wheel/tyre size change - mileage and speedo problem?

[Mrwhite2016]

Hi

I guess this is a general question, not just ford, but wonder if anyone can help?

Just got a new Focus with 18" wheels. Am changing down to 17" inch as I prefer the smoother ride of my old Focus.

Will this affect the speedo and mileage?

My current wheels and tyres are: 18", offset 55, fit with 235/40 R18

The ones I am about to fit are: 17", offset 50, fit with 215/50 R17

When I compare them here: http://www.wheel-size.com/calc there is only a 2mm difference in overall wheel diameter.

Should I be concerned?

Many thanks

P

 

 

[eddie eastwood]

Measure the rolling radius at the center line (highest point) of both tyres types and see if there much of a difference. Another check you could do is to compare the revs with both types at a certain speed.

[Mrwhite2016]

Thanks Eddie

I worked out the rolling radius here: http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC calculator.htm

17"   Static= 300mm Dynamic = 314mm

18"   Static= 302mm Dynamic = 313mm

Speedo deviation is 0.2km/h (at 100km/h)

Looks like I really don't need to worry?

[eddie eastwood]

Looking good then.

[Mrwhite2016]

How on earth are those figures so close? Weird.