Inaccurate Tachometer?

[sen2two]

I used an safc NEO and an autometer digital tach to check how the stock tach was reading. They both matched each other within 10rpm. So i trusted them for this test. Turns out the higher i go in the rpm's, the more my stock tach is off. At idle, the stock tach is pretty close to correct. And only a few hundred off in the lower rpm's. but close to 1000rpm's off by 7k! That's pretty bad if your trying to get good shift points to better your times.

So you are idling fine at 750 or so. But the higher you go, the more it's off...

*Also, if you changed from the stock tire height (not rim size, but full tire height) then your stock speedo is off.

[infernosg]

Originally Posted by sen2two

So you are idling fine at 750 or so. But the higher you go, the more it's off...

*Also, if you changed from the stock tire height (not rim size, but full tire height) then your stock speedo is off.

The idle "feels" fine to me. This is my first rotary so I don't know what a 450 RPM would feel like, but I imagine it would be rough. Based on my calculations the stock tachometer actually gets slightly closer to the expected engine speed the faster I go. So it's "off" 300 RPM at 30 mi/hr in 3rd, but by 70 mi/hr in 5th it only appears to be "off" by 250 RPM or so. Idle is something I cannot check because my calculations require vehicle speed, and therefore a digital tachometer would be needed. I'm beginning to lean toward the speedometer being off due to the idle condition, but I'll verify this with a GPS unit.

Regarding tire size, I know the diameter of my tire from the manufacturer website (Toyo T1-R), but I don't know the exact diameter of what stock was. I'm running a 225/50R16, which is supposedly like 0.7% larger than the stock 205/60R15 though...