Any - Most efficient RPM | Motoring discussion | Back Room Forum

What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks

Wed 4 Jun 2014 21:31

Any - Most efficient RPM - csgmart

According to the manufacturer of my car my engine produces the highest torque figure between 1,600 and 1,800 PRM which equates to around 70 MPH in 7th gear (automatic).

I generally drive between 60 and 70 on the motorway so the car tends to sit in the rev range where the torque output is in the 'sweet spot'.

Question - is the car running at it's most efficient when the engine is producing maximum torque?

As you can probably tell from my question I'm no engineer.....

Reply | Report as offensive | Link

Wed 4 Jun 2014 21:49

Any - Most efficient RPM - brum

You dont develop maximum torque when you sit at 60-70mph. You need to plant your foot to the floor to enable that.

If your underlying question relates to fuel economy then engine efficiency is only a small part of the equation.

For most vehicles, fuel economy decreases with vehicle speed, so the slower you drive, the higher the mpg is - up to a point where the engine efficiency dramatically drops at low revs. That of course assumes you are driving at a constant speed on a straight level road with no head/tail/cross winds and.............

Look up "Specific Fuel Consumption" to find out more about engine efficiency.

Edited by brum on 04/06/2014 at 21:53

Reply | Report as offensive | Link

Wed 4 Jun 2014 21:56

Any - Most efficient RPM - csgmart

You dont develop maximum torque when you sit at 60-70mph. You need to plant your foot to the floor to enable that.

I understand but if you plant your foot to the floor the RPM goes up and takes it out of the rev range where maximum torque is produced, so........ catch 22

Of course speed of the car is a factor in fuel consumption but I guess my underlying question was what combination of speed, RPM and torque produces the most efficient fuel consumption - it makes 'sense' to me that the RPM where the torque is the highest would play a factor somewhere.

Reply | Report as offensive | Link

Wed 4 Jun 2014 22:08

Any - Most efficient RPM - brum

If you want higher mpg, slow down, simple.

Overall efficiency - Engine, transmission losses, tyre rolling resistance and aerodynamic drag are all factors to consider.

Specific fuel consumption graphs for engines vary a lot, they are ditacted in modern cars by the mapping of the fuel and ignition systems - the design engineers can manipulate this a lot.

This is a typical graph taken from wikipedia

en.wikipedia.org/wiki/File:Brake_specific_fuel_con...g

you will see that efficiency drops off with low throttle openings (like you have travelling at 60-70mph on a motorway), but you do use less fuel at lower power/torque output, even though that power/torque is developed less efficiently.

Edited by brum on 04/06/2014 at 22:12

Reply | Report as offensive | Link

Wed 4 Jun 2014 22:15

Any - Most efficient RPM - csgmart

If you want higher mpg, slow down, simple.

True - although I often get over 61 mpg (on the computer ) so I'm not doing too badly.

Reply | Report as offensive | Link

Wed 4 Jun 2014 23:25

Any - Most efficient RPM - Mike H

If you want higher mpg, slow down, simple.

I don't buy into that. I'm thinking that my automatic is probably using less fuel at 50mph in 5th than at 40mph in 4th, for example. And the fuel consumption doesn't vary whether I drive at 70mph or 90mph, as found in practice over c. 140,000 miles in the same car.

Reply | Report as offensive | Link

Thu 5 Jun 2014 00:59

Any - Most efficient RPM - alan1302

And the fuel consumption doesn't vary whether I drive at 70mph or 90mph, as found in practice over c. 140,000 miles in the same car.

It will vary - you can't have the same MPG at 70mph as you do 90mph

Reply | Report as offensive | Link

Thu 5 Jun 2014 13:58

Any - Most efficient RPM - skidpan

If you want higher mpg, slow down, simple

Because of roadworks on the M1 and a 50 mph speed limit the wifes 85 mile weekly round trip to her mothers is currently being made at a lower average speed. 40 of the 70 motorway miles are covered at the lower speed. Before the road works her average mpg for the trip was between 55 and 57 mpg, since the road works it has never been less than 62 mpg.

She estimates that the lower speed limit has added about 5 minutes each way to her journey but over a year a fuel cost savings would be about £50.

For the record the car is a Kia Ceed SW 1.6 CRDi. At 70 mph in 6th its doing about 2200 rpm thus at 50 mph its doing approx just under 1600 rpm. According to the spec sheet max torque is produced between 1900 and 2750 rpm thus perhaps it would be even better at 50 mph if she dropped to 5th

Reply | Report as offensive | Link

Thu 5 Jun 2014 14:10

Any - Most efficient RPM - HandCart

Yes - I would suggest that. Travelling at 60 ish in my Astra diesel, I have to press the accelerator slightly further down in 6th gear than I do in 5th. On the face of it the engine seems happy at 60 in 6th, but the sound of it is slightly laboured. I tend not to bother with 6th unless I'm going downhill, or doing at least 70 on the flat.

I'm pretty sure that at 50 the Ceed would be better in 5th.

Reply | Report as offensive | Link

Thu 5 Jun 2014 14:19

Any - Most efficient RPM - HandCart

I agree. All else being equal, the power required to travel at a given speed goes up as a cubed-law as the speed increases.

For the typical car's frontal area and Cd, the air resistance starts to go up significantly once over 50mph, and very significantly once over about 80. I would think it would have to be a very slippery, very powerful, car that showed negligible difference in consumption at 90 compared with 70.

Reply | Report as offensive | Link

Thu 5 Jun 2014 14:44

Any - Most efficient RPM - skidpan

I'm pretty sure that at 50 the Ceed would be better in 5th

When we first bought the car i would have agreed, in truth it would not run at 50 mph in 6th, the engine was totally outside of its comfort zone. But when the car went in for its 3rd service the dealer downloaded an ECU update and the difference to flexibility was staggering, its now totally happy at 50 mph in 6th. But just because it feels better does not mean it is better. Will suggest she tries 5th on her next trip.

Reply | Report as offensive | Link

Thu 5 Jun 2014 15:09

Any - Most efficient RPM - Gibbo_Wirral

And the fuel consumption doesn't vary whether I drive at 70mph or 90mph, as found in practice over c. 140,000 miles in the same car.

It will vary - you can't have the same MPG at 70mph as you do 90mph

And then factor in gradients. To maintain 70 mph uphill you'll press the accellerator pedal more than 70 downhill, where it probably won't be touched at all.

Reply | Report as offensive | Link

Thu 5 Jun 2014 18:47

Any - Most efficient RPM - Mike H

And the fuel consumption doesn't vary whether I drive at 70mph or 90mph, as found in practice over c. 140,000 miles in the same car.

It will vary - you can't have the same MPG at 70mph as you do 90mph

I can assure you that, in real life, over a large number of tankfuls calculated brim to brim, it's a fact. I concede that there might be other factors involved that aren't immediately obvious, e.g. driving at 70 would often be due to the amount of traffic, and there might be perhaps a quantity of slowing down and speeding up, while being able to cruise at 90 would normally be constant throttle and steady speed.

Reply | Report as offensive | Link

Ask Honest John

Sell Your Car

Real MPG

Fuel Calculator