rpm at 70mph

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Thu 8 Oct 2015 22:45
rpm at 70mph - Mr Carrot Cake

Can anyone tell me some used cars that have a fairly low rpm at 70mph? It find engine noise quite annoying on long journeys and some cars do over 3000rpm at 70mph. I wish 6 speed gearboxes were more common.

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Thu 8 Oct 2015 23:01
rpm at 70mph - bernie123

Back to my VW Bora TDI 130 6 speed, 70 mph less than 2000 rpm :-) and at that speed 60 mpg.

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Thu 8 Oct 2015 23:04
rpm at 70mph - RobJP

It's all to do with torque.

Basically, you want a car that can run with minimal throttle input at a constant motorway cruising speed. To get that, you need the engine to be turning over at maximum torque (or turning force). Diesel engines develop very high torque from 1800 rpm (ish). Petrol engines don't develop max/high levels of torque until considerably higher up the rev range.

That's why diesels are the ultimate in fuel economy on motorway journeys.

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Fri 9 Oct 2015 07:56
rpm at 70mph - skidpan

It's all to do with torque.

As RobJP says, its all to do with torque and that is why diesels have the lowest RPM at a given speed.

But my Seat Leon 1.4 TSI is not at all bad. A genuine 70 mph is 2550 RPM and over long distances (recently did 430 miles in the day (all on Motorways and fast A roads) is very relaxing.

The wifes new Nissan Note DIG-S is equally good for a small petrol. For a small petrol car with only 98 PS and a 5 speed box 2800 RPM is very respectable. At this speed the car is very quiet, much better than the wifes previous diesel which sat at 2200 rpm at the same speed.

Basically there is no point having low PRM's if wind and road noise kick up a huge racket.

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Fri 9 Oct 2015 08:36
rpm at 70mph - RT

Turbo'd engines, diesel or petrol, and large capacity V8s give lots of torque allowing cruising at low rpm.

Edited by RT on 09/10/2015 at 08:37

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Fri 9 Oct 2015 09:29
rpm at 70mph - catsdad
I notice that HJ regularly quotes mph per 1000 rpm figures for individual cars.however despite googling I can't find any tables on the wiser net listing this data across a range of cars. Anyone here got a source they can share? Back to the OP I have seen very low rpm on automatic diesel audis at motorway cruising speeds.
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Fri 9 Oct 2015 10:40
rpm at 70mph - kiss (keep it simple)

I find it bizarre that the last car I owned with a good high top gear was a 1989 Cavalier 2 litre. Indicated 90mph (probably nearer 80) was just 3000. It would return 45mpg on a motorway run and felt very relaxed. Later cars have all been turning 3000 at indicated 70. They include Nissan Primera 2.0, Audi A4 1.8 and Mazda6. None could get over 40mpg under the same conditions. Mrs always kept trying to change up to a non-existent 6th gear!

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Fri 9 Oct 2015 13:01
rpm at 70mph - RT

I find it bizarre that the last car I owned with a good high top gear was a 1989 Cavalier 2 litre. Indicated 90mph (probably nearer 80) was just 3000. It would return 45mpg on a motorway run and felt very relaxed. Later cars have all been turning 3000 at indicated 70. They include Nissan Primera 2.0, Audi A4 1.8 and Mazda6. None could get over 40mpg under the same conditions. Mrs always kept trying to change up to a non-existent 6th gear!

The '89 Cavalier 2.0 was very efficient on fuel - it had virtually no emissions equipment on it and had plenty of power despite being tuned for economy - and that era were well-built by mass-market standards.

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Fri 9 Oct 2015 13:28
rpm at 70mph - kiss (keep it simple)

Mechanically it was sound. I flogged it at 9 years and 150k. The buyer took it to over 200k but it rusted away....

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Fri 9 Oct 2015 13:19
rpm at 70mph - SteVee

I don't understand this obsession with low revs - is it just because there's a big tachometer in front ?

The slower the engine runs, the bigger bang each cylinder has to contribute (for the same power etc). More cylinders is good, but I don't want to run turbos, DMFs etc as a simple 4 cylinder petrol engine can EASILY develop enough power to drive a normal car along a British motorway.

The move to multi-valve engines may contribute to the slightly higher revs in more recent cars.

A sports 600 motorcycle runs at incredible (to a car driver) speed - 8000 RPM is nothing, - it's smooth, relatively quiet and the engine's perfectly happy at those revs. It will run up to 15-16K without problems. A modern car engine will happily rev to 7K (and beyond). Why worry about 3K ?

