Diff gears

Submitted: Sunday, Jan 06, 2013 at 21:52
ThreadID: 99839 Views:2212 Replies:8 FollowUps:1
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Got an 80 series GXL, and wondering if it is possible to lower the diff ratio's to in turn lower the revs at 100 kph, to in turn get better economy. I am getting it turbo'd soon so power shouldn't be a great problem.
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Reply By: Member - Greg N (QLD) - Sunday, Jan 06, 2013 at 22:50

Sunday, Jan 06, 2013 at 22:50
I have an 80 series too, so I know where you're coming from; don't know how many times I've reached for a sixth gear:)

Can't help with the possibility of taller diff ratios though. You might have better luck asking on the LCOOL site.
AnswerID: 501880

Reply By: Ross M - Sunday, Jan 06, 2013 at 23:42

Sunday, Jan 06, 2013 at 23:42
Changing to a higher section tyre will get you someway there and possibly be suitable.
If you decide to change diff gears, if available, will be $2500 or more because you have to have both diffs done and the 80 series diffs are different size diffs/crown wheels too.
If turboing, it will possibly be best to do it first.
Find out at what speed your engine starts to make maximum torque. If the vehicle is used near that speed and a little faster than when max torque begins to be developed, it will probably give the best fuel economy in this zone.

The notion of gearing up for economy doesn't always work as it is a balance of fuel use rate at a certain revs/speed for a given load.

If the gearing up puts this balance, out of balance, yes it will rev less but you will be fuelling it more to give the power/torque you need to hold the desired speed.
The turbo may help attain this and may give a little better economy but judicious use of the turbo has the ability to use more fuel than normal.
Try to get some info from people who have re tyred for economy and rev lowering. They can give their account of results.

AnswerID: 501881

Reply By: Rockape - Monday, Jan 07, 2013 at 06:24

Monday, Jan 07, 2013 at 06:24
Neighbour had his done in his 80 series so the ratios are available.

As Ross said dropping revs many times doesn't work regarding fuel consumption.

Another thing to take into consideration is until that turbo comes on the boil our old girl will be a slug with taller ratios.

An example of torque verses fuel economy. My old troopie with a 6.5l chev diesel developed max torque at 1800 rpm. The sweet spot for the engine and fuel was between 2200 and 2300 revs.

Have a good one.
AnswerID: 501887

Reply By: Bazooka - Monday, Jan 07, 2013 at 10:29

Monday, Jan 07, 2013 at 10:29
I don't know specifically in regards to your vehicle Josh but as a few have already said it could well be significant money wasted.

Manufacturers set engine management systems to try and get a balance for "typical" use and fitting larger tyres etc, while raising gearing fractionally, interferes with this balance. So instead of improving fuel consumption it may in fact result in slightly worse consumption overall. At least that's what my mechanic b-i-l told me after I recently went from 245/70/16 to 265/70/16s. Interestingly, as others said on the internet about this change for my truck (I did a lot of reading beforehand) my petrol Jack seems to "like" the change (wishful thinking?) but I haven't noticed any improvement in fuel use.
AnswerID: 501897

Reply By: Member - Craig F (WA) - Monday, Jan 07, 2013 at 15:30

Monday, Jan 07, 2013 at 15:30
The best rev range for economy is normaly pre-set by the manufacturer. Tyre size etc is normaly the killer as even a small change will put thinks off. I recently changed the ratios in my hilux to raise the revs. It was on a later car than yours but sorted issues I was having with gear changes (Auto) and economy. As stated fit the turbo get some K's on it and colect some fuel data before and after then take it to a GOOD 4wd shop. They can then show you what is available. The gears range from 500$ to 1200$ per end and around 400$ per end to fit. Try "Marks adaptors" for specifications and more info..

