Yeah, but an electronic auto trans will force a shift before "redline" no matter what you do ANYWAY. Hell- go out and get in many new cars (one I know for certain- Ford Expedition) and floor it in neutral and see what happens. OK, I'll tell you- the throttle-by-wire revs it up to about 3600 RPM and it sits there happy as a clam.. Some aren't quite as graceful- a GM 3800 I had as a rental didn't have throttle-by-wire, so it just cuts the fuel and the engine will "surge" around 4000 RPM.
A tach would have been more useful in a 1970 car with an automatic than in a 2005 car with an automatic (not useful at ALL). Muscle cars frequently got tachs with automatics, because they do come in useful when manually shifting an auto. But back then, tachs went primarily in stick-shift cars where they REALLY belong.
Probably cheaper to produce one instrument cluster for everybody in models that can be bought with either an automatic or a stick, as is often the case.
At the higher end of the market, it's also useful for the growing number of automatics that you can optionally run by hand.
With LCD's getting cheaper and faster and brighter, I wonder when (at least in performance cars) they're going to start superimposing a virtual indicator on a display of power and torque curves, dyno-tested for at least the engine and driveline package if not the individual car.
Cheers,
--Joe Owner of two tach-less automatics, two tach-less manuals, and, until recently, one automatic with tach
The last time I looked (several years ago) it was rather difficult to find a manual option on most anything but the "sport" models.
I'm waiting for them to do a fully configurable LCD based dashboard where you can load a "skin" for the instrument style and layout you prefer. Pretty simple to do and flexible enough that the same guts could be used on nearly every vehicle in the lineup, just changing the display bezel for the particular vehicle.
But given that the bulk of models sold are ONLY available with an automatic..... its just bling-bling, as another poster already said.
And frankly, you have to be a pretty incompetent driver to actually
*use* the tach to decide when to shift on a routine basis. I always shift by engine sound, although I will check the tach a time or two the first time I drive a stick car that I'm not accustomed to (insert obligatory story about first-time RX-7 driving experience here...). Even drag racers don't use the tach- they generally use a shift-light triggered by a digital tach, not the tach itself.
What is critical in that case is that they equip the enigne with calibrated sensors that can provide real data to the "skin" software. Its no good having a super-duper skin that shows oil pressure, amps, coolant temp, oil temp, and transmission temp if all you have is a voltage probe, an oil pressure idiot light switch, and a 3-state coolant temp idiot light switch :-/
Well, my feelings on this subject are mixed. Your Otto Cycle engine in your auto-tranny equipped vehicle will be most fuel-efficient in the powerband range of certain RPMs. SO, if you know how to find that fuel-efficient point*, you can try to run your engine at a certain RPM to maximize fuel economy. Obviously, if you don't have a tachometer, you can't do this. But then again, for some vehicles, it doesn't matter whether there is a tachometer or not. Take my wife's car for example. I ran the numbers to determine where it would be most fuel-efficient. It should be most fuel-efficient from 3200-3600RPM. Problem is, I discovered that 75MPH was somewhere below 3000RPM with overdrive enabled. So to get to 3200 minimum would be ummmmm, not a good idea. With overdrive OFF, RPM jumped up above
4000. Basically, I discovered that my wife's car was geared incorrectly. So in the case of that specific vehicle, a tachometer is useless. I can't drive it legally AND keep the engine in the powerband. -Dave
Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at maximum horsepower RPM. In the case of my wife's car, this means it will be most fuel-efficient in a range from 3200-3600RPM.
The implicit assumption in that sort of "rule of thumb" is that you NEED all the power that the engine can produce. Its a good rule for, say, an industrial generator engine or irrigation well engine that is expected to run most of its life at maximum load Cars cruising down the highway NEVER need all the power that the engine can produce, so they play by a different set of rules.
Cars have engines that are capable of producing several hundred horsepower in order to achieve good acceleration, but when cruising steady-state they require a few tens of horsepower at most. So how do you run a 300-horsepower engine at an output of 35 horsepower and do it most efficiently? It turns out that the best way is to "lug" the engine. IOW, run it so that its way down on the lower RPM side of its torque curve, but do so with the throttle held relatively wide open at that low RPM. that's why overdrive works so well. Lugging the engine does a few things simultaneously:
1) it raises the starting pressure in the cylinders by decreasing the amount of manifold vacuum. Higher pre-combustion pressure -> more efficient combustion
2) it allows the spark timing to be retarded. Spark "advance" is really only needed because when you operate an engine against very high manifold vacuum (throttle mostly closed) you are LOWERING the starting cylinder pressure so much that the burn rate is reduced, so that in turn you need to start the burn very early in order to finish it before the exhaust valve opens. The downside to spark advance is that a portion of the combustion cycle is working against the movement of the piston. If you lug the engine with the throttle open, the pressure is high so the burn rate is high, and you don't need much spark advance, and therefore less of the combustion cycle is spent working against the crankshaft.
