This is a great question, and unfortunately I don't know the answer. I too would love to swap in a set of 3.64s, now that I know they exist! I've always felt that all of the gearing on my manual LCI E83 is too low, especially first and sixth gear.

There are definitely options in the DTC module for speed correction. IIRC, there are lots and lots of options. I've always glossed over those when playing around with the coding in NCS since I never had a reason to change the values. If I get a chance later this week, I'll take a closer look.

 

In my opinion it’s worth the sacrifice in torque to get better top speed or lower rpm in general on highway speeds

I will also take a look in to ncs to see if I find anything, if not, I’m planing on just yoloing and installing them and see how it goes

however if someone has any info I would greatly appreciate it

 

Due to the smaller engine, the gear ratio is probably optimal IMHO. And if you have significant road grades (slopes; there are no hills or mountains in Greece, corect? ), lowering the ratio will make it struggle and put more strain on your transmission, etc.

Slow down, reduce weight in and drag on the vehicle, and optimize its performance {tires, tune-up, etc.}. Or buy another, smaller vehicle that gets better fuel economy.

 

Due to the smaller engine, the gear ratio is probably optimal IMHO. And if you have significant road grades (slopes; there are no hills or mountains in Greece, corect? ), lowering the ratio will make it struggle and put more strain on your transmission, etc.

Slow down, reduce weight in and drag on the vehicle, and optimize its performance {tires, tune-up, etc.}. Or buy another, smaller vehicle that gets better fuel economy.

Sure I get your point

I guess we do have mountains but I normally stay on the highway.

I prefer to stay away from mount Olympus it’s really quite steep 😂🤣

the reason I’m considering the change it’s because my diffs are due for bearings they have become quite noise, they have 210k miles

We don’t have dependable shops that can rebuild differentials so the only option is to buy used parts.

so I thought since found two diffs for less than 300€ both I said why not.

I don’t thing I would slow down the car since 3.91 to 3,64 is not a big difference in my opinion, however I think that it will make a significant difference in fuel economy as I will be lowering the rpm from 3.4k to around 3.1k at around 80 mph and this makes quite the difference in the m54

Also the engine has been rebuilt recently and performing better than new

 

This ^^^
I wouldn't change it.

Is it a manual five speed or something?

 

It a 6 speed manual but the ratios are not really great, first gear is almost useless It’s 1000% torque

 

A six-speed manual with a short differential shouldn't be touched, in my opinion.
If you're all about highway rides - put the tallest tires you can fit under the wheel well (all four of them). This will give you the difference you need.
Your speedo might start to cheat as it will show lower speeds than what the real speed is, so you'll have to use any phone app.

 

Well I may be the odd one out, but I say go for it. People swap diffs all the time in trucks and Jeeps, so why not the E83? The difference between 3.91 and 3.64 isn't that big - it's only a 7.5% ratio decrease. Please report back how it goes as I am quite curious!

 

I would definitely not go to a higher (numerically lower) gear ratio. Engine RPM is only part of the story for fuel economy; the engine needs to be in its power band for most efficient operation. At 75 mph, mine averages 24-26 mpg…with 4.44 gears. The little M54B25 makes only 181 ft-lbs @ 3500 RPM, and 189 hp at a lofty 6000 RPM. That engine needs to rev and sing to make any power and motivate the vehicle. Gear ratios are matched to the rest of the drivetrain for the intended purpose, in this case every day ease of use and extended highway efficiency. The 2.5 E83 was only EPA rated at 15/22/18 city/highway/combined. These cars, especially the early ones, weren’t incredibly efficient: Too much mass, too much wind resistance, too much driveline drag and not enough power.

I’d be very surprised if fuel economy did anything but plummet with the change in diff ratio. Raising the diff ratio from 3.91-3.64 is going to lug the engine and make it even more of a dog getting up to highway speeds, as well as require more frequent down shifting while on the highway to maintain constant speed and especially when passing. Lugging the engine will decrease its lifespan.

If your engine’s fuel trims trend positive, the vehicle will use more fuel. Bring the fuel trims down, fuel is conserved. Changed all driveline fluids with new, full synthetics, and fuel is conserved. Idle less, warm up the car less, and fuel is conserved. Ensure that X drive isn’t fighting a wheel speed issue, fuel is conserved. Ensure the vehicle’s wheels are fully, and properly, aligned… and fuel is conserved.

 

The 2.5 sure does want to spin up. It's really lame at low RPM, almost as bad as the 3.0 V6 in my old Accord 6 speed, which hits peak torque at a higher RPM than contemporary Honda 4-cylinders. Lovely engine but not for lugging, and the M54B25 in a 4k pound brick is not so different.

The long term effects of low RPM lugging a small engine in a big vehicle shouldn't be ignored. That said, I'm all for experiments, and I'd like to see some objective efficiency tests before/after. If your differential bearings are going bad anyway, seems there's nothing to be lost but your own time. And even if 6th gear efficiency does plummet as duke suggests which is very possible, you might just end up spending more time in 5th gear.

 

Seems like plug and play. Speed sensors are in the abs ring by the wheels. I don’t think there are any sensors in the rear diff.