Wheels: Size vs Ride trade off

cunnifferous

Recently bought an Alfa 147 with 18 inch wheels on it and 225/40 tyres. The ride is fairly rough on it tho on non perfect roads and I'm wondering if fitting 17 inch wheels with the recommended 215/45 tyres would make much difference to the ride?

Reluctant to go down to 16 inch wheels as they don't look great imo.

colm_mcm

215/45 would have a slightly smaller outside radius than your existing tyre, the sidewall will be about 8% thicker which is good. 215/45 is an oddball size though, you'd be much better going for 225/45r17 as it's a really common size, easier to find in the tyre shops and cheaper. It retains the tyre width you have whilst giving you a 14% thicker sidewall than you have now while being the same outside radius as your existing tyre. You're basically adding an inch of sidewall.

You can calculate the sidewalls in this case by multiplying the width by the profile and dividing by 100.
215 X 0.45 = 96.7mm
225 X 0.4 = 90mm
225 X 0.45 = 101.5mm

cunnifferous

colm_mcm wrote: »

215/45 would have a slightly smaller outside radius than your existing tyre, the sidewall will be about 8% thicker which is good. 215/45 is an oddball size though, you'd be much better going for 225/45r17 as it's a really common size, easier to find in the tyre shops and cheaper. It retains the tyre width you have whilst giving you a 14% thicker sidewall than you have now while being the same outside radius as your existing tyre. You're basically adding an inch of sidewall.

Thanks for the detailed answer! Noob question, but are 225 and 215 interchangeable without any problems? I got the 215/45/17 spec from the owners manual.

colm_mcm

Yes, the car won't know the difference, the outside radius of the tyre will be identical to the old one and the width is exactly the same. It's effectively just the wheel size inside the tyre that's changing. The inch outer radius of wheel you're losing is being filled by an extra inch of sidewall.

Cee-Jay-Cee

cunnifferous wrote: »

Thanks for the detailed answer! Noob question, but are 225 and 215 interchangeable without any problems? I got the 215/45/17 spec from the owners manual.

Don't bother changing the wheels, keep the 18"wheels and just fit a slightly larger tyre, ie 225/45/18 instead of a 225/40. It'll give you a bit extra profile and increase comfort, you will retain the same tyre width and it'll actually leave your speedometer reading more accurate as they generally read slightly lower than actual speed (usually 3-5% lower) with OE size tyres.

colm_mcm

You could probably have problems with wheels rubbing if you go any bigger.

grogi

Cee-Jay-Cee wrote: »

Don't bother changing the wheels, keep the 18"wheels and just fit a slightly larger tyre, ie 225/45/18 instead of a 225/40. It'll give you a bit extra profile and increase comfort, you will retain the same tyre width and it'll actually leave your speedometer reading more accurate as they generally read slightly lower than actual speed (usually 3-5% lower) with OE size tyres.

The speedo always reads higher (you go 90 km/h, it shows 96 km/h). It is a type-approval requirement...

Cee-Jay-Cee

grogi wrote: »

The speedo always reads higher (you go 90 km/h, it shows 96 km/h). It is a type-approval requirement...

Yeah sorry, I meant read faster than actual speed.