Changing Diff Ratios in 2002 Turbo Troopy

I would try a larger set of tyres (preferably borrowed from a friend) to test the fuel economy and running acceleration etc, BEFORE changing to an RPM you may like/want to achieve, but won"t be any advantage and may be a disadvantage overall.
Carefully noting tyre size percentage increase and performance and fuel characteristics BEFORE any mechanical work ,will therefore allow you to determine the extent/limit you can reasonably achieve.

I think any more than a 200 to 300rpm reduction will cause more fuel use by loading the engine too much.

Im assumming you have to HDJ with the 1hdfte motor so you have the big strong gearbox h151 and 4.1 diffs .I haven't been able to find a taller 5th gear. However i have the same set up in my 105 series and i understand how the motor could easliy cope with less revs due to its engine power. You could change to 3.9 diff however not recommend if you frequent Hilly terrain as your crawling ability will be less.
-I calculated various diff ratios when i did my conversion including the 79 series h151 box i opted for the 4.1 diffs as the 105 came with 4.3 . I kept the file and pasted it below. The -R151-F Box is the tiny weak box found in the live axle 1hz 100/105 series and some hiluxs
From factory my hzj105 does 2274 rpm in 5th all below is with 33 inch tyres 285x75x16 actually 32.9 standard Hdj79 also 33" = 2280 RPM
2391 rpm Hbox with 4.3 diffs
2280 rpm Hbox with 4.1 diffs
2274 rpm Rbox with 4.3 diffs standard set up
2063 rmp Hbox with 3.9 diffs
2237 rbox 4.1 diffs
FORMULAR IS.. DIFF RATIO (4.1) X 62mph x .336 divided by tyre height in inches x final drive 5th gear Ratio-Hbox .881 Rbox .838 ( 62 IS 100KM/H )

from http://forums.overlander.com.au/viewtopic.php?p=601453

80 series:
H150F - FJ80, HZJ80 (4.0P, 4.2D)
H151F - HDJ80, FZJ80 (4.2TD, 4.5P)

100/105 series:
H151F - GXL V8, GXL 4.2TD,
R151F - GXL and Base model 4.2D

78/79 series
R151F: HJ79R, HJ78R
H151F: FZJ79R, HDJ78R (Oct 2002 on)



Current Hilux
TD's - R151F
V6's - R150F

H150F: 1st 4.529:1, 2nd 2.464:1, 3rd 1.490:1, 4th 1:1, 5th .881:1, reverse 4.313:1
H151F: 1st 4.081:1, 2nd 2.294:1, 3rd 1.490:1, 4th 1:1, 5th .881:1, reverse 4.313:1
R151F: 1st 4.313:1, 2nd 2.330:1, 3rd 1.436:1, 4th 1:1, 5th .838:1, reverse 4.220:1

I don't know if they are still available or not, but a bloke I knew had a Fairey Overdrive in his 45 series ute. When used on the highway, it was like having another gear.

Maybe this is what you are looking for.

Just because you get the revs down may not change the fuel consumption.
I know it’s a different motor but doing tests on my 2008 Troopy towing a caravan.
I did 300ks in 4th compared to 300ks in 5th - I couldn’t measure any difference in fuel consumption.

At the moment you get the worst fuel consumption in the best conditions!
Your worst consumption comes from a billiard-table smooth, level flat, almost camber-less road.
In all other conditions you have ple-enty of tractability available and economy to boot.
Perhaps increase tyre size and definitely decrease width

Try Marks Adaptors, I believe they do an electronic overdrive unit for the troopies/utes

This or similar questions pop up on pretty much all 4wd forums on pretty much a monthly basis ........ and the consensus is....... its simply not worth it.

One thing that must be understood is that with diesels a reduction in RPM does not equate to a reduction in fuel consumption.

Mostly toyota diesels are greared to put the engine at maximum efficiency at arround 100KMH.

Mostly maximum efficiency is where power is still continuing to rise and just before torque begins to fall.

In big high and square 4wds, at highway speeds the increase in fuel consumption over about 90kmh is due to rapid increase in wind resistance not increase in engine RPM.

Many people experience an increase in fuel consumption with a modest increase in tyre size ...... it can be argued if this is due to increased wind resistance, due to the higher body height or the increase in gearing or both.

Small, light, aerodynamic petrol cars are another story all together.

cheers

Thanks everyone for your ideas but I'm not too worried about the fuel consumption it was more for the noise ect if I sit on 90-95klms the car does it easily and comfortably and not so noisy that is what I'd like to achieve at 110 or thereabouts . Might be just a dream though so it seems cheers Drum