How accurate are our factory tachs??

[Charlie Allen]

Anyone have a good aftermarket tach and a working OEM tach in their car? How close are they?

 

[revhead347]

At low rpm they seem to be decently accurate. I have found them to about 400 rpm off on average at the top end. Pretty inaccurate I would say.

Kurt

 

[final5-0]

Yes ... I'd say Kurt has it about summed up

Over the years I've been hanging around on these Stang forums
I've seen this Q come up several times

I've seen a few say their tach was off as much as 600 to 800 rpm

Some have said only 100 to 150 rpm

Yea ... an average of 300 to 400 sounds about right to me

Almost forgot ... most have said they read ... too high

Datalogging has shown mine to be off by a bit less than 200 rpm

Grady

 

[Stopsign32v]

Would a shift light be dead on?

 

[4Jenna]

At low rpm they seem to be decently accurate. I have found them to about 400 rpm off on average at the top end. Pretty inaccurate I would say.

Kurt

My exp aswell

 

[Zero Signal]

I'm sure it varies. In my 95, the Tweecer always read the same as the dash, but they might be connected, I'm not sure. Our 93 GT had a tach when we bought it and it reads withing 100 rpm or so of the dash.

 

[vladasap]

i know mine is quite a bit of up high
at 5400 it reads 6000
(i compared it to a digital timing light)

would a shift light actually be accurate???

 

[Killercanary]

I still shift via the stock tach and I haven't noticed any measurable difference between it and my datalogs.

 

[Zero Signal]

Yeah and I would HOPE that the rpm's read from the computer would be accurate since ALL the timing and fuel curves are based on load and rpm.

 

[vladasap]

lol

true!!!

although knowing ford i wouldnt be surprised if that was off too

 

[moostangvert]

I still shift via the stock tach and I haven't noticed any measurable difference between it and my datalogs.

same here.

 

[HISSIN50]

I think the inaccuracy lies in the gauge itself. The EEC is nuts-on with its reading - it has to be for the minute (tiny, not a reference to time) changes it makes.

In my experience, the tachs tend to be off by a percentage. So they read off a couple hundred RPM at 3K and perhaps 400 RPM off at redline (as a random example).

I've noted my fox's tach to be off by 200 RPM at 3K RPM. The tach on the 94 is off by 100 RPM or so at 3K (I can't interpolate the stock tach further). I'm sure everyones' mileage varies on how far off they are.

Paul's method has always sat fine with me. Consistency is key.

 

[sage2k]

My old 94 vert was dead on perfect.
My current 94 coupe is 300 - 400 rpm off throughout the whole rev range.
but previous owner has put a white face guage setup on and might have messed up the needle.