AnswerID: 501921

Reply By: Member - Ian H (NSW) - Monday, Jan 07, 2013 at 17:36

Monday, Jan 07, 2013 at 17:36
I have an 80 series base model and I fitted a turbo 6 years ago. (should have done it sooner).
At 110 kph the engine is doing 2750 rpm with 265 75 x 16 BFG AT tyres and I get about 13 l/100 k's. If I drop to 100 kph the consumption improves to about 12l/100k's.
In the end you are still pushing a 2.5 tonne house brick through the air.
I guess what I am saying is that to improve fuel economy just slow down a bit.
Having said that the turbo makes the thing so much better to drive but you MUST keep the boost to about 8-9 psi as the engine compression ratio is still 22.5:1 where as later factory turbo diesels are 18:1. Mine has been absolutely faultless for about 150k ks.
You should also fit at least a 2.5" exhaust system to allow it to breath.
Good luck with it and post when you have it done.
AnswerID: 501930

Reply By: Member - Bruce C (NSW) - Monday, Jan 07, 2013 at 19:45

Monday, Jan 07, 2013 at 19:45
Hi Josh,

Changing the ratios as you suggest will not give you better economy in fact as other have said it will make it worse.

I had the 96 multi valve factory turbo diesel and I found running it in 5th gear when towing used more fuel. This is why they say "only tow in 4th gear, not 5th".
I have proved it time and time again, if only to myself.
You need more accellerator to maintain the same speed in 5th when towing. Normal road ratios for unladen use are pretty good I believe.
I also know what you mean about going for another gear as I also went for 6th gear a few of times. How embarrassment. (as Effie used to say) LOL

Fitting a turbo will definitely improve your economy and as has been suggested upgrade the exhaust system when you do.

My recommendation is a 3" madrel bent system if you want the best from the turbo. A turbo and 3" exhaust will give you the biggest bang for your buck.

But get the right person to fit the turbo as there is a fair bit of tuning to get it tuned right and a lot of mechanics do not know enough about it.

I used to get around 11/100K out of mine when tuned correctly.
You may not get that out of the 1HZ but you will go close I think.

As has been suggested, keep the boost at single figures or you will pay big time, most likely. That is unless you detune the motor by lowering the compression ratio down to the 18s and even then 12 pounds boost is a good conservative amount for longevity of the motor. That is the factory setting I think.

Hope this has been of some use.

Cheers, Bruce.

At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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AnswerID: 501946

Reply By: WBS - Tuesday, Jan 08, 2013 at 09:23

Tuesday, Jan 08, 2013 at 09:23
I would think twice before you do what you propose. I have an after market turbo'd 80 Series GXL and my experience tells me the following.

Fuel consumption when not towing has gone up albeit by just a little (11L/110k to 13L/100km).
Fuel consumption when towing has remained the same or improved slightly.

I tow in 4th gear and my speed is governed by two things. Firstly I drive on the engine revs and that I try to keep around the 2400rpm mark. Going faster at say 2600 revs increases fuel consumption and also oil consumption. I can drive for weeks at 2400 rpm all day for weeks and use no oil. At 2600 rpm for a sustained period and the oil level drops after one days travel.

Second. I have an EGT gauge fitted. When I tow I drive to keep the EGT as low as is reasonable. Ideally at 350 degrees C. By driving to these two criteria (revs and EGT) I generally sit on between 85 -90 kph. With head winds I can get down to 70kph and tail winds get me up around 95kph.

I have tried towing in 5th on a few occasions to see what happens. Generally I try this on long straight smooth flat highways and on all occasions, the EGT climbed noticeably above what I consider to be acceptable for cruising for a sustained period.

Higher revs do not necessarily equate to higher engine temperatures in fact it can be the opposite. 2000 -2400 rpm seems to be the happy zone for my 1HZ motor.

AnswerID: 501980

Follow Up By: WBS - Tuesday, Jan 08, 2013 at 09:23

Tuesday, Jan 08, 2013 at 09:23
That should read 11L/100 k.

FollowupID: 778313

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