What is the latest on European tyres for our C3’s

[Chuffer]

Calculate your tyre diameter to match OEM sizes and no changes will be required - in my case the OEM sized equivalent (I think) would be 215/70/15) at something like 27" diameter. The 285/40/18 rear and 255/45/18 front's are really close to original tyre fitment diameter. Calculation is 285 divided by 25.4 = 11.22 divide this by 100 - multiply this answer by 2 - then add 18 and you have diameter (26.97")

How can using the tyre width ( you quote 285 mm ) in the calculation have any bearing on overall rolling diameter ? It`s the profile of the tyre ie : 40 mm x 2 + wheel diameter that gives the overall rolling diameter of wheel and tyre configuation , or is there some dark art at play here that I am blissfully unaware of

 

[Roscobbc]

How can using the tyre width ( you quote 285 mm ) in the calculation have any bearing on overall rolling diameter ? It`s the profile of the tyre ie : 40 mm x 2 + wheel diameter that gives the overall rolling diameter of wheel and tyre configuation , or is there some dark art at play here that I am blissfully unaware of

Yes - sorry wrong calc - should be 40% of the width of the tyre - multiply by 2 for the total tyre height then add wheel diameter. Same calculation works for all sizes.
Should have read Calculation is 285mm divided by 25.4 = 11.22" - divide this by 100 - then multiply this answer by 40 (to get aspect ratio) - then double that figure and add 18" to get overall diameter (26.97")

 

[Moliere]

Found this.

 

[Daytona Vette]

Putting larger diameter tyres on the back of a C3 will have a negative effect on the Caster
A Tyre Diameter has been explained, but for actually calculating the Rolling Road measurement of a tyre: multiply the Diameter x 3.142

 

[CaptainK]

Again, late to the party, but my Corvette came over from the states with a speedo mis-read issue that was not told to me. I only found out when my missus was following me one day and asked why I was speeding through town (normally I'm good). Anyway, turns out my BFG tyres on my Corvette standard wheels were a bit too big, so when I got my 17 inch wheels I downsized the overall diameter a little deliberately to get the speedo right-ish. Its still not 100%, but meh, close enough.

 

[Roscobbc]

My '68 speedo has always read slow and a bit 'wobbly' from when first bought it (reads 50 ish when running 70 mph) - you can source different gearbox speedo gears specific to whatever rear end ratio your car has. They are colour coded relating to each ratio. Relative simple job to change. First double check what rear end ratio your car has and cross reference to correct colour for it. In my case it had the correct colour gear for the rear axle so the slow reading was probably down to a dodgy speedo. A bit of maths calculating the diameter and distance travelled with correct OEM sized tyres and your aftermarket tyres and see what the percentage difference is and perhaps source an alternative speedo gear. I used to memorise RPM's relative to speed/selected gear. Fo a number of years I simply have an old TomTom unit mounted in line of sight to 'check' real speed.

 

[CaptainK]

My '68 speedo has always read slow and a bit 'wobbly' from when first bought it (reads 50 ish when running 70 mph) - you can source different gearbox speedo gears specific to whatever rear end ratio your car has. They are colour coded relating to each ratio. Relative simple job to change.

Thanks. I did look into this years back when I got my C3, and made a post on the forum about it at the time. Ultimately, I couldn't work out which colour speedo I'd need and decided that I could live with it - like you just memorising gears and RPMs to legal speed limits. It just happened that as I was swapping my wheels out to 17s I thought I'd gear it down a little bit that way.

One day I may do it properly, but at the moment its within about 5 mph (at 70mph) difference and I use a GPS speedo app to check it too. So it doesn't bother me enough to fix it yet, when there are other more important jobs for me to do on the C3 when I get time.