Tyre Aspect Ratios

[Colin Pearcy]

I'm fitting a E Type fabs 5 speed gearbox to my E that currently has a 3.07 axle ratio.The cars original tyre size was 6.40 x 15 on original rims - 5" wide I believe.
I want to achieve circa 2500 RPM at 80 MPH / 130 KPH and an not sure what diff ratio I need to change to 3.54 OR 3.31 - I will be using 215 x 15 x 70 or 185 x 15 x 70 Avon ZZ tyres on 6" wide rims.
Does anyone know the formulator converting revs from original 6.40 tyres to 185 or 215 x 70 aspect ratio - Iknow that as I'm fitting wider tyres the I already compensating for reduced aspect ration ???
Any help appreciated as my original handbook bases its revs based on original 6.40 size not 70 aspect.
Colin.

[mtnjag]

Everything I see is 185 80 15 equals very close to orig diam. Often spec’d as 185R15. 205 70 15 also being very close.

The tires you ref; the 215 70 15 is bigger, the 185 70 15 is smaller.

But you don't need to convert from original anyway. Google a RPM calculator, have your diff ratio and trans ratio handy, plugin your tire diameter (easy to calc or go to an on line calculator), speed or rpm depending on how that calculator works, and play with the different variables until you get what you want.

[Heuer]

Use this: https://wahiduddin.net/calc/calc_speed_rpm.htm

[dougal]

The tyres you need are on here

https://www.longstonetyres.co.uk/classi ... -type.html

Crossplys are not so suited to modern motorways.

Stick with a proper period 185R15 tyre on your car. Michelin or Pirelli are best.

Going wider will not handle as well.

a 205/70VR15 is the same diameter as a 185R15 which is effectively a 185/80R15 (80% of 185mm is about the same as 70% of 205mm)

Yes those clever boys at Eagle E-Type fit wider tyres, and tyhey handle nicely, However that is a wholey different car. If your car is still basically an e-type then it will be much nicer to drive on a 185VR15 Pirelli Cinturato or Michelin XVS.