As above, wind noise etc can be far more of a problem, to say nothing of stupid low aspect ratio tyres and the noise they can contribute. NVH is not about running the engine as slowly as you can.

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Fri 9 Oct 2015 13:37
rpm at 70mph - kiss (keep it simple)

Point taken about multivalve engines, their torque curve is different so I suppose driveability and performance will suffer if the gearing is too high. I used to think that a full cylinder was more efficient hence lower revs being more economical for a given load, but I have read stuff about modern engines behaving differently. I had a hire car recently with a 1.4 engine and 6 gears. It was so gutless that you had to change constantly to make any decent progress, and it was still doing 3k at 70! There was such a huge gap between 2nd and 3rd it seemed a waste having 3 more after that. I wonder if the gear ratios are chosen to do the best possible in the mpg tests.

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Fri 9 Oct 2015 14:28
rpm at 70mph - SteVee

Yes - I think modern engines are designed for the best performance under test conditions. The trend has been to introduce short-stroke multi-valve engines which like to rev - however, Ford has bucked this trend with their EcoBoost engine which has a long-stroke engine. Although still multi-valve, it looks designed to run at lower engine speeds. I believe it has a DMF though :-(

I might have given the impression that I like high-speed engines, whereas it's simplicity that I really want (and a quiet, comfirtable ride!)

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Fri 9 Oct 2015 16:35
rpm at 70mph - mike hannon

When will people realise that it doesn't matter how many gears you have in the box? It's the ratio of the top one that matters if you are looking for low rpm at cruising speed isn't it?

My 6-litre XJS shows a whisker over 2,000rpm at 70mph in the top ratio of a 4-speed automatic. That's 22mpg plus and seems pretty reasonable to me.

The Rover SD1 V8 manual years ago was geared to do 38mph per 1,000rpm in top (fifth) gear but wasn't much more economical than that.

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Fri 9 Oct 2015 17:17
rpm at 70mph - focussed

The relationship between RPM and economy is quite simple-if you double the RPM you triple the frictional and pumping losses.

Pumping losses are the energy requirements of moving the crankcase gases and oil mist from one bore under the piston to the next bore.

At rated speed a highly efficient but high revving petrol engine is using 20%of it's power merely driving itself.

That's one of the big reasons diesels are so much more efficient - they deveop their power at lower RPM hence less frictional and pumping losses.

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Fri 9 Oct 2015 17:35
rpm at 70mph - RT

20% really - compared to all the other frictional losses in the drive train, especially tyres as well as aerodynamic losses ?

The main reason a diesel is more efficient is because the higher compression ratio allows a bigger % of theoretical energy to be transformed into power and not lost as heat.

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Mon 12 Oct 2015 20:45
rpm at 70mph - xtrailman

"The main reason a diesel is more efficient is because the higher compression ratio allows a bigger % of theoretical energy to be transformed into power and not lost as heat."

So why have Mazda reduced the comppresion ratio to only 14-1?

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Mon 12 Oct 2015 21:01
rpm at 70mph - RT

"The main reason a diesel is more efficient is because the higher compression ratio allows a bigger % of theoretical energy to be transformed into power and not lost as heat."

So why have Mazda reduced the comppresion ratio to only 14-1?

What's the effective compression ratio - actual ratio multiplied by the boost ratio?

Has that been reduced?

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Tue 13 Oct 2015 00:09
rpm at 70mph - Manatee

"The main reason a diesel is more efficient is because the higher compression ratio allows a bigger % of theoretical energy to be transformed into power and not lost as heat."

So why have Mazda reduced the comppresion ratio to only 14-1?

It reduces combustion noise apparently. The Outlander 4N14 diesel has a similar CR and among the reasons quoted are noise, and the reduced combustion pressures that allow a lighter aluminium block to be used.

Even if extra boost restores the overall compression levels, the use of intercooling has the potential to reduce combustion temperatures which wiill help to manage NOx levels and/or EGR use which will give an efficiency offset.

Edited by Manatee on 13/10/2015 at 00:11

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Fri 9 Oct 2015 23:06
rpm at 70mph - SteveLee

Four values per cylinder don't nececerily move power up the rev range the smaller valves and ports might have the same area as the two valve designs - the added benefit is you can have lighter valve springs and lift the valves less for the same port area meaning it's easier to achive higher compresion ratios without massive piston pockets to accomodate open valves - you will often get better swirl at low revs with softly tuned 4VPC designs improving low end torque. The 150bhp PSA 2 lite petrol is very torquey as was the Triumph Dolomite Sprint.

