help understanding diff gears and fuel economy

[shmick82]

Hi all,
I cant seem to get my head around how 4.56 would be more economical than 3.73. I understand less acceleration, but once up to speed its just cruising. Im contemplating gears and 35s vs just 33s.
Please explain.
Ps. Most of my km on freeway and realistically 35s just cos it looks cool.

[Rock God]

Don't get 35's for the looks get them for offroad use. They will drain your fuel tank very nicely expect about 16l/100+. Changing ratio to 4:56 will bring the jeep in line with factory setup a bit better, with improved stop start performance but you sacrifice the top end cruise range, so at 110km/hr might be another 100rpm higher then stock. The change in ratio on a average might drop your consumption by a litre if that but performance will be better don't confuse performance with economy.

[MDK]

Its not just about lower revs = less fuel usage.

If your gearing is too far apart or too tall you'll have to use a lower gear with higher RPM to have the torque to accelerate or maintain speed. So in gearing your diffs to a higher ratio your putting the best torque band back to the lower revs and maintaining the overlap between the gears.

Think of it like the difference in economy between a 3 speed and an 8 speed (thinking in extremes) gearbox everything else being equal. The 8 speed is able to better maintain the efficient low RPM high torque point through the entire speed range better. Where as the 3 speed you have to rev one gear past this point before you can change to the next gear or it may not have the torque at the lower RPM it shifts into to accelerate or maintain the speed. Its not just about pushing the same vehicle but the increase in mass to rotate of larger tyres.

If you don't regear you may find 5th and 6th now unusable as the engine doesn't produce the torque required in the low RPM to maintain speed to benefit with fuel savings. Whether economy is better one way or the other on a JK I have no idea, just trying to add my understanding to the conversation.

I'm no expert, just my .02.

[MightyMouse]

Engine efficiency ( and fuel consumption ) is governed by design choices like intake manifold volume, runner length, cylinder head design, camshaft etc etc.... and there's a lot of etc's.....

Over all the possible engine load / RPM conditions some will be more efficient than others due to the characteristics of the individual components "working together" and therefore the engine will be using the least fuel for a given RPM / Load - i.e. best economy

Move away from this "optimum" point ( e.g. a specific RPM for a cruising speed) and some elements of the design will no longer be "in sync" and more fuel will be used to produce the same power.

Its complex stuff... but lets just say that every engine is at its most efficient at a particular RPM for each load situation. Gear you vehicle so it doesn't operate in this optimum range when your driving and economy will suffer.

[shmick82]

Thanks for detailed replies.

Originally Posted by Rock God

They will drain your fuel tank very nicely expect about 16l/100+. Changing ratio to 4:56 will bring the jeep in line with factory setup a bit better

with petrol u mean?? Ive never seen the crd at 16l. Lies, 4 days in vic desert did it. Just.

So 35s without gears = more load n more fuel & 35s with gears less load less fuel. So the gears will bring torque band forward ie earlier in revs or just increase revs to be in torque curve?
Would 35s and 4.56 bring similar to stock fuel economy in crd? I think seacomms had a chart. From memory id be sitting 300rpm higher with 35s n gears over current at a gven speed. Sounds reasonable.