what is use of tachometer?

If the vehicle has a manual transmission, the driver can synchronize the engine speed with the speed of the car (using the speedometer) when shifting. This eliminates jerkiness when changing gears.

Ambrose

With an automatic transmission it has little or no value, since you have little or no option in selecting the gears.

With a manual transmission, it provides the information you need to select a gear that causes the engine to run in an appropriate power range.

"manou" wrote

From a technical point of view, the tachometer is more important for driving than the odometer. If you want to get the best out of your engine (torque, power or economy), the tacho is a must. Of course you need some specs of your engine to know what you are actually doing.

Thomas

I use mine to get the best mileage.

Every engine has a 'sweet' spot where it runs the most efficiently, in my case it is about 2300 rpm or where the timing is running full advance. (4.2 L straight six carb engine pushing a CJ7 that has the aerodynamics of a brick)

So when I am shifting between gears, I shift right around there and when on the highway, I try to keep it right around 2300 rpm.

For me this means no 5th gear because I have oversized tires. If I use

5th at 65 mph, my rpm drops to less than 1800 which lugs the engine and makes it drink gas.

Staying in 4th on the highway gives me a nice 19 US mpg, using 5th, I drop to 14 mpg or less.

Same for city driving, if I stay in 3rd tops for the city which is around 2300 at 40 mph, I get about 15 US mpg, use 4th and it lugs and drinks gas once again.

If I stay in the power band, I only need a twitch on the gas pedal to adjust for wind or a hill, if I am running too low an rpm, I need a lot of gas pedal to compensate.

It is also handy if you are stuck on ice or in mud. It tells you just how fast you are revving the engine. Low revs and a higher gear in the bottom of the power band will get you unstuck faster.

Or pulling out to pass on the highway, once again you drop a gear and give it up to the top of your power band in rpm, then shift up a gear. Winding the engine up higher than the top of the power area just wears the thing out and wastes gas, shifting up a gear puts you back in the power area of the next gear.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

manou wrote:

that was great guys, thanks a lot

With a manual transmission, it is useful to let you know how close you are to overrevving or lugging the engine, or letting you know that you can (or must) shift up or down a gear. With an automatic transmission, it is less useful.

Oddly, some car companies make tachometers standard equipment in some vehicles that come only with automatic transmissions, but do not include them on some vehicles which can have manual transmissions.

That lever that says "P-R-N-D-2-1" or "P-R-N-D-3-2-1" works just fine for avoiding lugging an engine on long climbs, or for engine braking on long descents. :-)

And anyone who looks at the tach when normally shifting a manual transmission should learn to shift smoothly by ear and keep his/her eyes on the road! :-)

Yep, it sure does! :)

My daddy literally beat that one into me when teaching me to drive... Him: "Eyes on the road! LISTEN to the engine! It'll talk to you when it's time!" Me:

Indeed, though familiarity with one's car is actually best for this.

An electronic limiter (fuel cut off) is even better for stopping you toasting the motor!

Generally, but this depends on the car and gear in question. V8s and up often pull merrily from fast idle even in higher gears (apparently; never driven one so as to taste the proverbial pudding), and my four banger runs well in the 1000-1500rpm range when limping along in 2nd gear under very light throttle (no engine bogging so long as you don't try to load the motor by nailing the gas or climbing any kind of hill).

Unless your vehicle is really well insulated, the best way to get a sense of the most appropriate gear etc. is by feel and gut instinct. One can tell when one's bogging the engine down (noise, shudder) or when one's overrevving (ECM prevents that from being dangerous anyway, assuming no misshift).

Héhéhé... what, no electronically controlled Studes? What were they thinking when they made those fine cars? :} (Guess it would take away half the fun though.)

Automatic users are warned not to shift into D4 when RPM is (circa)

1200 or above, as stated in user's manual. You have the option to shift to D1, or D2 for example.

Virtually all automatics prevent lugging automatically. It’s usually fool proof.

But slamming gears on D1 or D2 above 55 mph isn’t fool proof. At this moment the RPM gauge is useful for gauging the driver’s intelligence. :-)