changing diff ratios change economy

[hottiemonster]

if i change diff ratios to suit my tyre size (33s), would my economy improve.

eg

mine has 3.9s with 31s standard, now i am running 33s with 3.9s, say if i goto 4.3s (i think thats what i need) would my economy improve, as in more km out of a tank?

as when i went from 31s to 33s my economy decreased a fair bit. though i am running all terrains 33x12.5

[TUFFGU&GQ]

G'Day Peter, Is you SWB an Automatic?

3.9's in a Maverick /Patrol were only in 4.2 petrols with auto trans. I'm still come across a manual 4.2 with anything other than 4.1's stock.

If you are certain that the ratios haven't been changed the compliance plate on the inner guard (drivers side next to the battery) will show axle/trans the last digits indicates the diff ratio ie 39, 41, 43, etc.

--Andrew.

[bigbluemav]

To retain your STANDARD gearing, go to 4.1's.

4.3's will lower your STANDARD gearing. If you started with 3.9's (have you checked the plate behind the right hand headlight yet) and wish to retain standard gearing, you could run 35's with 4.3.

Most manuals (ie. mine) came standard with 4.1's, so when I put 35's on, to retain the standard gearing I put in 4.6's

In relation to the fuel economy, theoretically, it should stay the same. Practically, at least in my experience, it gets worse. What you have to remember to do when calculating fuel economy with your big tyres with the std. gears, is that you cover MORE km's than indicated by your odo due to the bigger tyres. You need to add the % difference between the old tyre height and the new tyre height, in relation to km's travelled to be able to formulate an accurate fuel economy (fuel consumed and km's travelled).

I found mine went from 11.9 litres per 100km to 14.5 litres per 100km. :( :( :(

Hope this helps :) :) :) :) :) :)

[hottiemonster]

yeah i realise the difference in distance travelled, but it still isnt as good.

mine is manual with 3.9s :cry:

at the moment i run 33s for daily driving and 34" JT2 for offroad but will goto 35s sometime soonish hopefully.

[MavTestPilot]

My truck is a '91' Mav SWB , manual with 3.9s as well. ID plate says HG39 and since lockers fitted I know for sure they are 3.9s

Last truck was a LWB with 4.1s that started with 31's then went to 33's then 35's. Economy got worse each tyre size increase. After putting in 4.6s and taking into account speedo differences etc was back to similar economy to 31s when running 35's. Economy difference from 3.9s to 4.1s would be bugger all for the amount of work involved. 4.3s would be just about impossible to pick up. If you want to run 35's ( legal issues aside )on road, the 4.6s are worth every cent for the power and economy compared to 4.1s, let alone 3.9s

[hottiemonster]

thanks mavtestpilot and others, i got the information i wanted to know

[Bartso]

i have a swb petrol auto if it is correct and i have 3.9's should i change to 4.6's cause i run 35 inch tyres

[BIG GQ]

i have a swb petrol auto if it is correct and i have 3.9's should i change to 4.6's cause i run 35 inch tyres

Yes John that is what you have (a petrol, swb, auto). Mate you really should learn more about your rig. :lol:

Cheers
Linc

[Bartso]

thanks mate :oops: i ment the 3.9 ratio's being correct in my 4wd :wink:

[procompman]

hello all i have a carb petrol 91 gq mwb running 32/11.5/15 bfg mud and on highway i only get 5.4 km/l and im sick of it so if i change my diff ratio it should increase my fuel ecc.
I have been thinking about puttting a diesel motor in it to c if its any better as i really love the little truck

[hottiemonster]

well the 4.6s seem to make the economy better with the pedes, but i just need to get some 35s for road duties as the 33 look puss and just dont cut it :twisted:

[MAV91]

My truck is a '91' Mav SWB , manual with 3.9s as well. ID plate says HG39 and since lockers fitted I know for sure they are 3.9s

My '91' LWB Manual has the same minus the lockers :cry:

[MavTestPilot]

