I was given yokohoma prada spec 2 tires but i have no idea if they will fit on OEM wheels 225/40Z/R18 92W I know stock is 225/45R18 91W
Does the 91W and 92W make a diffrence.
I did search but dont understand the size system Thanks for any help.

 

Explained...

The 91/92 is a load rating, they are close enough not to matter.

As for the profile being a little lower -

this Miata site

explains how that affects your speedo, ride height etc.

S

 

your moving to a smaller wheel diameter. too much change imo. you usually want at max 2% change.

 

Thanks for the help Is it bad to go smaller the only reason im gonna trade out my tires is because one of the oem have a screw and a replacment is 200+ so free new tires i hope work well on the stock rims

 

No not at all. Your gearing will be lower so you will accelerate faster with 40's over the stock 45's.