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[bigtow]

Hi all I have just layed my hands on a set of 18" 8j bling bling rims off another omega but they don't have any tires on at the moment . I have standard elite rims now so what size tires should I get for the new rims and can I get the speedo re cailbrated via tech 2 ?

[2woody]

pick the right tyre size and the speedo won't need re-calibrating. Moreover, the car will still be safe and predicatable.

the standard car is designed with 820 wrpm tyres, so to preserve the gearing and handling you need to be using

235/40 x 18
225/40 x 18
215/45 x 18

and those are in order grip at the top, handling at the bottom

[bigtow]

thanks for that i just found this site and their calculator says a 225/45/18 will only make the speedo read slow by 0.5 mph where as 235/45/18 will make it 1.3 mph out . 40 profile will make it read fast . not a major problem my main concern is clearence issues ?
http://www.performanceprobe.com/index2.php?redirect=http://www.performanceprobe.com/misc/index.php?do=calc&page=tire

[2woody]

225/45 is 2% out
235/45 is a whopping 3.6% out

moreover, the 235 will raise the whole car half an inch - more than enough to bu**er up the handling.

[NiallyV6CDX]

225/40s is closest to original manufacturer spec and probably cheapest and most popular of those sizes.
I once fitted 235/45/17s to my old 325i and they made it tramline terribly, so don't go too wide on the fronts.

[feeutfo]

225/235/245 relates to the width, how does this affect the gearing?

i thought the profile (40) was the important mesurement for the outside circumferance, or rolling radious, whatever its called...?

[Kevin Wood]

225/235/245 relates to the width, how does this affect the gearing?

i thought the profile (40) was the important mesurement for the outside circumferance, or rolling radious, whatever its called...?

Provide is specified as a proportion of the width, so a 245/40 has a higher profile than a 235/40.

Kevin

[2woody]

kinda....

there's actually only two really important figures related to tyres, wrpm or wheel revs per mile and hub height or static laden radius to be precise.

wrpm is a figure that is measured for each size of tyre. IT CANNOT BE CALCULATED, as it depends on the maximum load, tyre slip, temperature, etc. But, the figure is vital as it defines the gearing.

Most of the "ready-made" calculators just do a basic arithmetical calculation which can be up to 10% wrong.

hub-height or SLR is important as the whole suspension package is designed around the hub being at a certain height. If, for instance, the tyre is larger, the suspension geometry will be wrecked. SLR is again measured and cannot be calculated as it allows for the amount the tyre "sags" at the bottom when on the car.

if you were to do the calculations (wrong), then the nominal diameter would be ( width times profile % ) + ( width times profile % ) + wheel size.

[bigtow]

Now I am confused so what size do I need ?

[2woody]

235/40 x 18
225/40 x 18
or
215/45 x 18

215 will give you the best handling, whilst the 235 will give you the best grip.

[feeutfo]

245 40 18 sc3 fitted on the front, same size  linglong at the back that came with the wheels, handls better than ever, grip is good at the front, back keeps applying opposite lock in the wet for some strange reason. Linglongs are sh1te. Ride is firmer for the better.

Gearing is higher going by feel and sat nav speed reading. But would this give slightly better economy? Width gives some kerb protection as the spokes bow out past the rim on sports stars.

[NiallyV6CDX]

245 40 18 sc3 fitted on the front, same size  linglong at the back that came with the wheels, handls better than ever, grip is good at the front, back keeps applying opposite lock in the wet for some strange reason. Linglongs are sh1te. Ride is firmer for the better.

Gearing is higher going by feel and sat nav speed reading. But would this give slightly better economy? Width gives some kerb protection as the spokes bow out past the rim on sports stars.

Possibly, but slower off the mark too.
I find that going too wide affects the steering in a negative way.
It is a matter of personal taste as much as anything.

[feeutfo]

245 40 18 sc3 fitted on the front, same size  linglong at the back that came with the wheels, handls better than ever, grip is good at the front, back keeps applying opposite lock in the wet for some strange reason. Linglongs are sh1te. Ride is firmer for the better.

Gearing is higher going by feel and sat nav speed reading. But would this give slightly better economy? Width gives some kerb protection as the spokes bow out past the rim on sports stars.

Possibly, but slower off the mark too.
I find that going too wide affects the steering in a negative way.
It is a matter of personal taste as much as anything.

With new tyres as it stands now i'm really pleased, i was concerned re tram lining as its ine of my pets, but its fine, as the tyres wear however....well time will tell.

Did i mention those Slingshlongs are crap by the way? just checking.