Why are top gears in so many cars so short??
Discussion
This is kind of a pet hate of mine about road cars.
When you read fuel economy specs, they tell us an "urban" mpg figure, which is suppose to represent motorway driving mileage, and an "extra-urban" mpg figure, which is suppose to represent town driving mileage. And the universal law surrounding these figures is that the urban motorway mpg figure is higher than the extra-urban town driving figure. This is because you're driving at a steady pace on the motorway whereas there is more stopping and starting due to traffic when driving around town. Makes sense.
What I'd love to know is how they test this? I would have supposed they got the motorway mpg figure by running a car at 70mph in top gear on a smooth road or dyno of some sort. I've no idea whatsoever concerning how they calculate extra-urban mileage since traffic can vary so wildly in town. Perhaps just multiply the urban figure by some commonly used percentage in the industry?
My experience has been the complete opposite of what car manufacturers have been telling me. Infact, 100% of the time I get better mileage getting to a place when I set the sat nav to avoid the motorways because I fancy taking the scenic route. Not to appreciate the scenery, but to appreciate the machinery. That means driving with verve. That's supposed to mean decreased fuel economy...
But it doesn't. The key is in the revs. When Martin Lewis told me never to rev my car above 3,000rpm if I want my wallet to be reasonably thicker, I decided to take a look at what rpm my Suzuki Swift Sport revved at in 5th gear @ 70mph... 3,500rpm. Have you even tried keeping under 60mph on a 200mile motorway journey? Don't put it on your bucket list.
Apparently rpm in top gear @ 70mph is commonly between 3,000-3,500rpm even in more powerful cars such as the 2.0 Clio. At first I thought perhaps only peaky NA petrol cars were the victims here, but the Cooper S has the same issue, despite having a lot more low down torque to pull you along and even allow for a comfortable amount of acceleration in top gear at motorway speeds. I don't know for sure but if I still have more than enough power doing 1,500rpm in 5th @ 30mph then doesn't it follow that the car would cope doing 1,500rpm at 70mph too? Wouldn't the car benefit immensely from a 6th gear that was capable as such?
I know sporty cars are supposed to have close ratio gears, but I'm sure the vast majority of customers would greatly benefit from a top gear that didn't obey the close ratio rule, and instead went really tall. Afterall, I've never ever found myself hitting the rev limiter in 4th and being so thankful that I'm still in the power band once I switch up into 5th. If I had done such a thing and got caught, pretty sure that would be driving license bye bye. Unless of course we're talking about doing this on the track. I have no track experience so I can't comment, but what percentage of hot hatch consumers will end up taking their car to the track?
Car companies would surely benefit from greener credentials and a pat on the back from the EU if they did this simple thing: make top gears taller! How did they even achieve extra-urban figures lower than urban figures? Are they just flat-out lying?
P.S. I realize this probably isn't as pertinent a problem for dieselheads or huge-engine petrolheads. But if it doesn't have to be a problem for them, it doesn't have to be a problem for the rest of us (and there are more of us than them). Why are car companies (seemingly deliberately) missing out on this trick?
When you read fuel economy specs, they tell us an "urban" mpg figure, which is suppose to represent motorway driving mileage, and an "extra-urban" mpg figure, which is suppose to represent town driving mileage. And the universal law surrounding these figures is that the urban motorway mpg figure is higher than the extra-urban town driving figure. This is because you're driving at a steady pace on the motorway whereas there is more stopping and starting due to traffic when driving around town. Makes sense.
What I'd love to know is how they test this? I would have supposed they got the motorway mpg figure by running a car at 70mph in top gear on a smooth road or dyno of some sort. I've no idea whatsoever concerning how they calculate extra-urban mileage since traffic can vary so wildly in town. Perhaps just multiply the urban figure by some commonly used percentage in the industry?
My experience has been the complete opposite of what car manufacturers have been telling me. Infact, 100% of the time I get better mileage getting to a place when I set the sat nav to avoid the motorways because I fancy taking the scenic route. Not to appreciate the scenery, but to appreciate the machinery. That means driving with verve. That's supposed to mean decreased fuel economy...
But it doesn't. The key is in the revs. When Martin Lewis told me never to rev my car above 3,000rpm if I want my wallet to be reasonably thicker, I decided to take a look at what rpm my Suzuki Swift Sport revved at in 5th gear @ 70mph... 3,500rpm. Have you even tried keeping under 60mph on a 200mile motorway journey? Don't put it on your bucket list.