The proof is in the pudding. If you look at that (otherwise useless :-) ) tach in any modern car, you'll see that when cruising in O/D at 70 mph, the engine is only turning 2000 RPM or even less. And this is usually with engines that have redlines of 5000-7000 RPM and torque peaks up at 3000-4000 RPM. By any rule of thumb relating to their MAXUMUM power, the engines are being operated way, way off optimum. But for the NEEDED power, they're right in the sweet spot.
Ummmmm, no. The most fuel-efficient RPM of an Otto Cycle engine is well below the RPM that will produce the most horsepower. "Lugging" the engine is not fuel-efficient, as the engine RPM is too low. At low RPM, too much energy is wasted as heat. (a lesser percentage of energy is converted to kinetic energy while the engine is lugging) Overdrive is meant to improve fuel efficiency at higher speeds. (OVER drive speeds) It can't do this by "lugging" the engine. It does this by reducing the RPM into a more fuel-efficient range while still NOT lugging it. Good example: My car is at 3500RPM/43%(redline) and .6 (of max horsepower RPM) at 78MPH in OVERdrive, where it easily gets 42MPG. That's just the way it was designed. If I downshift it to drive, the most fuel-efficient
3500RPM/43%(redline)/.6(of max horsepower RPM) equates to exactly 55MPH. I have no idea how fuel-efficient it is at 55MPH in drive, as there is nowhere near me where I could safely test that. Even the highways posted at 55MPH have average traffic speeds well in excess of 70MPH, and it's just not a good idea to hold a steady speed 20MPH below the flow in any lane. BUT, I suspect my vehicle would be quite fuel-efficient at 55MPH, as that is how it was designed. The most fuel-efficient RPM of my engine happens to fall right AT 55MPH, when the car is in drive gear. No way was THAT a coincidence, so it had to be by design.
Yet since I learned how to find the most fuel-efficient RPM of an engine, I have discovered that a lot of vehicles are geared incorrectly. My wife's car is one of them. Her car should be most fuel-efficient at ~ 3400RPM, yet it is hard to get her engine to even hit 3000 (without driving fast enough to lose your license), which is the point of maximum torque. A lot of cars I've driven have been similarly geared incorrectly. In one extreme example, I drove a chevy with a 4-cylinder engine that rarely hit 2000RPM. It was horrible, as far as fuel economy goes. Now I know why.
If "lugging" the engine was such a good idea, I'd expect to see more 7 or
8-speed trannies, both automatic and manual. That is, there should be more "over"drive gears to keep the engine running really slow. But if the car is geared correctly, only one overdrive gear is needed. This allows the car engine to max out the EPA estimate at 55MPH (where it is tested, in drive gear), and yet still get decent fuel economy closer to speeds that the car is actually driven. Thus, more overdrive gears would be redundant on a street-legal car. -Dave
In practice, only one cruising-only gear is required for good highway economy (which, as Steve correctly informed you, is achieved by lugging the engine at highway speeds).
That's precisely how cars like late-model Corvettes can get close to 30 mpg with 300-400+ hp. The manual trans cars are geared for ~40 mph/1000 rpm in top gear, which allows just enough surplus power to climb most freeway grades at normal fuel mixtures without downshifting. That top gear is useless for any purpose other than maximizing highway fuel economy, but it serves that purpose very well.
Say, have you been able to figure out what kind of mpg a Corvette should get at 125 mph in 6th? According to your bogus (40% of redline/60% of power peak/x% of whatever) rpm-based rule, that should be the sweet spot for fuel economy, right? Whaddya think? 40 mpg at 125 mph? Maybe one day, you'll allow yourself to learn the difference between engine efficiency and fuel economy, but until then, I guess you'll have to consider Corvette fuel economy to be one of those weird Mysteries of the Universe.
All answers are correct, the tach reads engine Rpm, and is used by many racing drivers as a guide to shifting gears in a manual shift tranny at the most optimum time to maintain overall performance from their racing cars, other than that, it=92s just for sport, a gauge that tells how many times a minute the engine is spinning. Mike De Vito
Think what you want, don't let facts stand in your way.
The most fuel-efficient RPM of an Otto Cycle engine is well
Uh, that's what I SAID.
"Lugging" the engine
That contradicts what YOU just said!
And that's simple bullcrap.
Wordsmith the definition of "lug" however you want to, but it doesn't change anything.
Good grief, man! that's not even CLOSE to being a typical modern overdrive gear. A typical overdrive gear is a lot more like what I originally stated- say 1800 RPM at 70 MPH on a 5500 RPM redline engine.
An IROC (either Camaro or Daytona, you don't say which) is old enough that it might not have a rev limiter built in to the engine controller, and ghus goes along with my earlier comment that tachs on automatics are less useful *now* than they were years ago.
Most modern cars, will still rev-limit at the top end as well as in neutral or in lower gears (and no, I'm not talking about the artificial top-speed limiter that many cars have- even performance cars without a speed limiter will still rev limit at redline now.)
Race cars don't have speedometers and the tach is rotated so that the desired position is with the needle pointing straight up (at least they used to be) so the driver can maintain engine speed in the power zone. When racing you don't care how fast you're going and rev limiters keep drivers from over revving these days (quite unlike years ago).
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