm diameter. Still not brilliant from an insurance point of view. But personally, I’d be fine with that*, wouldn’t own up & claim ignorance as they were fitted by a dealer. I can’t suggest that you do the same, but you might.

I used this to calculate. They’re all pretty much the same thing.

*My track car has some weird size as you can no longer get the oem size.

One other consideration, going narrower could make your alloys more susceptible to kerbing damage, especially if the chosen tyres don’t have the lip that is designed to protect the alloy when kerbed.

So should I be fitting them with the Alfa specced 235’s – which are even MORE expensive that the 225’s/45/18.

So the previous owner fitted the wrong tyres?

IMO Put on the right size/rating/load tyre for the car – the only discussion really is budget; from no-name thru to Michelin etc. I’ve run Avon tyres for many years now on various cars, happy with them as they are a good combo of grip/wear/price.

What you really need to do is man up a little and do it properly….

hot_fiat – Member
its only 1.3% smaller, or 9mm diameter. Still not brilliant from an insurance point of view. But personally, I’d be fine with that*, wouldn’t own up & claim ignorance as they were fitted by a dealer. I can’t suggest that you do the same, but you might.

I used this to calculate. They’re all pretty much the same thing.

*My track car has some weird size as you can no longer get the oem size.

Sure, but in respect of “wrongness”, if the original manual specs 235 then with 225 on its gon 1.3% / 9mm the WRONG way (lower), than manufacturer spec?

Kryton57 – Member

And you are probably right…

From an another forum regarding exact tyre size:

“You could have a good arguing point with your insurance company – but in the event of an accident it could be an insurance investigator from an injured third party’s insurance company who is looking for the loopholes.”

When I worked with motorcycles, just about the first thing insurance assessors looked at was tyre type/size/condition/tread depth.

If your Alfa is specified with 235/45 R18 – it looks like they would be:

235/45 R18 98W

98 = 750kg load per tyre = 3000kg total
W = up to (approx) 168 mph

To be honest, for a tyre rated to carry the car plus 4/5 people plus luggage up to 168 mph, £150 ish seems justifiable

Nah it’s 2% up or down a size. Doesn’t matter which way. What you have to watch out for is those rims are an 8″j fitting. Which is 204mm. I believe a 235 is right on the max width that you could fit to that.

Camskill might have some decent options…

235/45/18s

Uniroyal Rainsports – £111.50
http://www.camskill.co.uk/m61b0s2057p111677/Uniroyal_Tyres_Car_Uniroyal_RainSport3_Uniroyal_Rain_Sport_3_-_235_45_R18_98Y_XL_FR_TL_Fuel_Eff_%3A_C_Wet_Grip%3A_A_NoiseClass%3A_2_Noise%3A_72dB

Bridgestone Potenza’s – £112.80
http://www.camskill.co.uk/m61b0s2057p110451/Bridgestone_Tyres_Car_Bridgestone_RE050A_Potenza_Bridgestone_RE_050A_-_235_45_R18_98Y_XL_TL_Fuel_Eff_%3A_E_Wet_Grip%3A_C_NoiseClass%3A_2_Noise%3A_72dB

Vredestein Ultracs – £116.25
http://www.camskill.co.uk/m61b0s2057p125621/Vredestein_Tyres_Car_Vredestein_Ultrac_Vorti_-_235_45_R18_98Y_XL_TL_Fuel_Eff_%3A_E_Wet_Grip%3A_B_NoiseClass%3A_2_Noise%3A_70dB

just need to get them fitted….the garage that does my servicing does my tyres for £10+VAT/tyre, but some places will only fit tyres that they supply.

Nah it’s 2% up or down a size. Doesn’t matter which way. What you have to watch out for is those rims are an 8″j fitting. Which is 204mm. I believe a 235 is right on the max width that you could fit to that.

I can’t win can I? If I fit the 235’s Im testing the limit of the tyre on the rim – what might happen? If I run 225’s my insurers may kick off.

Whats “right”?

Looks to me like you’re fine.

Equivalency table

Rim width Minimum tyre width Ideal tyre width Maximum tyre width

7,5 Inches 205 mm 215 or 225 mm 235 mm
8,0 Inches 215 mm 225 or 235 mm 245 mm
8,5 Inches 225 mm 235 or 245 mm 255 mm
9,0 Inches 235 mm 245 or 255 mm 265 mm
9,5 Inches 245 mm 255 or 265 mm 275 mm
10,0 Inches 255 mm 265 or 275 mm 285 mm

Hank’s are my go to mid range tyre, I’ve never met a garage yet that doesn’t want to fit a new tyre supplied by myself. It’s all money.

Is “8j” the 8 inch rim width? ( I appreciate I’m asking the bleedin’ obvious but just in case…

I guess having tyre AND wheel sizes that were originally specced would be the ideal.

Correct

Thats the 235/4518 then…