Why are top gears in so many cars so short??
Discussion
I believe both 5th and 6th are overdriove ratios in the Corvette. My Mustang has only 5 speeds but 70 is very low rpm. On the other hand, my Fiesta ST is annoyingly low-geared for the 0-60 and I would prefer a higher top gear for cruising which our Mondeo, using the same engine, manages perfectly well.
LuS1fer said:
I believe both 5th and 6th are overdriove ratios in the Corvette. My Mustang has only 5 speeds but 70 is very low rpm. On the other hand, my Fiesta ST is annoyingly low-geared for the 0-60 and I would prefer a higher top gear for cruising which our Mondeo, using the same engine, manages perfectly well.
Which sense of "over-drive"? I've heard it to mean three different things:1) That top speed is achieved in a lower gear
2) That the wheels are turning faster than the engine
3) That the prop-shaft is turning faster than the engine
kambites said:
If we look at a typical British* motorway mile muncher, what do you get. What RPM is a 330d sitting at in top gear at 80mph?
I suspect a diesel will be different to a petrol due to the different gearing they have.- That's the motorway that's British, not the car.
But I think much of the cruising rpm comes from legacy setups. When we only have 4 gears then 4th was likely 1:1 ratio. With the advent of the 5 speed gearbox we got an over drive, maybe 0.86:1 or something.
And many 6 speeders are double over driven. But instead of just making the gearing taller final drive ratio's have reduced. Partly because revvy 16v engines require more revs to make their power and to increase performance while retaining the same ability to cruise. Modern gearing is part of the reason that despite heavier cars and similar power to weight ratios, most 'normal' cars performance figures have improved over the years.
And despite what the op claims, you can't just have a huge ratio cap between gears, a really tall 5th/OD would mean that at altitude, on inclines or when fully loaded you'd never be able to use it. It also means you'd have to use higher revs at lower speeds because you can't use your OD gear until you are going faster.
(BTW - not all of the reply was aimed at your quote, I just got a little carried away typing....
)My ATR is a prime suspect for this, with 70mph at 3600rpm.
It's not even like the gears are evenly spaced though. That, I don't mind so much.
No, in the ATR, they actually fitted a short 5th.
Here's what it looks like, vs what I think it should have come with (5th gear from a Prelude. Can be fitted.)
Why they did that, I have no idea.
It's not even like the gears are evenly spaced though. That, I don't mind so much.
No, in the ATR, they actually fitted a short 5th.
Here's what it looks like, vs what I think it should have come with (5th gear from a Prelude. Can be fitted.)
Why they did that, I have no idea.
Edited by Kozy on Wednesday 19th June 09:17
Edited by Kozy on Wednesday 19th June 09:18
I went directly from one extreme to the other:
70mph at nearly 4000rpm in a Subaru 22B
70mph at 1600rpm in a Lotus Carlton
Both are silly really.
You're forever changing gear in the 22B and can even settle into 5th in a 30mph zone. That will take you to about 145mph if you wish. But I am sure greater spacing between ratios would benefit the car (I believe the few Prodrive supplied UK spec cars had a higher 5th gear for this reason).
Of course the Lotus has the ZR1 Corvette gearbox with 6th gear interstellar overdrive (wasn't that a Pink Floyd track?), which cannot really be used below 70mph and isn't much good for punchy overtaking. Thing is because it's there I do use it on motorway or dual carriageway runs, so it may be quite an economic advantage. Not that a 377bhp 3.6 litre 24 valve twin-turbo can ever be considered remotely economical!
70mph at nearly 4000rpm in a Subaru 22B
70mph at 1600rpm in a Lotus Carlton
Both are silly really.
You're forever changing gear in the 22B and can even settle into 5th in a 30mph zone. That will take you to about 145mph if you wish. But I am sure greater spacing between ratios would benefit the car (I believe the few Prodrive supplied UK spec cars had a higher 5th gear for this reason).
