Speedo and Tires

[particlewave]

The outer diameter of the tire won't change much and 18" will fit. That's not the issue, though.
The speedometers in these cars read about 5mph slow by design.

[marsheng]

Originally Posted by particlewave

The outer diameter of the tire won't change much and 18" will fit. That's not the issue, though.
The speedometers in these cars read about 5mph slow by design.

Using the same profile tire and going from 16" to 18" will change the speed by roughly 10%. To be exact 11.1 %.

Do I have 1" all around the existing tire to fit into the bodywork is the question.

[Deserion]

Originally Posted by marsheng

Using the same profile tire and going from 16" to 18" will change the speed by roughly 10%. To be exact 11.1 %.

Do I have 1" all around the existing tire to fit into the bodywork is the question.

The profile can change, but the overall diameter has to stay roughly equal. I take it the 225/50/18 is for the front? It's almost 2" larger in diameter compared to the stock 225/40/18s.

My Boxster displays (digital) 3mph faster than actual speed, fairly consistent.

[Rob175]

I agree.....my 98' reads 5 mph slower than I'm actually traveling (based on my gps reading).........so that means that instead of driving my typical +5 mph over the posted speed......I drive +10 mph over the posted speed......lol.....still gives me the 5 mph "margin" so the cops don't bother.