Black Knight Posted February 17, 2012 Report Share Posted February 17, 2012 Right, I’ve read a lot of the threads about tire and wheel sizes and, frankly, I’m still lost in confusion!?! So now I’m putting this out there for you lovely people to explain to me in plain language, (or as plain as is possible ). 1996 UK Gen 6. Currently rolling on original 14" rims with 195/65 R14 Tires. I’m soon going to be receiving some newly refurbed stock 5 point Gen 7 16" rims. So, what Tire size do I need to maintain the correct rolling diameter and not mess up the Speedo??? All help greatfully recieved Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 This might help mate.. http://www.etyres.co.uk/tyre-size-calculator.htm Link to comment Share on other sites More sharing options...
Euan Posted February 17, 2012 Report Share Posted February 17, 2012 So from that, 205/50/16 is almost identical size wise, 195/50/16 is 1.3% smaller, or 195/55/16 is 1.97% larger. Link to comment Share on other sites More sharing options...
Black Knight Posted February 17, 2012 Author Report Share Posted February 17, 2012 Yea that’s what I got, so looks like 205/50 R16’s are the way forward. Anyone know how larger or smaller affects the Speedo? Do larger tires read slower or faster etc? Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 larger tyres will make speedo read slower.. Link to comment Share on other sites More sharing options...
GreyRacer Posted February 17, 2012 Report Share Posted February 17, 2012 All you need is here http://www.miata.net/garage/tirecalc.html Link to comment Share on other sites More sharing options...
Euan Posted February 17, 2012 Report Share Posted February 17, 2012 larger tyres will make speedo read slower.. Other way around surely!Larger wheel circumference means more distance travelled per revolution means a higher speed at a given number of revs. Link to comment Share on other sites More sharing options...
Alan Clogwyn Posted February 17, 2012 Report Share Posted February 17, 2012 Dude, don't worry, no Celica speedo is remotely accurate anyway! 205/50 16 is standard for Gen 7 wheels, that's plenty good enough! Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 Other way around surely! Larger wheel circumference means more distance travelled per revolution means a higher speed at a given number of revs. it would give you a slower speedo reading ,wheel would be turning slower due to it being larger.. Link to comment Share on other sites More sharing options...
Bill T Posted February 17, 2012 Report Share Posted February 17, 2012 it would give you a slower speedo reading ,wheel would be turning slower due to it being larger.. I don't think so, the speedo is really just a post gearbox rev counter. So the wheel turning slower would be reflected throughout the drive train. If your axle indicates a speed then if the axle maintains that speed with a larger wheel you will have covered more distance. This would also explain why smaller wheels give better acceleration. Having said that I may be wrong However what I know for sure is tire is what you do as a result of excessive exertion,whereas a tyre is a rubber thing put around the wheel of a car Link to comment Share on other sites More sharing options...
Old Red Celica Posted February 17, 2012 Report Share Posted February 17, 2012 You may find it cheaper to fit 205/55-16's as that size is more common. Link to comment Share on other sites More sharing options...
Euan Posted February 17, 2012 Report Share Posted February 17, 2012 it would give you a slower speedo reading ,wheel would be turning slower due to it being larger.. If we moved the car at a set speed, say 30mph as measured by GPS, then yes, with larger wheels the speedo would under-read by more.But as Bill T says - the speed's factoring off the number of revolutions of the axle, and for a given number of revolutions of the axle, the bigger the wheel the faster the speed. Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 If we moved the car at a set speed, say 30mph as measured by GPS, then yes, with larger wheels the speedo would under-read by more. But as Bill T says - the speed's factoring off the number of revolutions of the axle, and for a given number of revolutions of the axle, the bigger the wheel the faster the speed. bigger the wheel,axle would be turning slower,than a smaller wheel.. Link to comment Share on other sites More sharing options...
Old Red Celica Posted February 17, 2012 Report Share Posted February 17, 2012 It's the overall size of the tyre that makes the speedo change. You could have 18/19/20" wheels fitted and the speedo would still read correctly if the tyres sizes were right. Link to comment Share on other sites More sharing options...
Bill T Posted February 17, 2012 Report Share Posted February 17, 2012 bigger the wheel,axle would be turning slower,than a smaller wheel.. But that would be reflected by the speedo as it measures the revolutions not distance travelled. Thus if the speedo (number of revs) is the same, the bigger the wheel the farther you travel each rev and so your speed is higher... perhaps Link to comment Share on other sites More sharing options...
_Chris_ Posted February 17, 2012 Report Share Posted February 17, 2012 Look at it either way, the result is the same. Car travelling at 30MPH, make the wheels 10% bigger diameter the axle RPM will be 10% lower so the speedo will read 10% low (27MPH) Axle RPM constant at an RPM which would give 30MPH with standard wheels. Make wheels 10% bigger, car will travel 10% faster (33MPH) while speedo still reads 30. Either way, the speedo reads 3MPH (10%) less than your actual speed. This all assumes the speedo is accurate to start with. If, for example, it reads 5% high (as most do) then a 5% increase in diameter will make it accurate. Link to comment Share on other sites More sharing options...
Euan Posted February 17, 2012 Report Share Posted February 17, 2012 If the car is going at a set speed, yes, bigger wheel = slower axle speed. But the car's speed isn't externally controlled, it's a function of the engine and gearbox making the axles turn through lots of revolutions per minute. Boring maths time-> Using http://www.etyres.co.uk/tyre-size-calculator.htm - 195/65/15 tyres give a diameter of 643mm, multiply by 3.14 (approximation of pi) gives a circumference of 2019mm. So for every rotation of the axle, the car will move 2019mm forward. 70 miles = 112 654 080 millimetres (=112.65km), so it takes 55797 revolutions of the wheel to go 70 miles, if you want to do that in 60 minutes (for 70mph) then that's 930 RPM near as dammit. So the speedo will be calibrated, more or less, to read at 70mph when it's doing 930RPM at the axle. Now, let's imagine we whack on some 17" rims, and go to 195/55/17s. These are 646mm diameter, so about 2028mm circumference. So at 930RPM, we go 1886040mm in a minute, or 113 162 400mm in an hour... 113.16km. Speedo will read the same, as the axles are going at the same number of RPM, but the actual distance travelled in a set time is further - so a higher actual vehicle speed (albeit we're still only dealing with very small percentages, and given that a speedo can be 10% out and still OK...) Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 So if you fitted massive tractor wheels to your car and drove at what showed on the speedo ,say 60mph would you actually be going faster or slower than the speedo says?I say speedo would read slower? The driveshaft would be turning slower with a tractor wheel compared to the stock wheels.. Speedo will read the same, as the axles are going at the same number of RPM, but the actual distance travelled in a set time is further - so a higher actual vehicle speed (albeit we're still only dealing with very small percentages, and given that a speedo can be 10% out and still OK...) so im correct then..speedo would be reading to slow.. Link to comment Share on other sites More sharing options...
Bill T Posted February 17, 2012 Report Share Posted February 17, 2012 so im correct then..speedo would be reading to slow.. Yes Following that through your rev counter would be reading lower as well. If engine revs remain the same then you would travel faster and use more petrol Link to comment Share on other sites More sharing options...
Biggy Posted February 17, 2012 Report Share Posted February 17, 2012 Yes Following that through your rev counter would be reading lower as well. If engine revs remain the same then you would travel faster and use more petrol I knew i was right..heres a little link http://www.kouki.co.uk/utilities/visual-tyre-size-calculator?wscr=1024x768 Link to comment Share on other sites More sharing options...
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