Of course the Lotus has the ZR1 Corvette gearbox with 6th gear interstellar overdrive (wasn't that a Pink Floyd track?), which cannot really be used below 70mph and isn't much good for punchy overtaking. Thing is because it's there I do use it on motorway or dual carriageway runs, so it may be quite an economic advantage. Not that a 377bhp 3.6 litre 24 valve twin-turbo can ever be considered remotely economical!
kambites said:
Which sense of "over-drive"? I've heard it to mean three different things:
1) That top speed is achieved in a lower gear
2) That the wheels are turning faster than the engine
3) That the prop-shaft is turning faster than the engine
In the sense that 4th is generally around 1:1 and the two gears above are 0.x:11) That top speed is achieved in a lower gear
2) That the wheels are turning faster than the engine
3) That the prop-shaft is turning faster than the engine
LuS1fer said:
kambites said:
Which sense of "over-drive"? I've heard it to mean three different things:
1) That top speed is achieved in a lower gear
2) That the wheels are turning faster than the engine
3) That the prop-shaft is turning faster than the engine
In the sense that 4th is generally around 1:1 and the two gears above are 0.x:11) That top speed is achieved in a lower gear
2) That the wheels are turning faster than the engine
3) That the prop-shaft is turning faster than the engine
70*1600 = 112000m/h
112000/60/60 = 31.1m/s
31.1/2 = 15.6 wheel rps
15.5*60 = 933rpm
So for a 1:1 ratio, the engine would need to be turning over at 933rpm at 70mph. That's VERY high gearing; especially for a 4th gear out of 6!
Edited by kambites on Wednesday 19th June 09:19
kambites said:
otolith said:
Cars which can't maintain motorway cruising speed up gradients in top gear are a nuisance. I don't mind dropping a couple of gears for hard acceleration, but I don't want to have to do it routinely. My old Civic Type-R was fine like that, adequate acceleration in top for normal purposes (and plenty more in 4th if needed). If it had been longer geared, that might not have been the case.
You'd have to have obscenely high gears (or steep motorways) to need to drop out of top for a two litre civic to not be able to maintain 70-80mph! 
otolith said:
Google NEDC urban cycle for details of how the urban figure is measured.
Gearing is a set of compromises and priorities vary with the function of the car.
You'd think the interent giving free access to information would turn everyone in to an expert, but no, instead the internet is used by people to whine about how they don't understand things.Gearing is a set of compromises and priorities vary with the function of the car.
OP - cars are optimised for performance or to get good numbers on the NEDC cycle or a compromise between the two (depending on the target market and how huge the fines for fleet average are). If you want to match the numbers for performance you need to learn how to drive the correct way. If you want to match or beat fuel economy figures you need to learn how to drive for that too (having a good hard look at the NEDC is a start).
Don't redesign your car until you've optimised your driving style to suit the car. The car may still be wrong, but by far the largest variable is the driver.
Kozy said:
Mine was done in miles. My 225/45/17s are .0001235 miles. At 70mph, they'd be spinning at 944rpm.
What you have wrong is that you missed out the final drive. That will be between 3 and 4 and spin the engine up to where it should be.
I was assuming that he was referring to the ratio after final drive is taken into account. It doesn't really make any practical difference to anything what speed the prop shaft is spinning at - if you use that definition you could have an overdriven gear that turns the engien over at a million rpm at motorway speeds if you wanted to. What you have wrong is that you missed out the final drive. That will be between 3 and 4 and spin the engine up to where it should be.
Top gear on my Elise is overdriven if you're talking about prop-shaft speed, but I still get well over 3000rpm at motorway speeds.
kambites said:
I was assuming that he was referring to the ratio after final drive is taken into account. It doesn't really make any practical difference to anything what speed the prop shaft is spinning at - if you use that definition you could have an overdriven gear that turns the engien over at a million rpm at motorway speeds if you wanted to.
