Inaccurate Tachometer?

[infernosg]

I originally posted this in the FAQ sticky, but that isn't the right place to ask questions (mods please delete that post) so I'm starting a new thread. For those who want to skip to the point of this thread, ignore the next part.

*Example calculations*

Based on transmission gear and final drive ratios I made a spreadsheet to calculate engine speed for a given road speed and tire diameter because I wanted to see the impact of switching to 5-speed Miata gears and a 4.30:1 rear end. To verify my spreadsheet I went out driving and recorded various gear/speed/RPM numbers. What I found was that the numbers I was calculating were significantly (to me) lower than what the car's tachometer was telling me. Below is an example.

Speed: 30 mi/hr
Tire diameter: 24.7 in.
Gear Ratio: 1.366:1 (3rd gear)
Final Drive Ratio: 4.10:1

So based on that information the engine speed should be calculated as follows:

Tire circumference = 3.1415926 * 24.7 = 77.597 in.
Vehicle speed (in./min) = 30 mi/hr * 5280 ft/mi * 12 in./ft * 1 hr/60 min. = 31680 in./min
Tire rotational speed = 31680 in./min * 1 rev/77.597 in. = 408.632 rev/min
Driveshaft speed = 408.632 rev/min * 4.10 rev/rev = 1673.879 rev/min
Engine speed = 1673.879 rev/min * 1.366 rev/rev = 2286.519 rev/min

So theoretically, at 30 mi/hr in 3rd gear my engine should be spinning at about 2300 RPM. However, when driving I notice the tachometer is actually reporting about 2600 RPM. This trend is repeated for just about every gear/speed I recorded.

*Question Below*

My question is, how "off" are our tachometers usually? Is there a way to recalibrate them or am I S.O.L. and forced to compensate in my mind (will make tuning later on a bit of a pain)? The other possibility is I'm completely screwing up the math, but the fact I'm off by ~300 RPM doesn't make me believe this is so.

[RETed]

I assume you're talking about stock Mazda gauge clusters?

The short answer?
Significantly.

I can only comment about stock FC tachs, since that's my experience.
Mazda has an "adjustment calibration screw" on the circuit board to compensate for such variations.
When we were messing with these things, we've seen these things regularly off by an average of 400 - 500 RPM's.
I've seen some off by almost 700 - 800 RPM's, but this was a rare case.

Readings were done at discrete 1,000 / 2,000 / 3,000 RPM increments.
Signals was generated with a custom built microcontroller.
Signals was monitored with a lab-grade o-scope.


-Ted

[infernosg]

^^^ Correct, stock Mazds S5 N/A cluster. I have read a couple things regarding recalibrating the system but I wanted to make sure that was the track to follow or if it could be something wrong with the trailing coil.

I was reading something written by Granny's Speed Shop regarding recalibrating the system with a 12V battery charger. Apparently this outputs a 120 Hz DC signal, when applied to a RX-7 tachometer and should cause it to read 3600 RPM - does that seem about right?

The other indicator that something is off is if I allow the car to perform it's cold start routine (rarely) it shows upwards of 3400 RPM. My biggest concern is the idle. Right now the tachometer is showing 750 RPM, but if mine is reading ~300 RPM high at speeds, does that mean my idle is also off and therefore actually ~450 RPM?!

[Mobius]

Here's a question, what did you use to measure your speed?

[infernosg]

Originally Posted by Mobius

Here's a question, what did you use to measure your speed?

Good question. I'm using the stock speedometer of course, and I suppose that can also be off, but that is purely mechanical correct? I suppose I can always go back and use my wife's GPS unit for another speed reading. I guess what I'm really saying is I really need a digital tachometer to know what the engine is REALLY doing. To bad no one around me actually carries something like that, let alone allows rental...

[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...

[Rotary#10]

Have a gtus with a vert wheels with tires of 205/55/15 my MPH is off by 4-5 mph. So 70mph on the gauge is about 65-67 mph according to my GPS. LOL. grr I need 16's ! I would agree when the car was completely stock the rpms would be off give or take a few 100. As long as its in spec of the FSM it should be alright if not. Happy hunting for the problem.

[infernosg]

I went through and cleaned some electrical connections a while back and that seemed to help slightly (maybe 50 RPM?). One of my trailing coils is pretty crispy so it is being replaced and maybe that will amount to something. I'm pretty sure this is a combination of the accuracy of the stock speedometer/tachometer and the poor charging system in the RX-7. The latter is fixable, the former, probably not so much unless you switch out for aftermarket.

[TitaniumTT]

There is alot of accurate information here regarding the inaccuracy of the stock gauges. Fact of the matter is that the stock tach, as Ted pointed out is eay off. I was seeing ~4-500 rpm difference compared to the S-AFC and the Autometer tach that I was using to test the accuracy as well much like the sentwo.

Your speedo is also off. I've said it before that I've never seen an accurate stock speedo. Don't know why companies do this but I've seen stock speedo's off by as much as 10%. Hell, I went from a 225/75/15 tire (28.28") to a 31", a difference of ~10% and my speedo up to about 70 FINALLY became accurate. After 70 the speedo actually shows slightly slower than what the GPS/police radar will show. Something to be aware of.

Other points of concern are how the speedo works, there is a spring involved, springs do fatigue, and the fact that tires are always slipping or rotating at different RPM's will cause reading's to jump around as well.

The most accurate way to get readings is not going to be from an analog gauge, but rather from high quality electronic sensors, really sheilded wiring, and digital displays.

Things the RX8 has. Can anyone comment on the accuracy of the the RX8 gauges?

[Rotary#10]

Time to make or buy a digital gauge cluster !

[sa22c]

My 82 sa22c tacho is out. I fixed my speedo by swapping in an underdrive speedo drive gear and putting a taller diff in and also taller tyres.

My speedo is actually under reading by about 10% at full speed ~124mph across the 1/4 speed trap. Its excellent because the speeding fine threshold is about there, so you actually try to make sure you are not speeding!

Anyway the tacho is notoriously bad. I know if I go past 8 really its probably only mid 7000s but not sure.

[My5ABaby]

My theory is they're both inaccurate on purpose. It gives the company and consumer a safety margin. If your speedo ran high, or even accurate, then people would be getting tickets more often, insurance rates would go up, and it may hurt the reputation.

Tach... 99.9% of consumers don't know the tachs are off. Those people think their engines are revving higher than they are which makes them think it's performing better. This gives the illusion of a cooler/high performing car and lets the company have a higher safety margin/more reliable engine. That difference of 100-500rpms makes a difference after 100,000+ miles. Is it annoying for the .1%? Sure, but if I were a company I'd cater to the 99.9%.

[Pete_89T2]

FWIW, my stock instrument cluster tach typically reads anywhere from 50~150 RPMs lower or higher than whatever RPMs my Rtek 2.x modified stock ECU reports out when I set it up to datalog & monitor real time via the palm device. This is on an S5 turbo.

[infernosg]

Thread revived!

Originally Posted by TitaniumTT

The most accurate way to get readings is not going to be from an analog gauge, but rather from high quality electronic sensors, really sheilded wiring, and digital displays.

I'm halfway there. The tach is now being run by a Haltech but I'm still using the stock speedo cable until I rig something up for a Hall effect sensor or the like. Too bad I didn't jump on those trigger wheels being sold a while back. I'm not sure about a digital display but one day the car will have a custom instrument cluster.