does anyone no what this will be or no of a way of conversion chart or something for a 245 to 275 wheel change?

 

http://www.miata.net/garage/tirecalc.html

might help

 

Are you comparing a 245/50-16 tire to a 275/40-17 tire? If so, the outside diameter is identical, and there is no impact on the speedometer.

You can't just compare the section width (e.g. - "245", "275"). You have to look at the complete specification of the tire, so you can calculate the outside diameter. Even a 315/35-17 will not impact the speedometer, if the speedo is calibrated for the stock (with Z-rated tire option) 245/50-16 tires.

 

Ya. But it's kinda confusing.