What disadvantages a car with 4-speed transmission have compared to 5, 6 or 7 speed with similar engine?

Question

This seemed to be most relevant site to ask my question. I am planning to purchase a new car and my shortlisted car has 4 speed automatic and 1.6 litre engine.

The thing that worries me is that it has 4 speed instead of 5 speed which is very common in similar cars here, and for some reasons I cannot go for the manual, and the top spec 7 speed 1.8 litre model is outside my budget.

I like everything about the car except the point in question. So my questions are:

1. Will it limit top speed on motorway? (limit here is mostly 120 or 140 km/h)
2. Will it limit acceleration, hamper overtaking etc? My search revealed that the 1.3 litre 4 speed automatic version is underpowered even within the city, but I have driven the 5 speed manual one and it was OK. Is it because of the smaller engine or the 4 speed transmission?
3. Any other limitations or issues I may have missed?

My use is almost entirely within the city.

For completing the question. The car I am considering is: Toyota Corolla Altis 1.6

And the alternate option is: Honda City Prosmatec (Honda's local website is not opening but the specs here are accurate).

Please let me know if this is the reason to go for the alternate, because except the doubt I have, I like everything about my primary choice.

Answer 1

For a car that has 4, 5, 6 and 7 gear options, if all else is the same, then your top speed will be the same; all that would change would be your acceleration:

In each gear you have a range where power is at it's highest. Below this gear speed your power is low (have you ever tried to accelerate in 5th from stationary?) And above it you hit the speed limitations of the engine.

So having more gears means you can spend more time in the power band - so acceleration efficiency is improved, however this may be offset by the time it takes to change between gears.

As an example, large trucks may have 12 gears - they need the power to accelerate heavy loads. But as the gear ratios are so close it takes a long time to get up to the top speed.

The top speed itself has nothing to do with the number of gears. All that is important for the top speed is the power and max speed of the engine and the gear ratio of your top gear.

So for question 1 - No Question 2 - probably the engine size/power Question 3 - far too broad a question to cover here. I'd suggest removing that one (you should only ask one question at a time)

Answer 2

There are two main reasons auto manufacturers have started putting more gears into their cars: acceleration and gas mileage. The main purpose of it is to keep the engine in its torque band. This is the area where the engine is working at its most efficient (improved gas mileage) and since it doesn't have to build back up to its torque band after a shift (it's already there), it accelerates faster.

Within reason, the top gearing for most automatic overdrive transmissions are going to be pretty much the same (within reason). This means the top speed is going to be about the same. The top end power is what is going to limit you, all other things being equal ... this pretty much eliminates the transmission as the deciding factor when looking at this.

Answer 3

Time and time again when improvements are introduced, even minor changes to a vehicles spec, the ultimate end today is fuel consumption. Lower fuel consumption means lower emmissions. When you have a vehicle with say 7 speeds, you will find the engine RPM at 70 MPH on the motorway is down to around 1500 RPM. This slower engine speed allows the engine controls to fully optimise power, fuel consumption and the all important emmissions. The slower engine speed gives the designer of the engine more time per revolution to implement a greater degree of control. They are counting in milli-seconds today. A vehicles performance, ie acceleration, is increasingly becoming irrelevant in todays cities. In London(UK) a ten mile journey by car during a working day will take at least an hour. A car for primarily city use today would be better gauged by cost of ownership, annual tax band, insurance band and projected maintenance costs.

Answer 4

tl;dr: More gears = higher acceleration and, possibly, better fuel economy in exchange for increased complexity and cost.

Let's imagine that all the cars in question are completely identical except for the primary transmission component of the system (so the final drive is still the same, engine power & torque are the same, etc). Given that, let's consider the three options for transmission: four, five and seven gears.

Will it limit top speed on motorway? (limit here is mostly 120 or 140 km/h)

Almost certainly. We haven't assumed that any of the individual gears match between the various sets. However, this is easy to check: look up the gear ratios for the gearsets. If all other properties are identical, an identical top gear ratio will give you an identical top speed.

Will it limit acceleration, hamper overtaking etc?

A smaller number of gears will almost certainly produce slower acceleration than a higher number of gears. The higher number of gears allows the designer to ensure that, after a shift, the engine will return to a productive place on the torque curve (in my car, this means keeping the revs up).

It's useful to imagine the degenerate case: imagine a car with only a single gear. It would have to chug up from idle speed, eventually reaching its peak torque somewhere around highway speed, finally running out of breath at top speed. This low end performance is why you want as many gears as possible: they allow you to select the best torque for the situation.

On the other extreme, it does take a finite amount of time to switch gears in the transmission. If the number of gears is so high and the changing gears time becomes too extreme, you could eventually see a reduction in the cost:benefit ratio of a many gears transmission. However, that doesn't sound relevant to your situation.

Any other limitations or issues I may have missed? My use is almost entirely within the city.

If you were driving a manual transmission, I would say that you might find seven gears to be annoying in primarily city driving. An automatic in the city, may feel much smoother and may give you better mileage. Depending on how the automatic is programmed, it may be quick to go to a high gear (reducing the revs and therefore the fuel consumed).