My '91' LWB Manual has the same minus the lockers

Have a mate with the same setup in his LWB. He is in the process of fitting crawler gears, so I'm gonna check how that changes it offroad. I run 35's on road with the 3.9s. I can put up with it to save the bucks, but when I put the camper trailer on, then its working the gears big time. SWB being a bit lighter makes it a bit easier on the 3.9s. Gotta love the lockers though :D

[hottiemonster]

My '91' LWB Manual has the same minus the lockers

Have a mate with the same setup in his LWB. He is in the process of fitting crawler gears, so I'm gonna check how that changes it offroad. I run 35's on road with the 3.9s. I can put up with it to save the bucks, but when I put the camper trailer on, then its working the gears big time. SWB being a bit lighter makes it a bit easier on the 3.9s. Gotta love the lockers though :D

i was running 34 JT2s on my 3.9s for a while with a rear locker and i put up with it, but changing the ratios save the clutch heaps, so when i went 35x11.5 pedes i stuck genuine 4.6s under them (expensive but good)

now twin locked works a treat, although i havent tested them much yet. tonight is the go!

[stripedtomato]

Although changing diff ratios will bring overall gearing back to standard you will most likely still find that economy is worse than orginally as there is more rolling resistance and additional weight in the over size tyres. If tyres are also more aggressive that too will affect economy.

Steve

[MavTestPilot]

i was running 34 JT2s on my 3.9s for a while with a rear locker and i put up with it, but changing the ratios save the clutch heaps, so when i went 35x11.5 pedes i stuck genuine 4.6s under them (expensive but good)


Yeah the clutch really struggles in the slow stuff. You have to ride it a bit, to keep the speed down. Thats why I'm considering crawler gears.

now twin locked works a treat, although i havent tested them much yet. tonight is the go!

And you thought you lifted wheels before :lol: , the twin lockers can really get you in trouble now.

[hottiemonster]

i really tested the locker only twice on friday night :cry:

once with getting massive air under a tyre like the one in your pic, but i was at full flex front and rear but i dont know exactly how much height as i was driving and it was about 10 pm at this stage. The crowd was very impressed and said it was heaps, one said approx 4 foot.

[hottiemonster]

so for my mav, would 4.6s get better fuel economy with 33 all terrains, or 35 all terrains, as a rough guide?

also running 33s with 4.6s does anyone know how much the speedo is out etc. and what speed i am actually doing if i am sitting on 100km/h for example?

[JK]

Running 4.1s, 35s with 6" lift and turboed 4.2

Just completed a 1,200km round trip over Chrissy with an average speed (from GPS) of 85km/h.

Accurate fuel figures using measured distance from GPS and fuel used (full tank to full tank) and returned 12.3L / 100km.

Driving involved some offroad work with about 2hours in low range, back country roads at 50 - 70km/h, lots of climbing up the mountain passes of the south coast and a good run up the Pacific Hwy from The Gong to Mid-Nth Coast and back sitting on the legal speed limit.

Must say I'm bloody impressed.

I need to drop back to 4th on steeper hils to maintain legal speed but could hold 5th up all but the steepest highway hills.

I won't be changing to 4.375s any time soon, would rather go transfer gears.

[BowTieGQ]

So why is it that my LWB TB42 carb, duel fuel, 4.1's and 32/11.5 AT's only gets 20lt/100k. I accept the right foot syndrome, but why am I so for from everyone else? Hence a future transplant!

[Bartso]

so for my mav, would 4.6s get better fuel economy with 33 all terrains, or 35 all terrains, as a rough guide?

also running 33s with 4.6s does anyone know how much the speedo is out etc. and what speed i am actually doing if i am sitting on 100km/h for example?

If your vehicle was originally fitted with 30" tyres and is now fitted with 33" tyres your speedometer will read 10% slower that the actual road speed and if your vehicle is now fitted with 35" tyres instead of the original 30" tyres then you speedometer will be reading approximately 16.5% slower.

this has been quoted from marks 4wd web page so your car would actually be doing 110km's

[hottiemonster]

thanks John

will check out the site and write down all the speed differences :D

[fatrolls]

That would be a carby. Thirsty little piggies they be.

[JemmyBubbles]

wtf dig