LiamGTR Posted July 20, 2011 Report Share Posted July 20, 2011 Hi all, As we know, some cars are atleast 10% out on their speedo, mine definitely is if you go by what my Sat Nav says (32/33-ish on speedo is only 30, 43/44-ish is really 40 etc. etc.) and we all know tyre's affect the speedo. So would it really be a fround upon thing to purposefullly go for 215/40/17's instead of the usual 215/45? Reason I ask is this site says it'll bring the speedo back to reality by 2%, and to be honest... I want that! If it was up to me my speed would be 100% accurate, I don't like this 'guessing' game shite I like to know exactly what things do and how they're doing it especially when it comes to speed and the law. I understand this would help slighly with accel.., but also increase fuel consumpion but i'd like to hear your thoughts please? Link to comment Share on other sites More sharing options...
rpsmith79 Posted July 20, 2011 Report Share Posted July 20, 2011 But you will also make you car slower on acceleration as you overall rolling radius has increased, you will however have a slightly higher top speed Link to comment Share on other sites More sharing options...
LiamGTR Posted July 20, 2011 Author Report Share Posted July 20, 2011 So what about 205/40 instead of 205/45? That will decrease RR and will also make my speedo 3% more accurate ionstead of 2% like 215/40's ? Link to comment Share on other sites More sharing options...
Kev 16v Posted July 20, 2011 Report Share Posted July 20, 2011 I was under the impression that Tyres with a larger rolling radius gave a truer reading Speedo wise. A few members on here (Aquazi for one) commented their Speedo was closer to the Sat Nav reading with 225/40/18. 205/40/17's are too small, the many disadvantages outweigh the increase in 0-60. 215/40/17 give a slight increase in acceleration and more protection from kerbing, but drawbacks are a harsher ride and a slightly bigger gap between Tyre and Arch compared to stock. 205/45/17 are the stock Tyre size for a gen 7. 215/45/17 will fill the Arches slightly better, give more Rim protection and a better ride. Downside is, as the rolling radius is increased over stock, acceleration will be slightly affected. After trying the bottom three on my 140, I settled on 215/40 front..215/45 rear. Link to comment Share on other sites More sharing options...
oMIKEYo Posted July 20, 2011 Report Share Posted July 20, 2011 I run on 40's and the speedo at a higher speed is about 5mph out to a sat nav and the things on the side of the road (these are set like a speed camera) However my arches look too small as youve seen. But if your wanting 40's I will swap you your current tyres, mine are all practically new khumo's as again you've seen Link to comment Share on other sites More sharing options...
Rich_482 Posted July 20, 2011 Report Share Posted July 20, 2011 iv got 205/40 tyres on my 17" t-sport wheels, when my speedo shows 30 my sat nav shows 26 which is exactly the same as my old standard 15" wheels. I would rather it be a couple of mph fast as a just incase with all the speed camera's i pass through. Link to comment Share on other sites More sharing options...
masiv Posted July 20, 2011 Report Share Posted July 20, 2011 I was under the impression that Tyres with a larger rolling radius gave a truer reading Speedo wise. A few members on here (Aquazi for one) commented their Speedo was closer to the Sat Nav reading with 225/40/18. I run 225/40/18 and my speedo is almost spot on. Link to comment Share on other sites More sharing options...
Old Red Celica Posted July 20, 2011 Report Share Posted July 20, 2011 You need to increase the aspect ratio or go for slightly wider tyres to make the speedo more accurate. Link to comment Share on other sites More sharing options...
Boynxdoor Posted July 20, 2011 Report Share Posted July 20, 2011 (edited) I find 215/45/17 the closest to the stock 205/50/16 Try to look on the tyre calculator to see How big is the diff.. corrected, Was at work earlier so only a rush msg... Edited July 20, 2011 by Boynxdoor Link to comment Share on other sites More sharing options...
