I have a B16 in my 95 CX. I'm using a cluster from a 92 SI, and 195/50/15 tires. Stock tire size on the SI according to TireRack.com is 185/60/14. According to miata.net's tire calculator, my speedometer would only be off my .03% with the 195/50's. I know the cluster plays a role in converting the signal from the VSS into the actual vehicle speed, because it says on the back of the gauge how many revolutions equals how many mph. I'm wondering now if maybe I should try swapping the B16 Vehicle Speed Sensor with an SI one? I haven't tried calling Honda yet about getting it calibrated. I know you can with most vehicles, not sure about Hondas though. If anyone knows if swapping to an SI VSS will help or something else, let me know. I would like me speed to be correct and my Odometer and Tripodometer to increase at correct rates. Thanks

 

By what percentage are you off?

A lot of factors can affect speedometer accuracy; it's not that precise, even under the best of circumstances. Different models/makes of tires labeled with the same size can vary in size. Heck, the difference in diameter between a tire that is brand new and one that has worn down to the treadwear indicator bars is usually around 2 percent.

BTW, the difference in diameter between 185/60-14 and 195/50-15 is 0.3 percent, not 0.03 percent. Still a small number.

 

I meant .3%... but mine is currently off by ~2.5-3.5% somewhere in that range.

 

That's not an unusual amount, for any car. You're talking roughly 2 mph at most highway speeds. I wouldn't worry about it.

As I mentioned above, just the difference between new tires and worn tires accounts for a variance of 2 percent.

 

The difference at highway speeds ranges is about 2 at 55mph, 3 at 75 mph and about 4 at 80-90mph. I'm need new tires so I'm going to start out by getting 205/50's and see how much that helps, then I'll try a diff speedo if it's not close enough to actual.