Apparently rpm in top gear @ 70mph is commonly between 3,000-3,500rpm even in more powerful cars such as the 2.0 Clio. At first I thought perhaps only peaky NA petrol cars were the victims here, but the Cooper S has the same issue, despite having a lot more low down torque to pull you along and even allow for a comfortable amount of acceleration in top gear at motorway speeds. I don't know for sure but if I still have more than enough power doing 1,500rpm in 5th @ 30mph then doesn't it follow that the car would cope doing 1,500rpm at 70mph too? Wouldn't the car benefit immensely from a 6th gear that was capable as such?
I know sporty cars are supposed to have close ratio gears, but I'm sure the vast majority of customers would greatly benefit from a top gear that didn't obey the close ratio rule, and instead went really tall. Afterall, I've never ever found myself hitting the rev limiter in 4th and being so thankful that I'm still in the power band once I switch up into 5th. If I had done such a thing and got caught, pretty sure that would be driving license bye bye. Unless of course we're talking about doing this on the track. I have no track experience so I can't comment, but what percentage of hot hatch consumers will end up taking their car to the track?
Car companies would surely benefit from greener credentials and a pat on the back from the EU if they did this simple thing: make top gears taller! How did they even achieve extra-urban figures lower than urban figures? Are they just flat-out lying?
P.S. I realize this probably isn't as pertinent a problem for dieselheads or huge-engine petrolheads. But if it doesn't have to be a problem for them, it doesn't have to be a problem for the rest of us (and there are more of us than them). Why are car companies (seemingly deliberately) missing out on this trick?
I honestly don't understand why my Clio 197 has the sixth gear ratio that it does. Whilst being the updated box with a "longer" sixth, it still, like you say, is running at about 3,500rpm at 70mph. 2,500rpm would be perfect (despite the flatspot there in the torque curve, which can be mapped out but is probably there due to emissions etc).
You're doing 100mph when you redline it in fourth, and fifth will take you (I suspect, the track wasn't long enough) to 120mph I'm sure. So even for track use, I struggle to see the use of a sixth gear that is matched in terms of ratio to the smaller five cogs.
However, my Haynes manual for an older car says that the most efficient engine speed is that at which it generates its maximum torque. I believe that the more "correct" statement is that it is the engine speed that maximises the brake specific fuel consumption (fuel consumption rate divided by power produced) which should be considered.
I would hope that car manufacturers actually test it empirically and work out the engine speed that's most fuel efficient and set that as 70mph (probably 130kph in my Frenchie) in top.
You're doing 100mph when you redline it in fourth, and fifth will take you (I suspect, the track wasn't long enough) to 120mph I'm sure. So even for track use, I struggle to see the use of a sixth gear that is matched in terms of ratio to the smaller five cogs.
However, my Haynes manual for an older car says that the most efficient engine speed is that at which it generates its maximum torque. I believe that the more "correct" statement is that it is the engine speed that maximises the brake specific fuel consumption (fuel consumption rate divided by power produced) which should be considered.
I would hope that car manufacturers actually test it empirically and work out the engine speed that's most fuel efficient and set that as 70mph (probably 130kph in my Frenchie) in top.
As said, you have confused urban and extra-urban.
Anyway, i agree that some cars rev frustratingly high in top gear. In fact, it was one of the main reasons that i chose the 7-speed DSG model of my car (Ibiza 1.2 TSi) over the 5-speed manual, as my car spends 90% of its time on motorway commutes. The extra long 7th gear is great for cruising, with 70mph coming in at around 2300-2400 rpm.
However, i suspect the main issue in choosing gear ratios is the overalll ability. 7th gear in my car does not bring very quick acceleration, the 'box changing itself to 6th and sometimes 5th when a burst of acceleration is required. For most people, using a manual, this would make the car feel sluggish and an unecessary chore to change gears often on the motorway (read the main complaint of manual Lexus IS220d drivers, the car in which 6th is pretty much useless below 80) which is a potential reason for manufacturers using higher ratios.
Anyway, i agree that some cars rev frustratingly high in top gear. In fact, it was one of the main reasons that i chose the 7-speed DSG model of my car (Ibiza 1.2 TSi) over the 5-speed manual, as my car spends 90% of its time on motorway commutes. The extra long 7th gear is great for cruising, with 70mph coming in at around 2300-2400 rpm.
However, i suspect the main issue in choosing gear ratios is the overalll ability. 7th gear in my car does not bring very quick acceleration, the 'box changing itself to 6th and sometimes 5th when a burst of acceleration is required. For most people, using a manual, this would make the car feel sluggish and an unecessary chore to change gears often on the motorway (read the main complaint of manual Lexus IS220d drivers, the car in which 6th is pretty much useless below 80) which is a potential reason for manufacturers using higher ratios.
