Speedo and Tires
 

New to this forum. My 'new' 1998 986 speedo reads under by 10%. I wondered why everyone was passing me when I was sticking to the speed limit.

I can correct the speedo by using 18" rims instead of the original 16". Just wondering if the 225x50x18 will fit in the body shell.

Thanks Wallace

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.

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.

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.

Digital read 78. Analog read 75ish. Washington County Sheriff said radar read 78.

My car was about 5-10% slower than the speedometer with stock 17" twists. Now have 18" Carrera light wheels, and it's a lot closer, maybe 2-3% slower now.

I guess it's probably helped me avoid speeding tickets!

If it is for the fronts, there is no way 225/50/18 will fit, that tire has a 26.9" diameter (vs. approx 25" diameter for stock sizes) and will hit the lower spring perch (if you have OEM struts) in the front for sure, and probably the back too.

Edit - and my digital speedo consistently reads around 3 mph faster than actual, measured at highway speeds (70 mph). I haven't measured it at lower speeds like 35 mph.

My S reads about 2 MPH too low and my base reads 3-4 MPH too low.

I have a 2001 base with relatively new tires and stock wheels. My speedometer reads about 5 to 6% higher than actual GPS speed, shows 74 mph speedo/70 GPS. I tested with a copilot so I could drive and watch for cops! LOL, it seemed to be true to that range from 55 MPH to 100 MPH.

I've used this chart frequently when looking for rims for various P-Cars:

Porsche Wheel Fitting Chart and Fitment Guide | Alloy Wheels Direct