Please ignore my ignorance but .....

[Pablo]

I ran santa pod in my (still) standard Galant VR4 auto last weekend an scored a 14.697, which i thought was a great time for a totally stock car. However i thought crossed my mind an wanted to clarify it with you guys.......


The rear tyres are standard 255 an fronts 205 which leaves me asking the question, although it was designed this way, will the different sizing put a strain on the diff, etc????
As you can tell this is my first galant and indeed the first car ive run with mixed tyre sizing, so like the title says ignore the ignorance! so views advise suggestions anybody???

[AlanDITD]

the tyres should be the same on all four corners?

[Pablo]

this is the clarification im talking about! thanks alan! so basically the last owner has put bigger on the back namely 255s an left the front as 205s so from this my guess is im hurting the diff????

[gareth001]

the tyres should be the same on all four corners?

Not just the same size, but the same age/tred depth as well - 4x4 cars should have all four wheels changed at the same time. Otherwise the diff is constantly fighting to move some wheels faster than others. This will eventually start braking apart the internal gears.

So worst case you purchase 4 brand new tyres and drive them 5000 miles and have worn away 2mm of tread. You then get a blowout - it's not 1 tyre to replace, but all 4.

Afraid Pablo if the front's are not brand new - it's replacement of all four.

[elnevio]

Decent time there for a stocker!


What are the other numbers on the tyres, as these affect the rolling radius of the tyre (which is really the important factor)?

[Pablo]

i think its 255/50/16s rear an 205/55/16 front i'll need to double check but it does explain a few things

[AlanDITD]

Does anyone have the rolling radius of both those two tyre sizes?

It might actually be about the same i dunno. The width doesnt matter, as long as the rolling radius is the same!

[gareth001]

Does anyone have the rolling radius of both those two tyre sizes?

It might actually be about the same i dunno. The width doesnt matter, as long as the rolling radius is the same!

This website has a nice calculator for it:

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

Works out at 4.7% out

255/45/16 would have been better at only 0.6% out

[Ghost_2008]

Although the rolling radius may be the same the fact that you have more contact area at the rear therefore more resistance, this could be putting more pressure however marginal on the diff........... best play it safe and change the tyres all round........

But in essence there is no real need to have different widths front and rear on a four wheel drive vehicle, rear wheel drive is a different story.

[gareth001]

...rear wheel drive is a different story.

Just look at drag cars!

[AlanDITD]

I disagree ghost

I think that fact we have the AYC and a 60/40 split bias to the rear is a very good reason to have wider tyres on there

It will not cause more friction, if you had the 255/50/16 on the fronts aswell the rears would still have the same amount of friction anyway!

Its the rolling radius that maters.

[Pablo]

Having read this i tend to agree, so new rubber all round but in my defence i was lead to believe this was normal so thanks for the heads up!

[Pablo]

I disagree ghost

I think that fact we have the AYC and a 60/40 split bias to the rear is a very good reason to have wider tyres on there

It will not cause more friction, if you had the 255/50/16 on the fronts aswell the rears would still have the same amount of friction anyway!

Its the rolling radius that maters.

So as i have AYC whats the ideal sizing for tyres then?

[elnevio]

I disagree ghost

I think that fact we have the AYC and a 60/40 split bias to the rear is a very good reason to have wider tyres on there

It will not cause more friction, if you had the 255/50/16 on the fronts aswell the rears would still have the same amount of friction anyway!

Its the rolling radius that maters.

At the same pressure, the contact patch is actually the same size (in area) anyway.

[Gowf]

At the same pressure, the contact patch is actually the same size (in area) anyway.

In theory yes, the patch is the same area, but in a different orientation.... but this is the static case. The contact area doesnt matter as much as the friction coefficient produced whist under dynamic loading, which will be very different due to the orientation of the patch area.

So on a solid surface with friction you would have more usuable grip in both directions.

Irrespective of that, it is the rolling circumference that matters, so yes....... same size on all 4 and replace all 4 if not new.

[chris g]

To avoid differences in rolling radius just go for same size tyres!

AYC and other mechanical bits are too expensive to risk messing up just by playing boy racer/chav in attempt for more grip/performance...

Car does not need them and benefits are prob marginal if not absent...

[John TheAntique]

Nip down to Hemel to the Herts meet on Sunday and get advice straight from the horses mouth so to speak. You're welcome.

[chris g]

Horses mouth...?

Advocating mixed size tyres on VR-4...?

[bradc]

Are you sure they are 255's? That is really really wide!

As long as the sizes are close you will be fine. For example 205/55/16 and 225/50/16 are the two stock sizes and work fine together. 225/45/17 and 255/40/17 is another good option.

[Turbo_Steve]

If the rolling radius is the same (i.e. use a tape measure to check the diameter) then I can't see it's going to abnormally load up a diff in any major way: the gearing is the same. Yes, there's a little more in terms of torque split, but it's no different to the rigours of cornering etc: the gear-ratio change as the tyres wear is likely to have more impact.