Tyres and rolling radius

[anubis]

I could have posted this in the wheels and tyres section but I kind of wanted some UK opinions...

Ok so I now have 19x9.5 wheels at the front and 19x11 rear and am looking at tyres... I have a question about rolling radius's and all that...

I want to go quite a bit wider than stock at the rear and my local performance tyre place explained to me that I really need to keep the rolling radius as close as possible to stock, to ensure things like speedo, gearing and differential are unaffected.

Plugging it in to his computer he said that 305/35/19 is the way to go on the rear for this reason. That's great as I want 305 on the rear, but the only tyres in that size are P zero. I wanted MPSS so that's issue number one.

If I go for a 305/30/19 there are loads of options including MPSS but then everything will be off apparently

At the front, I really wanted a 275 width but again, due to the fact that the tyre profile (eg 30/35/40) is a ratio to the tyre width, that would put everything out in terms of rolling radius. They have said I need to keep the stock size of 255/40/19 at the front despite the slightly wider wheels. Even going to 265 will be slightly off in terms of radius.

What's confusing me is that on the wheels and tyres section I can see loads of examples of people with all sorts of wheel widths, profiles and wheel sizes. Maybe they're only concerned with stance and how the car looks, as opposed to what tyres should actually work properly for the way the car's designed?

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[daltron]

The only thing that is affected is the speedo will be off (and your gear speed will be different). A Whipple flare tool can fix that. There is nothing "bad" that will happen if the speedo is off. I ran a different tire diameter for quite some time without correcting the speedo.

 

[Gibbo205]

Your doing track days mainly with your car so a square setup would be beneficial of say 285/35/19 all round.

Differential will only be harmed if running different tyres on rear axle in size or type. Yes gearing changes but it has no negative impact on the car, will either just accelerate quicker or slower. Speedo can be fixed by flashing the part of the ECU that stores the revolutions, anyone with a Whipple flare device can do this for you.

I'd go 285/35/19 all round with Cup 2s and then another set of wheels with some Rainsport 3's for wet days.

 

[daltron]

Your doing track days mainly with your car so a square setup would be beneficial of say 285/35/19 all round.

Differential will only be harmed if running different tyres on rear axle in size or type. Yes gearing changes but it has no negative impact on the car, will either just accelerate quicker or slower. Speedo can be fixed by flashing the part of the ECU that stores the revolutions, anyone with a Whipple flare device can do this for you.

I'd go 285/35/19 all round with Cup 2s and then another set of wheels with some Rainsport 3's for wet days.

It's the BCM and each Flare tool is tied to a VIN, he would have to purchase it for his car and his car alone. A better upgrade would be the Whipple SC which comes with the Flare tool :cheers:

 

[Gibbo205]

It's the BCM and each Flare tool is tied to a VIN, he would have to purchase it for his car and his car alone. A better upgrade would be the Whipple SC which comes with the Flare tool :cheers:

A good excuse to Whipple, Anubis if you do Whipple forget the square setup and throw some wider tyres on the back. ;)

Though for a track car I'd advice an NA build with over 500 horses crank and some weight reduction.

 

[anubis]

A good excuse to Whipple, Anubis if you do Whipple forget the square setup and throw some wider tyres on the back. ;)

Though for a track car I'd advice an NA build with over 500 horses crank and some weight reduction.

Yep I remember you saying that :-) I did consider it and with square setup but I am going to whipple it anyway. Just gotta do it you know. So wider tyres on
the back it is... And a bit of a loss handling wise.

So.. eventually I'll have the whipple tool which will sort the speedo

Sounds like I don't need to worry too much about the gearing and differential...

So in short it sounds like I probably can do 305/30/19 and get MPSS on the rear (or Cup) and go a bit wider than 255 on the front... That's good if so. Seemed a bit weird to have 255 front and 305 rear...

 

[Enoch]

On the front, if you go from the standard 255/40-19 to a 275/35-19 the difference in speedo readout is negligible...... And it'll be in you're favour anyway...

Ie speedo will reed slightly faster than you're going. So for example if your speedo was 100% accurate and read 60 at 60 with the standard size, it will read 61 with the new size, but the car would still be travelling at 60 IYSWIM..

I have these sizes on my car, and the speedo error is undetectable against my Garmin....

275/35-19 Front

295/35-19 Rear

 

[daltron]

Yep I remember you saying that :-) I did consider it and with square setup but I am going to whipple it anyway. Just gotta do it you know. So wider tyres on
the back it is... And a bit of a loss handling wise.

So.. eventually I'll have the whipple tool which will sort the speedo

Sounds like I don't need to worry too much about the gearing and differential...

So in short it sounds like I probably can do 305/30/19 and get MPSS on the rear (or Cup) and go a bit wider than 255 on the front... That's good if so. Seemed a bit weird to have 255 front and 305 rear...

