Stew Posted November 8, 2008 Share Posted November 8, 2008 The difference isn't 5mm. The difference is 5% of 225 and 245. Quote Link to comment Share on other sites More sharing options...
Greekman Posted November 8, 2008 Share Posted November 8, 2008 The difference isn't 5mm. The difference is 5% of 225 and 245. Its a combination of maths and physics.... the tacho mesures the spins of the front wheels and automatically shows the result....(the whole set up of the appliance converts it to mph) d = π x r x 2 , where π=3.14 as the distance travelled because d is the perimeter of the wheel.... and r is the radius.... if u change profile and/or rim size then the r changes.... As a result for the same number of wheel spins (thats what the tacho reads) you will be travelling less distance if you change only to lower profile because the r will decrease... the tacho will show still 100mph cos its set for a certain radius but the actual speed will be less... From 40 to 45 the difference should be.....(18inc=45.72cm) 2(40-45+45.72)πr / 2(45+45.72)πr x 100% = -6.2% So your actual speed when the tacho shows 100mph will be 93.8mph.... and you have to deduct from that the false reading from the tacho as well....(3-5mph for the 313 roadster i v got)...which then means a bit less than 90...! When you increase the width of the tyre you increase friction which means that if you only use wider tyres the car should be actually slower....in acceleration and top speed...however if u only use lower profile the whole weight of the tyre should be marginally less which means that the car should feel a bit faster.....a tiny bit..... Quote Link to comment Share on other sites More sharing options...
Greekman Posted November 8, 2008 Share Posted November 8, 2008 ooooops answered wrong comment..... However the car will be lower by 5mm...thats quite straight forward... no percentage there....although depending on the absorbers' preload (i dont exactly know the english term for the first bit of the absorber damping which is usually a bit softer than the rest of the damping) the actual loss of height may be slightly less! Quote Link to comment Share on other sites More sharing options...
M13KYF Posted November 8, 2008 Share Posted November 8, 2008 got 40's on the front of mine and have no issues. Looks better than 45's but I wouldn't change the rears due to the TCS Took the car for abit of an test, don't know if its the tyres or the lower profiles but ride and handing has improved. Also did not have the TCS come in once even when I pushed it alittle harder than usual, but then I've only had the TCS come in once before and that was in the rain and on a big roundabout The speedo won't be accurate though.... at 30 and 40 i've spotted no difference with other cars. Easily checked when going through speed cameras Quote Link to comment Share on other sites More sharing options...
Sarnie Posted November 8, 2008 Share Posted November 8, 2008 got 40's on the front of mine and have no issues. Looks better than 45's but I wouldn't change the rears due to the TCS Took the car for abit of an test, don't know if its the tyres or the lower profiles but ride and handing has improved. Also did not have the TCS come in once even when I pushed it alittle harder than usual, but then I've only had the TCS come in once before and that was in the rain and on a big roundabout The speedo won't be accurate though.... at 30 and 40 i've spotted no difference with other cars. Easily checked when going through speed cameras How can you tell? Quote Link to comment Share on other sites More sharing options...
Chris`I Posted November 9, 2008 Share Posted November 9, 2008 In terms of slows speeds you wont notice much difference, as the number of revolution the wheel makes at that speed is not much different to the "standard" as remember its a percentage change of the RR, so given a 5% change - at 30MPH isnt much - 1.5MPH, but at 70 it is much more noticeable at 3.5MPH. Remember that by reducing the RR the speedo will over-read this much so it doesnt really matter. Use GPS and be happy Quote Link to comment Share on other sites More sharing options...
Greekman Posted November 11, 2008 Share Posted November 11, 2008 In terms of slows speeds you wont notice much difference, as the number of revolution the wheel makes at that speed is not much different to the "standard" as remember its a percentage change of the RR, so given a 5% change - at 30MPH isnt much - 1.5MPH, but at 70 it is much more noticeable at 3.5MPH. Remember that by reducing the RR the speedo will over-read this much so it doesnt really matter. Use GPS and be happy The difference is 6.2% as i explained...not 5%.... Whilst on motorway doing 84mph with cruise on, if i reset the av consumption and speed, then it shows 80mph as av speed which is exactly what i got from the road angel....! Anyway...just got the car back from the tyre shop...with ps2s on at 40 prof....didnt notice any difference from the ESP...handling a bit better (they are still new tho) but ride is amazing!!!! The car is so quiet...the ride so bloody smooth!!!! Think the steering is slightly less accurate...but i can get used to that.... I dont know if its the ps2s.....or the nitrogen in the tyres.....i remember my maserati drove much better when i changed to nitrogen....but not that much!!! Quote Link to comment Share on other sites More sharing options...
matth76 Posted February 14, 2009 Share Posted February 14, 2009 Hiya all I had to change 3 tries due to Wear and nails. So I went onto Blackcircle to order an full set of Falken 452's but instead of ordering the standard 45 profiles I accidently order 2*225/40 and 2* 245/40 which I put them on anyway due to the guy telling me it would be slight drop in ride height and wont be any problems. My question is does anyone know of any problems I may encounter if continue to use 40's instead of 45's? such as scaping the bottom going over speed bumps??? Thanks Steven Your speedo will be slightly out due to the smaller overall diameter. Quote Link to comment Share on other sites More sharing options...
jon Posted February 26, 2009 Share Posted February 26, 2009 Lots of information, only some of it right (rears) Old sidewall: 110.2mm, new 98.0mm (fronts) Old sidewall: 101.2mm, new 90.0mm The speedo is driven off the front axle i believe, so will be now read 3.4% faster. This means when your speedo reads 60mph, you'll now be doing 58mph (by the old speedo) As for the TC, I've 40 profile front and back on mine with no problems at all. I usually run with TC off but on my commute it's on a fair bit and there's been no change to how quickly it cuts in. Quote Link to comment Share on other sites More sharing options...
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