Is Your Tachometer Truthful? | Mythbusters

Picture this: you just spent big money tuning your streetbike with the hottest parts and you’re headed for a ride to test them out. You get out of dodge, find a lonely road and let it rip through the gears. In a blaze of glory you catch every shift just as the needle touches redline, right at the power peak where your hard-earned money is making the most steam...or is it? Like most arguments around the SSB HQ, this one started during a late-night wrenching session and the debate escalated so quickly that a proper investigation was in order. Rumor has it that many modern sportbikes have inaccurate tachometers that are overly optimistic. In other words, the actual engine redline is achieved well into the red zone on the face of the tachometer.

The SSB staff placed their bets and a trip to the dyno was in order.

The Test: Since verifying tachometer error can be tricky, the SSB SuperFlow dyno was called into commission for accuracy’s sake. Each bike was tested at a steady 5,000 and 10,000 RPMs in addition to monitoring the tach in relation to the actual redline during a loaded dyno run. The last scenario was done to test the accuracy of the tachometer while in motion since some claim the tacho-meters read high at steady throttle in order to keep up with a rapidly accelerating motor–like an electronic head start.

This phenomenon stems from the sportbikes of yesteryear that had tachometers so archaic the motors would literally out-accelerate the tach needle. The needle was so far behind engine RPM that the rider had to shift short of redline on the tach to prevent bumping the rev-limiter.

In the order of fairness, SSB tested three sportbikes from different makes and even brought a 2012 Suzuki V-Strom along to represent the wildcard. Here’s what went down...

Myth: Confirmed

Just as the staff suspected, most of the tachometers were inaccurate. No matter if the RPM was sampled at a steady figure or under acceleration, all of the sportbikes had tachometers that were roughly 500 RPM high at redline. This means that to shift at the actual engine redline, the tachometer must be several ticks inside the red zone.

Bucking the trend, the wildcard V-Strom not only had the lowest redline, but it also had the most accurate tach.

After several fruitless attempts to get an OEM manufacturer to confirm or deny the findings, we gave up. Instead we’re left to wonder if the 500-RPM buffer is to keep the high-revving sportbikes under redline, thus prolonging engine life. If most riders believe they’re shifting at redline without actually getting there, it’s possible the OEMs figure it will extend engine longevity. While 500 RPM might not sound like much, it’s significant at engine speeds over 11,000 RPM. We suppose we’ll never know, but either way, understand that many modern sportbikes have tachometers that read several hundred RPM higher than the actual engine speed.

SSB doesn’t condone over-revving a motor, but in the rare occasion you’re running fork-to-fork with a competitor and you’re nearing redline, realize you have some wiggle room until the next shift. And ultimately if that’s the difference between winning and losing, well, you be the judge.