gear ratios

[Captcav]

I have a 04 TD5 auto and was wondering if there is a way to lower the ratio in top gear by about 500 rpm to improve fuel economy for highway driving. Cheers

[jafir]

Are you trying to lower the gears, and raise the RPM or are you trying to lower the RPM?

You could get 1.003 gears put in the high range of the transfer case to lower the RPM on the highway...

or you could get 1.410 gears in the high range to raise the RPM.

See: Ashcroft Transmissions (http://www.ashcroft-transmissions.co.uk/index.php?act=viewCat&catId=70)

[Captcav]

Trying to lower the rpm in top gear. Currently top gear sit at about 2500rpm at 110kph. I want to get that down to around 2000rpm at the same speed..

[Tombie]

Trying to lower the rpm in top gear. Currently top gear sit at about 2500rpm at 110kph. I want to get that down to around 2000rpm at the same speed..

Based on what logic?

What RPM range do you think its most efficient at? :)

[Captcav]

About 2000 - 2200 its at its most effecient, which is eqivulent to 90-100 kph.

[Blknight.aus]

thats more of an areodynamics thing than an engine revs thing.

[Tombie]

About 2000 - 2200 its at its most effecient, which is eqivulent to 90-100 kph.

Considering the TD5 engine in stock form makes most of its torque at 2400 rpm dropping below that by 400 rpm will result in worse efficiency than stock.

As Blknight has commented, the vehicle is more efficient at the speeds you listed above due to aerodynamics....

I had 4.11 gears put in, and for a short while was running wheels too small a diameter for that gear set (was doing about 2900rpm at 110km/h)...
Fuel use was only 0.2L/100km worse...

[Slunnie]

When I changed from 29" tyres to 33" tyres the fuel economy improved on the highway but it became worst around town. Running at the standard gearing makes the economy a lot more consistent also than with altered gearing.

This is for a TD5

[trog]

It is based on these arguements I , cash gods willing , would like to see about a six speed gearbox. Probably not economically viable but an interesting idea.

[Captcav]

Considering the TD5 engine in stock form makes most of its torque at 2400 rpm dropping below that by 400 rpm will result in worse efficiency than stock.

As Blknight has commented, the vehicle is more efficient at the speeds you listed above due to aerodynamics....

I had 4.11 gears put in, and for a short while was running wheels too small a diameter for that gear set (was doing about 2900rpm at 110km/h)...
Fuel use was only 0.2L/100km worse...

I'm not sure this argument has much merit, as a car that produces all its torque at 8500rpm cannot claim that driving it at that level would result in ideal fuel economy. The best fuel economy figures that can be produced with a TD5 are at speeds of 100kph, which in standard tune is 2200rpm. The problem with this is it's 10kph below our national speed limit and would put me 50km behind on my 5hr journey to our holiday destination.

[gavinwibrow]

I'm not sure this argument has much merit, as a car that produces all its torque at 8500rpm cannot claim that driving it at that level would result in ideal fuel economy. The best fuel economy figures that can be produced with a TD5 are at speeds of 100kph, which in standard tune is 2200rpm. The problem with this is it's 10kph below our national speed limit and would put me 50km behind on my 5hr journey to our holiday destination.

Would be cheaper and easier to have a set of oversize tyres and rims for long distance travel and leave it as is for city/urban work.

[dusty disco]

I think it is better to be cruising at an engine speed faster than the torque peak - in this case say at 2500rpm. Then when a hill or wind gust is encountered, the engine slows but also gets more torque to overcome the extra resistance.

[Tombie]

I'm not sure this argument has much merit, as a car that produces all its torque at 8500rpm cannot claim that driving it at that level would result in ideal fuel economy. The best fuel economy figures that can be produced with a TD5 are at speeds of 100kph, which in standard tune is 2200rpm. The problem with this is it's 10kph below our national speed limit and would put me 50km behind on my 5hr journey to our holiday destination.

