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Observations on original tachometer and speedometer accuracy
(as of Sept. 2000)

Speedometers, on an "as delivered" bike.

1. Original speedos tend to have a constant optimistic % error.

2. Nearly all speedos read high. A minimum of 5%, often 10% optimistic.

3. Few speedos are properly calibrated. (no oem speedos?)

4. Front wheel speed and rear wheel speeds are similar when coasting.

5. At full throttle, high speed, rear wheel speed will generally be 3% to 4% higher at full throttle, high speed. That is a constant with every tire that we've seen. It's likely that you will see 160mph front wheel speed and 168mph rear wheel speed at full throttle / high speed. There is a slight (+/-1% difference between different types of tires). That would mean that if your rear wheel driven speedo was optimistic by 10% at cruise, and you were topping out your bike, the speedo would now read +13% to +14% optimistic. The 3% to 4% is caused by creepage.

6. Creepage is defined as "controlled slippage".

7. Slippage is defined as "uncontrolled creepage".

8. If you are watching a dyno run and you see smoke coming off the rear wheel, that is slippage and the dyno is incapable of accurate information at that power level.

9. Our EC997 Dyno Eddy Current Dynos have patented slotted drive rollers and transfer  minimum 37% greater power transfer than other dynos (all other mc chassis dynos are limited by knurled drive rollers). (That's how we know this creepage / slippage information).

10. Speedos on the non-drive wheel (that would be the front on a motorcycle) are more consistently accurate than rear wheel (drive wheel) speedos.

11. It's worse to have an inconsistently accurate speedo than a consistently inaccurate speedo.

12. BC700 bicycle speedos, mounted on the front wheel, properly calibrated, are very accurate.


1. Almost all japanese tachs read high.

2. Guess at +10% optimistic.

3. The error is generally NOT consistent between different models and brands.

4. Late 90's and early to mid 2000 Suzuki GSXR "Kit" tachs are one of the goofiest, non linear error devices around - with an error of about 10% (pretty normal and expected) at 9,000 rpm, then smoothly  adding almost 20% error at 13k. The stock gsxr tachometer has a relatively constant error rate in comparison. I'm mentioning them because they are the most common "Kit" instrument sold.

5. As of September, 2000, the reasonably priced, attractive "Stack" brand tachs will break their needle off when used on a Suzuki gsxr racebikes. It makes some difference as to how they are mounted, but there is no reliable mount for race motorcycle use. Perhaps, "Stack" will strengthen the needle and provide some sort of vibration proof mounting at some time. The Stack Tachometer is reasonable priced and looks good - so I hope so. It is more consistent and accurate than the OEM tachometers when it has a needle!

6. "Scitzu" manufactures a battery operated tachometer that picks up a signal through a wire wrapped around a high tension lean and as very accurate. The "Scitzu" comes complete with an excellent (read as proven over the years on Nortons, BSA's, Triumphs and other nasty, vibrating classics).

7. Honda tachometers seem to have a  pretty consistent error and a bit more realistic on most models (though cbr250 don't really rev to 19k and cb-1's are sky high optimistic, too).

8. Yamaha yzf600 and yzf750 tachs ((93-97 vintage) have a "2 stage" error. A smaller error below 6100 rpm and magically at 6200 rpm, they change to another, more optimistic error rate. If you have one, just try to get the tachometer to read 6200 rpm!

9. Indicated RPM, define


Any observations, corrections, updates, suggestions on tachometers and speedometers?
Email Factory Pro about instruments!



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