6th Gear in X-Trail

[DavidM.L.]

I have read previous posts where people have said that towing in 5th is actually more fuel efficient than towing in 6th in various cars. Does any one have a definitive answer to this particularly in regard to the 134Bhp (post 04) diesel X-trails.

At 60mph in top the revs are at 2000rpm which would seem to be ideal. In 5th I think its probably more like 3000rpm (?). We have a long trip through France planned in the summer when I expect to just sit at a fairly constant speed for long periods of time and might as well use the 'best' gear available.

 

[rob_jax]

Hi David,

This was reported by one poster, and it may be true for that person's particular vehicle and driving style.

But it's not something I've experienced in any of the last handful of cars I've bought (with fuel computers - which give a moderately reasonable overview of fuel consumption) - but then I've tended to purchase cars with fairly large and powerful diesel engines - so my E-class 320 CDI does give the best motorway fuel consumption in 7th

Generally you would expect a diesel engine, running at the lowest speed but still compatible with producing maximum torque - so 2000 rpm from your X-Trail would comply with this - should produce the best fuel consumption.

If you have a fuel computer - make sure you are driving on a flat level section of motorway - reset the computer, maintain a constant speed and check the value after a mile or so. Then exactly repeat in a different gear.

You need to exercise care so that you don't influence the results - which can make an enormous difference - particularly if you drive with your foot off the "gas" in one test.

Robert

 

[JeremyClarksson]

Wrong car - Seat Alhambra - but I find towing in 5th or 6th makes no difference to MPG so I use 5th - less gear changes and more torque for overtaking, hills etc and more engine braking for going down hills.

I was using cruise at 60 mph the other day and found I was doing 70 on a gentle downhill slope in 6th because of the lack of engine braking.

 

[Garfield]

It was me!

We have found that solo between 50 - 80 mph our 2.0tdci Ford s-max is more economical in 5th rather than 6th and when towing, 4th is the most economical, however 5th in the s-max is about the same ratio as 6th in the xtrail (~60mph @2000rpm) 6th gives us nearly 70mph @2000rpm.

Every engine has its optimum gear ratio to give best efficiency for a particular duty (ie solo or towing) too low a gear and the engine is wasting fuel to simply keep the engine spinning, too high a gear and the cylinder pressures required to produce the necessary torque are inefficient, keeping the car in a gear to keep the engine near it's maximum torque revs (usually 2000 for a td) is not necessarily the most efficient.

The optimum gear ratio is very much dependant upon engine size as larger engines take more power to keep them spinning so it is no surprise that Roberts 3.2 gives best fuel economy in a high gear compared to my 2.0.

If my engine gives best fuel economy @2800rpm towing at 60mph then I would expect a 2.2 (10% larger) to be best at about 2500rpm and Roberts 3.2 to be best at about 1750rpm this is without taking into account the subtleties between manufacturers and vehicle styles.

The bottom line is that with smaller engines, towing in a lower gear can use less or no more fuel and means that cruise control stays on up the hills.

 

[Rich]

Engines do NOT have an optimum gear ratio for efficiency. The do however have an optimum RPM which the engine works best at. This is or can be different for EVERY make of engine.It is usually between Maximum Torque and Maximum Power. This is when a Turbo Diesel engine produces close to maximum boost and the engine has a good supply of fresh air. The engine has power in reserve and as the vehicle slows due to a gradient the engine will reduce RPM but the torque will increase without the need to "chase" the maximum torque by using exessive throttle use. So a slightly bigger engine with maximum torque at low RPM of about 1500-1800 RPM will not have too much trouble using top gear unless you allow the RPM to drop too much. Big turbo diesel engines of 3 liter plus capacity have an abundance or torque so they also have no real problem using top gear purely down to this over kill of power.

 

[Garfield]

Surely you use the optimum gear to get the optimum rpm.

many drivers, like I did, think that keeping the car in the highest gear it will pysically pull is the most economical, my recent experience is that "It aint necessarily so" I have found that, although the car will happily tow in 6th near peak torque revs, less fuel is used by letting the engine use less torque and more revs in 4th or 5th

 

[Rich]

Garfield,

The confusion is people feel the highest gear and lowest RPM will give maximum efficiency. The engine can be kept only as close to it's most efficient RPM range as the ratios in the gear box will allow. Some cars have a big jump in ratio between 1st and 2nd...Others may have even spacing through the box. Each car is different. But the common thing is the engine needs to be kept above maximum torque and the engine needs to keep up it's boost on the turbo so any decrease in both vehicle and engine speed will bring the motor up to maximum torque.

 

[SimonB]

I recently experimented with the gears in our 2.5 tdi caravelle 174bhp in Holland on flat roads and found 6th gear gave 28mpg and 5th gear gave 30mpg at 60mph with cruise on and if i dropped to 55mph and followed the lorries i was averaging 40mpg with a 6 berth twin axle on tow. Having said that it obviously varries from car to car.

 

[Rich]

OK the easiest way to think about it is the car manufacturers have designed the cars they build to be at their most efficient at between 70 and 75MPH(allow for speedo error) in top gear. Look at the engines RPM at that speed and you will see it is above maximum claimed torque RPM. If you are traveling and need to keep a good average going and the road starts to incline you probably are better to go down a gear so the engine is revving close to the same engine speed that will sustain 70MPH in top gear. In our Renault this is about 3250RPM and we average between 31 and 36 MPG on a run.