Excerpt from article "How to drive for economy"


Watch the Tachometer

Because tachometers are no longer limited to performance models, more drivers than ever have a chance to pay attention to engine speed as well as road speed. This allows you to find the engine's most efficient rpm and stay close to that point whenever feasible. What speed is that?
The exact figure depends on the engine but is typically the speed at which it produces the greatest torque output. For economy's sake, it's generally wise to remain below 3,000 rpm most of the time and to shift into the next gear before the engine gets much beyond its optimum rpm level. Too low an engine speed does nothing for your finances, so running below 1,500 isn't ordinarily a good idea.


Ding ding ding.... We have a winner...

[twr7cx]

I'm not sure this argument has much merit, as a car that produces all its torque at 8500rpm cannot claim that driving it at that level would result in ideal fuel economy.

Generally torque is made in the lower RPM. Peak power output may be in the higher RPM - the rotary motor in a Mazda RX8 or the high reving VTEC in a Honda S2000 spring to mind as examples of motors that rev out to near 9,000rpm and make their power up high, but their peak torque output is still much lower.
I cannot think of a motor that produces maximum torque anywhere near 8,500rpm, and I don't believe it would be a very drivable... But hypothetically, if there was such a motor, yes, I would drive it at 8,500rpm, as the amount of torque at lower revs such as 2,000rpm would likely be so low and pathetic you wouldn't move anywhere!



The best fuel economy figures that can be produced with a TD5 are at speeds of 100kph, which in standard tune is 2200rpm. The problem with this is it's 10kph below our national speed limit and would put me 50km behind on my 5hr journey to our holiday destination.

Reread what you have written - it's not the lower RPM that is causing the fuel economy improvement, it's the lower speed which means less resistance from the air - that additional 10km/h speed increase from 100 to 110km/h brings with it some significantly greater resistance.

For the record I have 32" tyres fitted and so rev at around 2,100rpm at 110km/h. I found absolutely no fuel economy improvement. If your TD5 is running correctly they are a pretty economical motor already!
The other issue you need to consider with playing around with the gearing is that the vehicle is more fuel efficient when cruising when your torque converter is locked. When it is not locked it is slipping which is causing additional heat and some of the energy is not being transfered down the driveline. The torque converter is more likely to lock at higher RPM and unlocks more frequently at lower RPM.
There's a lot of different factors involved and there are reasons why Land Rover set it up as they did...




I think it is better to be cruising at an engine speed faster than the torque peak - in this case say at 2500rpm. Then when a hill or wind gust is encountered, the engine slows but also gets more torque to overcome the extra resistance.

If your driveline is locked (i.e in a manual you do not have your foot on the clutch pedal or in an automatic the torque converter is locked [which it should be when crusing]) then your road speed is directly proportionate to your engine speed (RPM). As a driver if you see a hill coming up there's methods of preparing for it rather than hitting the hill at your flat highway cruising throttle and having the vehicle slow down...

[Mundy]

There are a number of related but different issues here which are confusing. There is a difference between engine efficiency and the vehicle fuel economy. Engine efficiency is based on the work output (not power, not torque) for energy input (ie fuel). Vehicle fuel economy depends on not only engine efficiency but also movement losses in the drive train and, most importantly, from vehicle speed. Wind resistance is proportional to the velocity squared, so travelling at 110kph generates 50% more wind resistance than 90kph.

My recollection of Uni days is becoming dim but IIRC, whilst the volumetric efficiency of the engine remains the same for a given RPM, the engine efficiency also depends on the load or work output.

So, maximum fuel economy depends on how fast you are going, how close to the peak efficiency point of the engine and what load is on it, and there is a trade off for these. So, most fuel efficient speed on the flat may be different to that going up a hill. From my experience, my TD5 gives me best fuel economy on the flat at 90kph.

Finally, peak power must occur at or above peak torque. Power is torque x RPM. So at the point of peak torque, if you increase speed by 5%, power output can increase if the drop off in torque is less than that. So power output can continue to increase until the rate of torque drop off is greater than the rate of speed increase.