Wheel & Tyre Sizes

Just changed the original 19" Style 5's for 20" Style6's and to my eyes the style 6's don't fill the arches the same as the style 5's. So my question is, should the overall wheel & Tyre size of the two be the same, I have placed the 19" up against the 20" and they look bigger, but the 20" are On the car so it may not be accurate.

depends on the tire manufacture. some tires have slightly different diameter so it may appear smaller.

I believe they should be the same. The profile of the tyres on the 19" compensate for the smaller wheel rim (with the 20" tyres having a lower profile). However, I stand to be corrected!

It will depend upon the profile of the individual tyres.
You can work out the total diameter by using the profile and wheel diameter. It's possible to have 19" wheels with a higher profile tyre, that stand taller than a 20" wheel, with a lower profile.
ANT

They were 235/55 19 and are now 245/45 20 so by my reckoning should be the same

I work out the 20" wheel & tyre diameter is 28.68 inches and the 19" wheel & tyre diameter is 29.18 inches, so the bigger wheel is 1/2" smaller diameter once those tyres are on.
However, I stand to be corrected!!!!
ANT

each manfacture has different diameter even thought they go down a series and up in size. some are big some are small just depends on the manfacture. Michellin is usually bigger than most. ive dealt with them in the past and they are vary company to company.

(21-09-2012 07:35pm)ant Wrote:  I work out the 20" wheel & tyre diameter is 28.68 inches and the 19" wheel & tyre diameter is 29.18 inches, so the bigger wheel is 1/2" smaller diameter once those tyres are on.
However, I stand to be corrected!!!!
ANT

Assuming you are correct which I think you may be, what will this do to the speedo readings

I don't think the size of a tyre differs from manufacturers. 235/55 19 is the same whether it's made by Pirelli, Michelin or Mickey Mouse.
235 is the width across the tread in mm.
55 is the size of the sidewall as a percentage of the width, in mm.
19 is the diameter of wheel the tyre will fit.
All those dimensions must be consistent.

Thus the size of the sidewall in this case is 55% of 235 which is 129.25mm. There will be two of these making up the diameter, thus 2 x 129.25 = 258.5mm. In inches, this is 10.18". Add this to the diameter of the wheel and it comes to 29.18".

The larger diameter wheel/tyre will travel a further distance for each revolution, and thus the speedo reading will differ with different sized wheel/tyre combinations. Don't ask what happens in this case, as I'm in an airport lounge waiting for a plane, on my third vodka, so my brain isn't up to that at the moment! Maybe somebody cleverer/soberer than me, can tell us if the speedo is going to over-read or under-read.

I'm sticking to my 19s!!!

ANT

All wheel/tyre combos should have the same overall circumference (within tolerances) to keep the speedo accurate to within it's tolerances.

Has anyone also considered that a set of unloaded 19's when compared to the loaded 20's fitted to the car will look larger due to no sidewall loading by the weight of the car on the 19's?