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Fri 9 Oct 2015 23:49
rpm at 70mph - Manatee

Point taken about multivalve engines,

My thoughts too, when I saw the reference to the '89 2 litre Cavalier. I had one from 89-91, and just before that a Carlton estate with (I assume) a similar engine, 2 valves per cylinder and rated at IIRC 115bhp. I loved them both.

Again from memory, the 16 valve engines that followed were rated at 130bhp but felt less powerful - as they almost certainly were at the low to middling rpm used for 90%+ of normal motoring.

Edited by Manatee on 09/10/2015 at 23:50

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Sat 10 Oct 2015 14:46
rpm at 70mph - Mr Carrot Cake

I don't understand this obsession with low revs - is it just because there's a big tachometer in front ?

The slower the engine runs, the bigger bang each cylinder has to contribute (for the same power etc). More cylinders is good, but I don't want to run turbos, DMFs etc as a simple 4 cylinder petrol engine can EASILY develop enough power to drive a normal car along a British motorway.

The move to multi-valve engines may contribute to the slightly higher revs in more recent cars.

A sports 600 motorcycle runs at incredible (to a car driver) speed - 8000 RPM is nothing, - it's smooth, relatively quiet and the engine's perfectly happy at those revs. It will run up to 15-16K without problems. A modern car engine will happily rev to 7K (and beyond). Why worry about 3K ?

As above, wind noise etc can be far more of a problem, to say nothing of stupid low aspect ratio tyres and the noise they can contribute. NVH is not about running the engine as slowly as you can.

I don't understand this obsession with low revs - is it just because there's a big tachometer in front ?

Because on the motorway lower revs = quieter, more relaxed journey. Obviously other things affect noise like the road surface and soundproofing, but engine noise can be annoying. It also should be more economical revving at 2900rpm rather than 3400rpm.

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Fri 9 Oct 2015 18:23
rpm at 70mph - steelghost

My 1.8 petrol Avensis with CVT gearbox turns over at about 2050 RPM at an indicated 70mph.

Of course, the revs jump up if you are maintaining speed up an incline, or overtaking, but for cruising, it's nice and quiet :)

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Fri 9 Oct 2015 19:20
rpm at 70mph - gordonbennet

My old bosses XJ6, manual/overdrive was ticking over at just over 2000rpm at 120mph, i was a 19 year old passenger, well impressed.

Smoothness depends to an extent on the number of cyls, a 4cyl will probably be harsh at 3500 rpm whilst a straight six will be silky smooth.

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Fri 9 Oct 2015 23:10
rpm at 70mph - SteveLee

My old bosses XJ6, manual/overdrive was ticking over at just over 2000rpm at 120mph, i was a 19 year old passenger, well impressed.

Smoothness depends to an extent on the number of cyls, a 4cyl will probably be harsh at 3500 rpm whilst a straight six will be silky smooth.

I suspect the tacho and speedo were faulty! My first Jag XJR (supercharged torque monster) was "only" geared 40mph per thousand and will have double to triple the torque of that old XJ6 at low revs - I find 60mph per thousand hard to beleive!

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Sat 10 Oct 2015 22:51
rpm at 70mph - John F

My old bosses XJ6, manual/overdrive was ticking over at just over 2000rpm at 120mph, i was a 19 year old passenger, well impressed.

I doubt it. Many years ago I had an old manual series 1 4.2 XJ6. I seem to remember it was about 35mph per 1000rev, certainly less than 40.

The best ratioed g/box I have ever had was the Audi A6 2.8 with the ZF 5 HP 19 which had 4th gear at 24mph /1000rev for a possible downhill 150mph at the 6.25 red line and 32mph/1000 5th for relaxed motorway drive. There is a problem with having a very high top gear with powerful engines as it's difficult for the g/box to handle the torque.

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Sat 10 Oct 2015 12:32
rpm at 70mph - Mr Carrot Cake

Considering buying a Toyota Auris 1.6 from 2007 and the only thing that puts me off is it does 3400rpm at 70mph. Only 5 gears, why they didnt give it 6 gears goodness knows.

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Sat 10 Oct 2015 14:58
rpm at 70mph - Manatee

My 16 year old MX5 is doing 3500rpm at 70 in 5th, which sometimes has me reaching for another gear that isn't there.

On the other hand, the red line is at 7000, so the top speed must be 140. (Downhill anyway).

All a bit academic as I don't keep it for speed, and I can manage quite happily with about 4500 rpm.

