I got my car with 19 inch standart wheels.. 255 50 19 size and after sometime i go for 22 inch 285 35 22 and i put winter tires on my original 19s. after using 22s all summer time i put 19s for winter time and i experienced that there is a big performance difference between them.my ride is a 2.7 diesel with superchip remap with inbox bmc air filter and sprintbooster i see 210 kmh and the limitor did not let me go for faster but with 22s i have never seen that... have you ever evperienced such thing??? KML4EVER

The 19 inch tyre has a slightly smaller overall diameter (about 2.7% or 20mm). This gives the following:

19in = 2318.49mm circumference
22in = 2381.33mm circumference

Putting on 22 inch rims instead of the standard 19 inch rims means that the speedo has been reading about 2.7% slow. It says 100km/h when you're doing 102.7km/h. The 22 inch tyres have lengthened the gearing.

The car tells you the speed based on the rate of rotation of the wheels, not by how far you actually travel in a given time. It assumes that one rotation of the wheel is a given distance. If you change the distance that the wheel travels in each rotation then you change the displayed speed.

Top speed is limited by a combination of gearing, drag and power. Drag increases with the square of the increase of the speed. Power absorbed by drag increases by the cube of the increase in speed. That's why supercars require silly amounts of power to go at 200mph. By changing the final gearing of the vehicle (by changing the diameter of the tyres) you will affect acceleration and top speed. Logically, lengthening the gearing should give a higher top speed but, if in the process of lengthening the gearing we prevent the engine getting to its peak power revs in top gear, we can actually reduce top speed. Indeed, I remember driving cars that hit top speed in 4th gear, not 5th. 5th gear was chosen to give economy at cruising speeds rather than ultimate top speed.

If you can find out the Cd and frontal sectional area of your RRS, you can determine the power absorbed at a given speed:

power absorbed = (Cd x A(sq ft) x v^3 (mph)) / 146600
or
power absorbed = (Cd x A(m^2) x v^3(m/s)) / 1225

Obviously, if you have a power plot of your engine and know the gearing (from the manual) then you work out whether you run out of power or gearing at top speed.

In your case, I would suggest that the difference in indicated maximum speeds is down to the speedo reading being changed by the change rather than by the gearing change making a meaningful difference. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Phew! Excellent write!
I need to lie down for a while after reading this!!

 Malaysia Boleh!
4.2SC Chawton White, Stormers
Tasmod's Sills, Wind Deflectors, LR Sills
Clear Side Repeaters, Towbar
Larini Sports Exhaust, K&N Air Filter
______________________________
WINNER - 2008 'Tech-Head Of The Year' Award
WINNER - 2009 Outstanding Contribution
WINNER - 2009 'Tech-Head Of The Year' Award

Thanks for information... But at the end of the story it seems my cars acceleration can be better with 275 40 20 s.Beceuase there ise a big difference with 22 s. its around 4.1% am i right? KML4EVER

I'm not ashamed to admit, I have no idea what half of that actually means but love the fact that someone asks a question on here and someone else actually takes the time to explain in such detail when they could quite easily just skip to the next thread. I can honestly say this is one of the best forums I've used

RRSTDV8

While RRSTDV8's science is OK and right in theory its only is partially correct because the airflow under a 'Sport' is pretty messy given the airflow separators under the front apron in front of the wheels so the change in frontal area is not likely to make a significant change and the theory also asssume the air is still whereas the wind will probably be different on different days, as is wet/dry road and air temp. (so engine efficiency)!!.

More likley is the varying rolling resistance because of the tread type, tyre wall stiffness etc etc add to that different air temperature, wind direction and you're going to be chasing your tail to establish why you 'think' the performance is different.

I suspect there may be a physcological effect because one set of tyres may be quieter or there' a tail wind.

@drdelrrs Hence my last line - the difference between tyres is small but measurable - if you can be bothered to do it.  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

imo its not psycological and its not abt the weather because i just changed the 22 to 19 and then next day started to feel the difference weather difference was may be 1 or 2 degrees and olso the wind etc.Because of the smaller wheels i think the acceleraiton is better and on the other hand if i use 275 40 20 s the difference is higher like 4% they are smaller then the 22s.so imo if i go for 20s may be the acceleration will be better and olso the feeling because with 22s its always a little bit shaking the steering wheel... anyway thanks for all giving really valuable information  KML4EVER

A smaller overall diameter tyre will give better acceleration because you shorten the gearing. But the effect won't be huge.

As for the steering wheel shake with the 22s, has your Sport got the steering damper that was introduced to allow for larger wheels? Not sure when it was fitted as standard across the range.

Check this thread http://rrsport.co.uk/forum/viewtopic.php?t=264 for some further info regarding damped steering racks and how to check yours  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

RRSTDV8 thanks alot really helped me too much  KML4EVER

With 19's car using less petrol and is quicker Big fun of RRS-my one WHITE-RED with wide body kit plus RS alloys all from KAHN