Both my friend and myself were running 205/50-15's on our N/A's. Of course this causes the speedometer to read faster than you're really going. But now, I've purchased 'vert wheels with the stock sized 205/60-15 tires on them. Now, when I had my SP5000's, which were just slightly oversized tires, my speedometer read off by exactly how much it should have according to every radar sign I passed. Yesterday though, when my friend was following me in his car...he claims that at 60mph (According to my newly correct speedo), his speedometer was reading 61mph.

He's got to be smoking crack. Even my Revs-to-speed seems accurate now.

Not sure if I'm looking for an answer really, just felt the need to vent that. I'll probably just end up getting an officer to help me out with his radar gun.

 

I had a little bit of a hard time reading/understanding this but I'll try. How much of an inaccuracy could you tell when you where running the larger tires? Are you sure you and your friends spedos aren't just a little bit off due to the age of the car? I saw this because my 93 Miata has stock rims/tire size and is off by about 4mph.

 

Our cars are the same year, and they're both N/A...but his has 250k on it versus my 188k. When I was running the Dunlops, at 60mph my speedo would read 61mph. Hardly no difference at all.

With the 205/50-15's we've worked out that for every 10mph, the speedo will read 1mph too fast. So at 60..well you do the math.

 

if you look in the FSM the tolerance for the guages is like 2 needle widths, so 60 or 61 mph is within the stock specs.