Kev 16v Posted July 20, 2011 Report Share Posted July 20, 2011 I find 214/45/17 the closest to the stock 205/50/16 Try to look on the tyre calculator to see How big is the diff.. 205/45/17 = 0.8% larger 215/45/17 = 2.3% larger Link to comment Share on other sites More sharing options...
LiamGTR Posted July 20, 2011 Author Report Share Posted July 20, 2011 Right so looks like it's 215/40's or 225/40's so get my speedo more accurate. I completely understand people want a 'safe' speedo i.e. reading a few mph fast, but I want mine to say the speed i'm doing so I KNOW if i'm speeding or not Link to comment Share on other sites More sharing options...
Boynxdoor Posted July 20, 2011 Report Share Posted July 20, 2011 205/45/17 = 0.8% larger 215/45/17 = 2.3% larger I see, so it is 205/45/17 closer... I went for 215 so alloys have extra protection once being hit at pavement... Link to comment Share on other sites More sharing options...
NJay Posted July 20, 2011 Report Share Posted July 20, 2011 Right so looks like it's 215/40's or 225/40's so get my speedo more accurate. I completely understand people want a 'safe' speedo i.e. reading a few mph fast, but I want mine to say the speed i'm doing so I KNOW if i'm speeding or not The rim size matters too. 225/40/18 and 225/40/17 are different rolling radiuses. And to concur with Masiv, my 225/40/18's are practically spot on GPS vs Speedo. Link to comment Share on other sites More sharing options...
Boynxdoor Posted July 22, 2011 Report Share Posted July 22, 2011 Hi all, As we know, some cars are atleast 10% out on their speedo, mine definitely is if you go by what my Sat Nav says (32/33-ish on speedo is only 30, 43/44-ish is really 40 etc. etc.) and we all know tyre's affect the speedo. So would it really be a fround upon thing to purposefullly go for 215/40/17's instead of the usual 215/45? Reason I ask is this site says it'll bring the speedo back to reality by 2%, and to be honest... I want that! If it was up to me my speed would be 100% accurate, I don't like this 'guessing' game shite I like to know exactly what things do and how they're doing it especially when it comes to speed and the law. I understand this would help slighly with accel.., but also increase fuel consumpion but i'd like to hear your thoughts please? I think You can calibrate Your speedo with your satnav when You fitted Your 215/45/17 wheels or tyres... I find it hard to calibrate it at first but with patience and eagerness. It was Done after several attempts... 30 on speedo and 29 to 30 on satnav... almost perfect... Link to comment Share on other sites More sharing options...
LiamGTR Posted July 22, 2011 Author Report Share Posted July 22, 2011 I can calibrate my speedo!? How!? Link to comment Share on other sites More sharing options...
GreyRacer Posted July 22, 2011 Report Share Posted July 22, 2011 I can calibrate my speedo!? How!? Yes you ca. I did mine for change of dials and tyres. Plenty of info in the Gen 7 forum and Hawker has a wiki. Link to comment Share on other sites More sharing options...
Boynxdoor Posted July 22, 2011 Report Share Posted July 22, 2011 Here You go Mate... I did it my way though as I was struggling with this guide. http://www.celica.talktalk.net/ Link to comment Share on other sites More sharing options...
Rad Posted July 23, 2011 Report Share Posted July 23, 2011 The rim size matters too. 225/40/18 and 225/40/17 are different rolling radiuses. And to concur with Masiv, my 225/40/18's are practically spot on GPS vs Speedo. Do many people use 225's on 17" rims as I thought about sticking them on next time I need tyres - only problem I can think of is being a lot shitter in ice/snow. Rad Link to comment Share on other sites More sharing options...
spotv6 Posted July 23, 2011 Report Share Posted July 23, 2011 The rim size matters too. 225/40/18 and 225/40/17 are different rolling radiuses. And to concur with Masiv, my 225/40/18's are practically spot on GPS vs Speedo. me too Link to comment Share on other sites More sharing options...
ams Posted July 23, 2011 Report Share Posted July 23, 2011 I run 225/40/18 and my speedo is almost spot on. Yep me too Link to comment Share on other sites More sharing options...
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