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Sat 10 Oct 2015 15:26
rpm at 70mph - Engineer Andy

My 16 year old MX5 is doing 3500rpm at 70 in 5th, which sometimes has me reaching for another gear that isn't there.

On the other hand, the red line is at 7000, so the top speed must be 140. (Downhill anyway).

All a bit academic as I don't keep it for speed, and I can manage quite happily with about 4500 rpm.

Not sure if your post was meant to be jokey, but in reply, engines don't work as you stated, that's why they have power and torque curves.

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Sat 10 Oct 2015 16:03
rpm at 70mph - Manatee

Not sure if your post was meant to be jokey, but in reply, engines don't work as you stated, that's why they have power and torque curves.

Sorry, I was being slightly droll...but if I could get it to ~7000rpm in fifth it would be doing ~140 - gearboxes do work like that.

The claimed top speed is IIRC c. 122 which is reached in 5th, at 6450 rpm, rather than 4th. I am unlikely to test it, it's too old and knackered like its owner, but the joy of a small, low car with decent handling and the roof down is that anything over 50 feels fast.

Edited by Manatee on 10/10/2015 at 16:05

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Sat 10 Oct 2015 16:12
rpm at 70mph - RT

Not sure if your post was meant to be jokey, but in reply, engines don't work as you stated, that's why they have power and torque curves.

Sorry, I was being slightly droll...but if I could get it to ~7000rpm in fifth it would be doing ~140 - gearboxes do work like that.

The claimed top speed is IIRC c. 122 which is reached in 5th, at 6450 rpm, rather than 4th. I am unlikely to test it, it's too old and knackered like its owner, but the joy of a small, low car with decent handling and the roof down is that anything over 50 feels fast.

It used to be relatively easy to get a car up to it's hypothetical maximum geared speed - empty motorway downhill, I've had a Vauxhall redlining in top gear pushing 140 on the speedo despite the power curve preventing the car getting past 115 on the level.

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Sat 10 Oct 2015 22:09
rpm at 70mph - Haydn24

Wish my 1.2 Corsa had a 6th gear! Think its between 3000-3500rpm at 70mph. So annoying. Still get a decent mpg of about 56 though

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Sun 11 Oct 2015 16:19
rpm at 70mph - SteveLee

My missis's Citroen C3 1.4 had very tall gearing in 5th - it would cruise all day long at 80mph - okay you had to drop to 4th to overtake at 60ish but the relaxed cruising made it worth it and it eaily returned 50+ mpg if you kept the speed down.

Edited by SteveLee on 11/10/2015 at 16:20

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Sun 11 Oct 2015 21:59
rpm at 70mph - daveyK_UK

The absolutly gutless Vauxhall zafira (current model) with the 1.6 petrol engine combined with the 1973 designed 5 speed manual gear box was horrific.

From memory, it was 3900 RPM at 70

they are very good value for money and that is why they are still selling in decent enough numbers, but on a long run fuel economy will never be its strong point.

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Sun 11 Oct 2015 23:05
rpm at 70mph - Mr Carrot Cake

The absolutly gutless Vauxhall zafira (current model) with the 1.6 petrol engine combined with the 1973 designed 5 speed manual gear box was horrific.

From memory, it was 3900 RPM at 70

they are very good value for money and that is why they are still selling in decent enough numbers, but on a long run fuel economy will never be its strong point.

The engine noise must be horrendous.

My ex has just bought a Corsa diesel with a 1.7 litre engine and 6 speed box. 6th is very high.

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Mon 12 Oct 2015 20:57
rpm at 70mph - skidpan

combined with the 1973 designed 5 speed manual gear box

What Vauxhall had a 5 speed box in 1973. The first I remember (other than the Droop Snoot Firenza and Chevette HS which both used dog leg Getrag boxes) was the Cavalier in about 1983.

From memory, it was 3900 RPM at 70

The engine noise must be horrendous.

Had worse. A 1975 Hillman Avenger 1600 with a 4 speed box was about 4400 rpm but at the time it was perfectly normal.

Mate had a Fiat 127 Sport and that was doing near 5000 rpm at 70 mph.

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Tue 13 Oct 2015 00:41
rpm at 70mph - diddy1234

I had a fiat 127 (1049cc variant) and the revs were very high for 70mph.

However, the engine loved to be revved and its red line was 7,500 rpm !

Still got very boring and tedious on a long journey with only a 4 speed gearbox

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Tue 13 Oct 2015 15:03
rpm at 70mph - balleballe

Honda civic 2.2D is at 2000rpm when doing 70mph

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