3.9 Diff reduction gears, thoughts please.

[Hendrik]

Hi All
I have been giving changing my diff ratios a lot of thought lately. Most people seam to be going for 4.11, but what about 3.9? The main reason why I say 3.9 is I still want to run two sets of wheels to keep the car legal for most of the time. I have heard that stock tyre size with 4.11's isn't a good idea as the engine revs too high at highway speeds. Now I am currently running 265/75/r16 tyres and will never go any bigger than a 285/75/r16. I just want the torgue back offroad and on road for that matter. Another option is a chip, but I don't know if it will make a difference.
Would love to hear people's thoughts on this matter.

[fatman]

Thats the roote i took (the 3.9 one) BUT i didn't install them yet:( - i have to take some diff(erential) decisions first.
The reason for no 4.11's for me,is that on my previous disco with 4.11's and 32's,gearing was a tad to low for my taste(albeit great offroad),engine reved higher(and noiser) plus i couldn't go back to smaller wheels.
From my calculations with 3.9 and stock tyre size,gearing becomes almost like standard Defender Td5 gearing - with stock tyres also(not too bad),so u can use a 2nd set standard tyres and have decent cruising speed-revs.
And if u want to run some 33's-33.5's,performance will be like now with stock gearing and 32's - acceptable but not optimum.
IMHO thats the best combo.
Cheers.

[Baffle]

What about changing transfer gears for low ratio. Price would probably end up similar at the end ?

[disco_thrasher]

What about changing transfer gears for low ratio. Price would probably end up similar at the end ?

good idea but always remember what alot of people seems to forget about when you doing modifications to drive lines eg.if you get reduction gears you a several upping your torque which is putting massive stress on diff internals,which you might get away with it a few time untill a week spot will break ,as with me having all my upgrades in the drive train i have left my drive shafts the weak points as they readily available and cheaper than entire diffs /axels /cv's/transfer case /gearboxs

so by upping transfer case ratio 's you upping strain on diffs ,so the best line to take will maybe a GBR option of a 4;10 or speak with Bill to see if he has lower
cheers kelvin

[Hendrik]

Have also considered the Defender transfer case, but I will make a few calculations and see. I was thinking going for the KAM 3.9's which LRA sells, and fitting diff locks at the same time. There is no point reusing the old centers, as you have to set up the crown wheel and pinion anyway when installing a diff lock and fitting the 3.9's. So might as well do it all in one go.
Not too worried about axles and CV's yet, as I have heaps of spares. I can pull apart the hubs to get to the cv's pretty much in my sleep now after doing the front knuckle balljoints and replacing the CV boots. So if I was adventurous, I could change them in the field provided the axle broke in the right place. But realistically, it would be far more beneficial to upgrade the axles and CV's as well.

[disco_thrasher]

provided the axle broke in the right place.

:Rolling: