Relative tyre sizes

[simonsparrow]

Got a question about relative tyre sizes (if anyone knows). I know the speed 6 Cerberas came with the same size tyres front and rear (225/50x16), I also think the Cerbera GT did too (245s all round), but all other Cerberas have had wider tyres at the rear than the front.

What is the effect of having the same width tyre on all corners?

I know Jes Firth has the same size R888 Toyos on his car with no problems.

The reason for asking is I'm thinking of changing to 17" or 18" wheels and if I go for the same rim width front and rear, was thinking about doing the same with the tyres.

[gazzab]

the 18s originally came on slightly less wide tyres (at the rear I think) than 17s as noone made the same width at that wheel size.

[bilton_d]

17 fronts 235/40 rears 255/40
18 fronts 225/35 rears 255/35

i have often thought about fitting 255's to the fronts in the same way the Tuscan S has

>> Edited by bilton_d on Tuesday 23 August 16:10

[Tam Lin]

bilton_d said:

17 fronts 235/40 rears 255/40 18 fronts 225/35 rears 255/40 i have often thought about fitting 255's to the fronts in the same way the Tuscan S has

Other than looks, why? It would give the marginal power steering more to cope with, increase the supertanker turning circle, and the car is quite overtyred as is, so lateral grip tends not to be down to lack of rubber. It would take longer to get enough heat into the front tyres.

With equal sizes front and rear, you'd get less initial understeer (depending on suspension set-up), but presumably at a cost of more likelyhood of aquaplaning in the wet.

17 fronts 235/40 rears 255/40 & Joolz set up works just fine, IMHO.

[simonsparrow]

Interesting comment about the car being overtyred, I was thinking if the Speed 6 was ok with 225/50x16, then could you run 225/40x18 front and rear for instance?

Are the differing tyre sizes front and rear there to promote understeer to some degree?

I read with interest in the latest Evo that the new Marcos Coupe has quite modest sized tyres (215 front and 225 rear) with 475hp as standard......

[apguy]

Can I hijack this thread?

My RL7 (4.5 wheels) equipped Cerbera is sporting 255/40/17 Toyos at the back, the fronts are knackered due to excess camber and I need a pair for the front. I could have sworn that the fronts were 225/40/17. But my tyrefitter is saying:
a) Toyo Proxies aren't available in a 225/40/17
b) It should be a 225/45/17 to get a near matching rolling radius.

Anyone got the std size for a 4.5 front tyre?

Edited: Reason for asking is that the car is 30miles away from me at the mo' Otherwise I'd just go and look

>> Edited by apguy on Tuesday 23 August 10:46

[simonsparrow]

I think 235/40x17 is the correct size for your wheels.

[Tam Lin]

simonsparrow said:

Are the differing tyre sizes front and rear there to promote understeer to some degree?

..or possibly a compromise to cope with the output of the more powerful cars? Remember some of the press cars seemed well endowed. Wouldn't do to have Jeremy Clarkson sitting there in a cloud of smoke.

It is interesting that the Speed Six Cerberas (in all the road tests I've read) were deemed to have the better ride/grip compromise for the road.

simonsparrow said:

I read with interest in the latest Evo that the new Marcos Coupe has quite modest sized tyres (215 front and 225 rear) with 475hp as standard......

Looks quite nice, too. Can't wait to see one for real

[simonsparrow]

Just an update on this question, has anyone run 'speed six' sized tyres on 17" or 18" wheels? (i.e. same width front and rear and slightly higher profile than the V8 sizes....)

So,

16"=225/50 front and rear
17"=225/45 front and rear
18"=225/40 front and rear

All answers on a postcard....