Well, yes, in that sense it makes no sense.An overdrive gear in my mind is one where Vmax is achieved in the gear below.
kambites said:
If we look at a typical British motorway mile muncher, what do you get?
As it happens... BMW publish the gear ratios for the 1/3/5 series cars both manual and auto and you get the following (picked the 5 series facelift to do the calcs...)520d - 35mph/1000 (Manual) or 39mph/1000 (Auto) = 2000/1800rpm respectively at 70
525d - 35mph/1000 (Manual) or 41mph/1000 (Auto) = 2000/1700rpm respectively at 70
530d - 48mph/1000 (Auto) = 1450rpm at 70
535d - 45mph/1000 (Auto) = 1550rpm at 70
520i - 30mph/1000 (Manual) or 37mph/1000 (Auto) = 2300/1900rpm respectively at 70
528i - 30mph/1000 (Manual) or 37mph/1000 (Auto) = 2300/1900rpm respectively at 70
535i - 29mph/1000 (Manual) or 39mph/1000 (Auto) = 2400/1800rpm respectively at 70
550i - 45mph/1000 (Auto) = 1550rpm at 70
even the shortest geared BMW I can see (at a quick glance - a 125i Manual) is geared at 28mph/1000 or 2500rpm at 70... most other similar cars will be likewise, not least because of the demand for diesels for high mileage use. For me I see cars geared either for motorway cruising (as above) or for town driving where short gearing makes the car feel nippy with less power.
LotusOmega375D said:
I went directly from one extreme to the other:
70mph at nearly 4000rpm in a Subaru 22B
70mph at 1600rpm in a Lotus Carlton
Both are silly really.
You're forever changing gear in the 22B and can even settle into 5th in a 30mph zone. That will take you to about 145mph if you wish. But I am sure greater spacing between ratios would benefit the car (I believe the few Prodrive supplied UK spec cars had a higher 5th gear for this reason).
Of course the Lotus has the ZR1 Corvette gearbox with 6th gear interstellar overdrive (wasn't that a Pink Floyd track?), which cannot really be used below 70mph and isn't much good for punchy overtaking. Thing is because it's there I do use it on motorway or dual carriageway runs, so it may be quite an economic advantage. Not that a 377bhp 3.6 litre 24 valve twin-turbo can ever be considered remotely economical!
Whats the carlton like on the motorway? I suppose you use 5th as much as 6th? 70mph at nearly 4000rpm in a Subaru 22B
70mph at 1600rpm in a Lotus Carlton
Both are silly really.
You're forever changing gear in the 22B and can even settle into 5th in a 30mph zone. That will take you to about 145mph if you wish. But I am sure greater spacing between ratios would benefit the car (I believe the few Prodrive supplied UK spec cars had a higher 5th gear for this reason).
Of course the Lotus has the ZR1 Corvette gearbox with 6th gear interstellar overdrive (wasn't that a Pink Floyd track?), which cannot really be used below 70mph and isn't much good for punchy overtaking. Thing is because it's there I do use it on motorway or dual carriageway runs, so it may be quite an economic advantage. Not that a 377bhp 3.6 litre 24 valve twin-turbo can ever be considered remotely economical!
LotusOmega375D said:
Of course the Lotus has the ZR1 Corvette gearbox with 6th gear interstellar overdrive (wasn't that a Pink Floyd track?), which cannot really be used below 70mph and isn't much good for punchy overtaking. Thing is because it's there I do use it on motorway or dual carriageway runs, so it may be quite an economic advantage. Not that a 377bhp 3.6 litre 24 valve twin-turbo can ever be considered remotely economical!
That's sort of the point though and why it has a double over drive. For normal driving 5th will offer the same flexibility and performance as most 5 speed boxes do. Good mpg and enough power to use it.However at speed you have another cog you can use too. It's things like this that have meant historically cars like the Corvette have always avoided the US "gas gusler tax" that almost all Euro fast cars have had to face in the US market.
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