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Top speed vs transfer box ratio?


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Exactly that. My Tdi is very happy at 60mph on the motorway and will return 30-32mpg all day long at that speed. When the roads are busy with lorries however (so almost always!) it is far less stressful to sit at 70mph as you pass them so much quicker and don’t have to deal with so many idiots in white vans up your bumper while doing so. 

The overdrive allows cruising at this speed with minimal noise increase compared to 60mph without; actually a decrease in engine noise but a slight increase in wind/tyre noise so probably all in about even. The price you pay is fuel economy, it’s a struggle to achieve 30mpg if cruising over 70mph and I seem to manage about 28 on most runs. The Discovery transfer box would achieve the same effect, albeit on a permanent basis. 

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Discovery transfer box is a good cheap option, however it does of course raise the overall gearing of the vehicle in every ratio, l had one in my previous 90 TD5 and found that first/second gears were a bit high.

But at 1/4 the price of an overdrive it was the only real option. I believe Ashcroft do a 1.3 ratio transfer box internals which may be a good compromise as it’s between the standard Defender 1.4 and Discovery 1.2
 

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On 5/3/2020 at 11:08 AM, Lightning said:

Discovery transfer box is a good cheap option, however it does of course raise the overall gearing of the vehicle in every ratio, l had one in my previous 90 TD5 and found that first/second gears were a bit high.

But at 1/4 the price of an overdrive it was the only real option. I believe Ashcroft do a 1.3 ratio transfer box internals which may be a good compromise as it’s between the standard Defender 1.4 and Discovery 1.2
 

The 1.2 works quite nicely on my remapped Td5, though without that extra umph I think it would be just as you describe.

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I think the biggest problem is your tyres, like others said,  but Tdis are noisy at high speed and the drag off the vehicle (increases logarithmically, remember) means that acceleration above 60 mph is a sedate affair.  
 

A 90 is typically light enough to use a 1.22 transfer case, but if it’s frequently laden or towing, forget it.  A Tdi driven empty 90 with no roof rack should be ok with it, I think, but it’d benefit from a modest tweak of the fuelling.

As John says, a 1.22 transfer case is a lot cheaper than an overdrive, and the overdrives seem too steep in their gearing increase for Tdis, meaning they have to spend a lot of time disengaged and labour the engine when engaged, so don’t save significant fuel.   A standard number of gear sets and shafts should also be more efficient and easier to maintain, and certainly the GKN overdrives are not synonymous with reliability or longevity.

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Since a short time I have a 1.1 transfer box in my Oneten from 1983 with a 200Tdi engine.

If the engine is tuned it is ok in my eyes.

Why?

It now feels like a car with a 4-gear box but additionally from 80 km/h you can engage the 5. gear and it is significantly quieter. 4-gear box, because Switzerland is limited to 80 km/h, except motorways. This is with 255 MT's. This works even in the Swiss mountains. Everything is one gear lower than previously

I enjoy the low noise level a lot and wouldn't do it different. 1.22 will have almost the same effect. A slightly tuned 200Tdi will do it.

I will see how the spline of the LT77 will do with it.

 

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