Yep my current petrol car revs away at 3500 to 4000 & I'm always looking for another gear. Corvettes run a very high top gear and seem to get remarkable mpg in comparison to other v8s. On a run to Le Mans Classic last year with a Monaro & a.V8 Vantage, the Corvette was sitting near 30 mpg with the V8 Vantage about 22 mpg, think the Monaro was about 24 mpg.
B'stard Child said:
Marvib said:
soda said:
Mines about 1600 at 70mph, nearly too low in my opinion. I'd be happier with just over 2k.
As soon as I saw this I thought "He's got a monaro" 
It's got the torque for it but still too low I reckon. 3rd gears like 5th on a supermini. Shorter ratio diff going on at some stageWhen you modify cars (as I do) and you start playing with gearbox calculators to work out what gearbox / diff ratios you want it becomes very obvious that loads of cars have absurd gearing.
I'm guessing that manufacturers generally want to use as few components as possible and also what is easily available because given a box of bits you simply wouldn't use the ratios they have.
I good example of this would be the Mk2.5 6 speed MX5 - an overdrive 6th was wanted and instead they made the close ratio 5 speed even closer !
The opposite being a Monaro which is handicapped by ludicrously long gearing.
I'm guessing that manufacturers generally want to use as few components as possible and also what is easily available because given a box of bits you simply wouldn't use the ratios they have.
I good example of this would be the Mk2.5 6 speed MX5 - an overdrive 6th was wanted and instead they made the close ratio 5 speed even closer !
The opposite being a Monaro which is handicapped by ludicrously long gearing.
Don't assume that lower revs for the same road speed automatically means more mpg.
A car takes x power to sustain a set speed, regardless of the engine rpm. The difference mpg between two different rpms is down to the efficiency (ie losses in the engine) at a given rpm. You need to know at what rpm the engine is most efficient.
A car takes x power to sustain a set speed, regardless of the engine rpm. The difference mpg between two different rpms is down to the efficiency (ie losses in the engine) at a given rpm. You need to know at what rpm the engine is most efficient.
98elise said:
Don't assume that lower revs for the same road speed automatically means more mpg.
A car takes x power to sustain a set speed, regardless of the engine rpm. The difference mpg between two different rpms is down to the efficiency (ie losses in the engine) at a given rpm. You need to know at what rpm the engine is most efficient.
Lower engine speeds and higher throttle openings will most always be more efficient under cruise conditions.A car takes x power to sustain a set speed, regardless of the engine rpm. The difference mpg between two different rpms is down to the efficiency (ie losses in the engine) at a given rpm. You need to know at what rpm the engine is most efficient.
BadBanshee said:
if I still have more than enough power doing 1,500rpm in 5th @ 30mph then doesn't it follow that the car would cope doing 1,500rpm at 70mph too?
Almost certainly not because increased air resistance at 70 will mean it takes a lot more energy to accelerate from 70-80 than from 30-40.I think you'd find that you'd be flooring it on hills and still losing speed or pulling out into lane 2 and not being able to overtake lorries before the guy behind catches up with you. Obviously that happens in some cars anyway. Imagine how much worse it would be if you halved your available torque.
fido said:
Maximum power at maximum rpm. I can't remember the last time I drove a car with an overdrive top gear. Also modern engines can cope with being driven at higher rpms by design e.g. variable timing, injection, ignition.
Whilst they can, my wife's Mazda6 2.3 sits around 4500rpm at 84mph - which makes more vibration and noise than if it was at lower revs.It also must take more power to spin the engine at 4500rpm than at 3000rpm.
Right... So what we need to know is what is the average rpm @ 70 mph...
My skyline 2.0 gtst 1990 non adjusted is at 3450 @ 70mph..
My Volvo V70 diesel auto 1999 is at 2450 @70mph..
As far as I am concerned, the volvo is just fine. However, the Skyline could do with an extra gear rather than a longer 5th gear ratio.
So if we gather info on a 5th gear basis,( rather that a top gear) We may be able to corralate some figures and work out the perfect answer.. (until we hit drag c/e and power to weight ratio and etc etc....)
My skyline 2.0 gtst 1990 non adjusted is at 3450 @ 70mph..
My Volvo V70 diesel auto 1999 is at 2450 @70mph..
As far as I am concerned, the volvo is just fine. However, the Skyline could do with an extra gear rather than a longer 5th gear ratio.
So if we gather info on a 5th gear basis,( rather that a top gear) We may be able to corralate some figures and work out the perfect answer.. (until we hit drag c/e and power to weight ratio and etc etc....)
Gassing Station | General Gassing | Top of Page | What's New | My Stuff