gear box g240 with variation of diff ratio, which is better?

Post Mon Jun 08, 2015 4:22 pm

may this topic has been covered many times, some may be boring, but choose diff is not easy as it is expensive and rare also.

I have made calculation about speed and rpm, but I don't know which will give me best fuel efficiency and good engine sweet spot?

now I with m40 engine I can have about 1 liter for 12 km combination, 1 liter for 8 in town and 1 liter for 14 km in highway and enough satisfied.

the reason to go to 325i engine is because sometime when i go to very high hill I feel no power and I really afraid with my car loss power.

I go short and long journey with my wife and 2 children with full stuff.

I still have no money to change my diff so I will still use my 4.10 diff with g240 now, does it will give me worst fuel efficiency? may be I still get 1 liter for 12 in highway?

below is my calculation, and info me which is better for me? also do i need ls or not?

3.64rpm 2500 60mph or 97km/h, 1 liter for km?

3.72rpm 2500 59mph or 95km/h, 1 liter for km?

2.91rpm 2500 56mph or 90km/h, 1 liter for km?

4.10rpm 2500 53mph or 85km/h, 1 liter for km?

thank you
markus

Post Mon Jun 08, 2015 7:14 pm

You might find better answers if you changes your criteria

No-one in the UK measures kilometres per 1 litre, so you need to start thinking in miles per gallon (gallon = 4.5 litres) for people to help you here.

To get started, 1litre/12km is about 34mpg. I have NEVER had that sort of economy from my 325i, and that has a G240 gearbox with a 3.64 differential, which is the most economical combination you can get. If I drive very gently (65mph on a long road) I get about 28mpg. In the city, with lots of traffic, I get 20mpg.

So I think 1 litre for 10km is good, but expect it to be as low as 1 litre for 7km, especially if you regularly have full passengers and luggage.

Post Tue Jun 09, 2015 9:18 am

thank you Grrrmachine, Ok I will try to measure in mil.

28mpg is very good, and that is what I'm looking for in 325i engine and in the city 20mpg is also very good.

today, I compare with my m40 318i with 4.10 diff, at 2500rpm in 5 gear it get 60mph, so it is same with 325i with 3.64 diff in 5 gear also that get 60mph.

but the problem is now I still use my 4.10 diff, so can you please tell me just an rough estimate if i still use my 4.10 diff with 325i engine, how much mpg I will get?

and do you use lsd or open? does it is important for everyday car?

Post Tue Jun 09, 2015 11:44 am

I don't have an lsd, and I don't think I need one. My car is fine even in snowy, icy conditions, so you won't need one - it's just a nice extra.

I can't help you with your mpg estimates with a 4.10 diff, but it's definitely not an ideal ratio. Your 1st gear must be almost useless, and the engine noise at 60mph will be unpleasant. At 60mph you will need 2800rpm with a 4.10 diff, compared to only 2500rpm with a 3.64.

Post Tue Jun 09, 2015 12:29 pm

ok, thank you Grrrmachine, I think I should go to 3.64 but is it good for climb the hill? still have power?

so it is only in medium case/188mm? or has 168 small case/168mm? I think if it is available in small case I can only buy the final gear and then use my old case.

may I ask one question again? does your car is transform from 318i or 316i to 325i ?

if yes, do I will have problem with my gearshift selector/gearbox arm linkage from m40 318i? may be too long and should I cut and weld it? and is it safe or not to welding it as it is alumunium?

Post Tue Jun 09, 2015 12:41 pm

Unfortunately my car was a 320i, so I just upgraded the engine and differential; I did not need to change anything with my gear linkage.

For you, a 3.64, 3.73 or 3.91 diff will all be fine to use, so buy whichever one you can find and afford. The difference in fuel usage between a 3.91 and 3.64 will not be noticeable. And as long as you do not try to drive up Everest, there will be no problems with power.

All three were medium case diffs, although I think the 3.91 was available as a small case as well.

Post Tue Jun 09, 2015 1:20 pm

thank you for the info,

I think your original diff from 320i is 4.10? have you tried use 4.10 with 325i engine before you change?

here locally I have friend got 3.91 but only the final gear in 1.68mm so I can rebuilt mine.

But in my mind the different between 4.10 that I got now to 3.91 is very little different, may be no noticeable, I think I will use my 4.10 first after a month to feel it and go to 3.64? how much price in the market for 3.64?