Why are tachometers next to speedometers?


Well shoot, if that’s the reason why not go for broke?


I use it as a fuel economy aid. I know that I get the best gas mileage if I keep the RPMs below 2000. My 2017 Accord has a ring around the speedometer that glows a lovely shade of emerald green when I am driving economically. Still, it will glow green even if I’m over 2000 RPM. I have to go about 70 to get over 2000 RPM, so it also helps me keep the speed down. I used to do this with my 2005 Accord, though it did not have the emerald light show.


Not a fair comparison, shadowfax. Many of those instruments are required equipment per 14 CFR 91.205… :slight_smile:

BTW: There is a compass deviation card in that plane, no? It’s required also. I was once ramp checked by the Feds. He actually peeked through the Thermacon covers on my parked Seneca 1, and thought I didn’t have the deviation card. (It was to the right of the plane’s glovebox) .

BTW: Is that a Sigma-Tex attitude indicator with the vac warning flag? When I bought one (in 1986) the vac flag was a $50 option. Now it costs hundreds more!


I guess that the OP would have REALLY loved the dashboard of the 1955 Plymouth, which had the oil pressure gauge and the ammeter directly in front of the passenger, and too far away for the driver to be able to accurately read the tiny gauges. I remember this very well, because it was my job as a child to relay the readings of those gauges to my father every few minutes while he was driving.


@VDCdriver. The 1956 Plymouth replaced the ammeter and oil pressure gauges located on the passenger side of the 1955 Plymouth with warning lights for low oil pressure and no charge to the battery. I guess I would rather have the 1955 Plymouth with the ammeter and oil pressure gauges on the right side of the dashboard than the warning lights for oil pressure and battery.


I agree… as long as the driver had someone to monitor those ridiculously distant gauges.


One of the biggest problems here is that the ideal shift points are different at different throttle openings. If you are accelerating gently with the throttle only about 1/8 of the way open, the ideal shift point will occur at a much lower rpm than a car driven with the throttle half open or fully open.


When learning to drive a truck, the method we were taught was to upshift at 1,500 RPM and downshift at 1,000 RPM.

In terms of a car, you can get a feel for the lowest RPM at which you can shift without lugging the engine.


I’ll take some exception to that. I’ve been driving manuals since I was 13 and think I know how to drive. On a 61 Chevy six, or a Morris Minor, or 59 VW, it was quite easy to hear the engine and the RPMs to know when to shift. But in my Pontiac when I use the manual mode for pulling a trailer, I always look at the tach when shifting. I shift just before 3000 to not lug the trans and slip the clutches. It’s very hard to hear the engine to know the revolutions. Maybe too many gun shots or loud music or rattling cans but hard to hear the thing running any more.

So yeah a tach is nice. In the old days, guys mounted their after-market tachs right on the steering column so it would be seen before the speedometer-it was that important. I also find the voltage regulator reasonably useful to see what the general output of the alternator is. Unfortunately those have pretty well been eliminated now in favor or computer messages. So I have to use the plug in type once in a while.


I don’t know about your car, but mine (2015 Jeep Cherokee) has all of those gauges accessible via the screen between the tachometer and the speedometer. You do have to scroll through to get the one you want.


Texases, what with all the cars now, including high performance cars, that have automatic transmissions with a sport mode and shift paddles or a separate gate for the shifter, a tachometer does make some sense. But it seems to me for cases like those, a shift light like drag racers use might be better. On the other hand, modern automotive electronics are so sophisticated now that some cars with manual transmissions (the Nissan 350Z that I know of; there may be others) have rev matching, wherein when you downshift the computer automatically increases the rpm so the shift will be smooth (like double-clutching for those of us old enough to remember that).


Cars have had tachs on the dashboard for years. Many cars from the 60’s and 70’s had them. But I agree with your basic premise, for most drivers tachs are not the best use of valuable dashboard space. I concur with the post above by @B.L.E , if there had to be another gauge, I’d prefer a manifold vacuum display rather than a tach.


You learned to drive a truck a lot later than I did. The early six cylinder Cummings engines operated between 1800 to 2100 rpm. It wasn;t until the turbocharged Maxadtne engines from Mack that I drove anything that you could run as low as 1200 rpm. Indeed you had to operate them that low because all the ones I drove only had 5 speed transmissions compared to the 10 to 16 speed trucks I was used to driving before the 70s.


Diesel engines are different than gasoline engines. Since there is no throttle on the air intake, the intake manifold pressure is atmospheric, nearly the same as the exhaust and even at low power, the engine does not have to pump air “uphill”, i.e. from a low pressure intake manifold to a high pressure exhaust manifold.
Instead of throttling the air fuel mix, a diesel’s injectors just spray less fuel into the cylinder.
In a gasoline engine, there is a manifold pressure where the engine makes zero torque. You can find and mark that pressure by revving the engine in neutral and noting the vacuum gauge reading while the engine is revving at that throttle opening. My observation is that manifold pressure is pretty much the same, whether it is idling or revving 2000 or 3000 rpm. Find and mark that manifold pressure with a marker.
If your engine is producing zero torque, you need to upshift to a higher gear. If you are already in the car’s highest gear, you need to disengage the clutch and let the car coast with the engine making zero torque at idle speed.
Learn to shift with manifold pressure, not at some sacred rpm. It’s why I ignore the tach and shift by feel. As your engine approaches the zero torque manifold pressure, it becomes less and less efficient, using the energy of the fuel mostly to pump air uphill.


With a CVT, It’s interesting watching the RPMs move up and down as I go down and up hills under cruise control.


The only reading I can get on my touch screen are tire tire \pressure. I cannot imagine anything more useless than a screen I would have to toggle through while driving. Holy distraction Batman!


Many cars have a display below the speedometer to display trip information (MPG, DTE), outside temperature, compass etc. You don’t have to scroll though it while you drive, you can leave it showing your preferred display.


Yes but my original comment was complaining about not having oil pressure or temperature gauges when I have a tach and 3 different ways to be told my fuel mileage. Why don;t all cars have an oil level gauge? That one would be a lot more useful than having an odometer readout, a gauge and a touch screen all telling you your gas mileage.


GM cars used to have a low oil level sensor, but dropped it some years ago. My guess…they didn’t want to get stuck replacing engines when the sensor failed. Same thing with GM radiator level sensors.


I rode in an intercity bus years ago that had a tachometer and didn’t have a speedometer. The tachometer had a scale for the road speed in each of the five forward gears. The bus was. Flxible body on a GM chassis with a diesel engine. This was back in 1960.
Back in 1969, I rented a U-HAUL truck to make the 125 mile move to go to graduate school. The truck was. 4 speed manual. The engine had a governor to control the rpms of the engine. I don’t remember that the truck had a tachometer. I would start off in 2nd gear and accelerate until I felt the governor kick in and then shift to 3rd gear. I would do the same thing–accelerate in 3rd gear until I felt the governor limit the engine and then shift to 4th gear. The exhaust system on the truck was noisy–I think the muffler was shot–so by determining the shift points with the engine governor, I knew I wasn’t lugging the engine. The truck was so noisy I couldn’t shift by engine sound.
I rented the truck on a return basis and stenciled on the front bumper were the word “For Local Use Only”. I had to double clutch between each shift to keep the gears from grinding. When I filled the truck with gas for the 125 mile return trip, I had to add two quarts of oil. When I returned the truck 125 miles later, the attendant at U-HAUL added another 2 quarts of oil. When I handed him the receipts for the oil I had added, he repaid me and then said calmly, “I think this truck seems to use a little oil”.