tach question

[boot]

If i'm reading 3000 rpm's on the boats stock tach under no load and a hand held phototach reads 2800 at the same time,is it safe to say if I was cruising at 2800 rpm's under load I would actually be running at 2600 rpm's.Would the 200 rpm difference be consistent throughout different variables of load and throttle.

 

[BillD]

If i'm reading 3000 rpm's on the boats stock tach under no load and a hand held phototach reads 2800 at the same time,is it safe to say if I was cruising at 2800 rpm's under load I would actually be running at 2600 rpm's.Would the 200 rpm difference be consistent throughout different variables of load and throttle.

IMO not safe to say at all.
Tachometers that are "off" to actual rpms are not always lineal in error.

You'd be best to phototach the engine in increments of say 200 rpms from idle to WOT and note the difference with the analogue tach.

Or install an Aetna digital tach...accurate to +- 1 rpm.

 

[Tuna Pursuit]

What he said. Bill helped me over last winter spec out the Aetna digital tachs after I bought a photo tach & realized I had no idea what my rpms were. Great investment!

 

[petrel]

It's probably a linear progression as RPMs increase, so does your differential. That's what I've seen w/ the tachs in my DMR.