You can still handle extremely well with a staggered setup. Granted, the tire is inherently stickier on my new setup (R888 vs my MPSS square set), but I handle better with my 295/325 setup than I did with my 295 squared setup. Going FI, you WILL want a beefy rear. Since these cars are so front heavy, just getting a much wider tire on the front will do wonders. Don't worry about understeer, that is what sway bar adjustments are for.

 

[MustangMustHaves]

I could have posted this in the wheels and tyres section but I kind of wanted some UK opinions...

Ok so I now have 19x9.5 wheels at the front and 19x11 rear and am looking at tyres... I have a question about rolling radius's and all that...

I want to go quite a bit wider than stock at the rear and my local performance tyre place explained to me that I really need to keep the rolling radius as close as possible to stock, to ensure things like speedo, gearing and differential are unaffected.

Plugging it in to his computer he said that 305/30/19 is the way to go on the rear for this reason. That's great as I want 305 on the rear, but the only tyres in that size are P zero. I wanted MPSS so that's issue number one.

If I go for a 305/30/19 there are loads of options including MPSS but then everything will be off apparently

At the front, I really wanted a 275 width but again, due to the fact that the tyre profile (eg 30/35/40) is a ratio to the tyre width, that would put everything out in terms of rolling radius. They have said I need to keep the stock size of 255/40/19 at the front despite the slightly wider wheels. Even going to 265 will be slightly off in terms of radius.

What's confusing me is that on the wheels and tyres section I can see loads of examples of people with all sorts of wheel widths, profiles and wheel sizes. Maybe they're only concerned with stance and how the car looks, as opposed to what tyres should actually work properly for the way the car's designed?

Tell your tire guy 305/30r19 and 255/40r19 would look horrible. Cause the rear would me smaller (total height) than the fronts. Can't be good for handling either btw. It would look like this

305/35r19 and 255/40r19 on the other hand works perfect though (running that myself and loving it). They are the P zeros though, so gotta check if Michelin makes them (am on holiday).

If you want a wider tire in front as well to go with that 305/35r19 in the rear, I'd go for the 285/35r19. Michelin makes them in the MPSS for sure and they recommend it on a 9.5-11" rim.

Don't worry about the speedo (or any of those things). The difference stock is about 8kph between real speed and the speedometer. With 305/35r19 it's 3kph. So if anything, it's finally more accurate :lol:

 

[anubis]

Tell your tire guy 305/30r19 and 255/40r19 would look horrible. Cause the rear would me smaller (total height) than the fronts. Can't be good for handling either btw....

....
305/35r19 and 255/40r19 on the other hand works perfect though (running that myself and loving it). They are the P zeros though, so gotta check if Michelin makes them (am on holiday).

You're completely right! I've just edited my original post as your post made me realize I had a typo... My tyre guy recommended 305/35/19 and
the issue there is that only P zero are available... I had originally typed that he recommended 305/30/19 which was wrong

Ok... So I probably will stick with the original recommendation. Best to get the radius all correct probably

 

[Gibbo205]

You're completely right! I've just edited my original post as your post made me realize I had a typo... My tyre guy recommended 305/35/19 and
the issue there is that only P zero are available... I had originally typed that he recommended 305/30/19 which was wrong

Ok... So I probably will stick with the original recommendation. Best to get the radius all correct probably

305/35/19 MPSS is available from US I believe, but not here in UK/EU. :(

305/30/19 would be too skinny for sure, look odd and probably make the ride rather harsh and make putting Whipple power down harder.

If your doing Whipple and track days, do make sure you upgrade your OPG/Sprocket and I would recommend the cobra jet tensioners as well, they are quite cheap and I have a set I am not using, drop me a PM if interested.

 

[daltron]

On the front, if you go from the standard 255/40-19 to a 275/35-19 the difference in speedo readout is negligible...... And it'll be in you're favour anyway...

Speedometer is only affected by the rear, so go as crazy as you want in the front! :cheers:

 

[wildcatgoal]

I have 275/35-20 front, 305/35-20 rear. If you have a M/T and those tire sizes, the only "issue" is your speed will be off by about 2 mph (speedo says 78, you're actually going 80). No big deal and, apparently, counts mileage a teeny bit slower as a result, haha.

If you have an auto, I'd be more worried but plenty of folks have zero issues with similar +sized tire radiuses. Just don't go driving around with different radiuses on each side of the car; that'll bugger things up after a while.

 

[Enoch]

I have 275/35-20 front, 305/35-20 rear. If you have a M/T and those tire sizes, the only "issue" is your speed will be off by about 2 mph (speedo says 78, you're actually going 80). No big deal and, apparently, counts mileage a teeny bit slower as a result, haha.

If you have an auto, I'd be more worried but plenty of folks have zero issues with similar +sized tire radiuses. Just don't go driving around with different radiuses on each side of the car; that'll bugger things up after a while.

Why would those sizes worry you with an auto

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