differential swap question for 1991 535i

[Robert K]

In these days of high priced fuel (especially the premium I need for my EAT chip), I think I'm willing to sacrifice a little zip in my 1991 535i in return for better fuel mileage. My car was built in 10/90, so I'm not sure if it has a 3.91 ratio (1990) or a 4.27 ratio (1991). At any rate, the manual trans equipped cars had either a 3.46 ratio in the 535i and 3.23 ratio in the 525i. I'm figuring changing to one of these diffs hopefully should move me from my 23 mpg interstate mileage up to 25-27 mpg. The only real question is whether it's a straight swap or not. Anyone out there know if I'll encounter any problems if I do this? I've been told I wouldn't have to do anything in regards to the speedometer because the sensor is in the diff. Is that correct too? Any help would be appreciated.

Grace and peace,

Robert K
1991 535i

[pundit]

In these days of high priced fuel (especially the premium I need for my EAT chip), I think I'm willing to sacrifice a little zip in my 1991 535i in return for better fuel mileage. My car was built in 10/90, so I'm not sure if it has a 3.91 ratio (1990) or a 4.27 ratio (1991). At any rate, the manual trans equipped cars had either a 3.46 ratio in the 535i and 3.23 ratio in the 525i. I'm figuring changing to one of these diffs hopefully should move me from my 23 mpg interstate mileage up to 25-27 mpg. The only real question is whether it's a straight swap or not. Anyone out there know if I'll encounter any problems if I do this? I've been told I wouldn't have to do anything in regards to the speedometer because the sensor is in the diff. Is that correct too? Any help would be appreciated.

Grace and peace,

Robert K
1991 535i

Changing your diff may not improve your mileage as much as you think. Depending on your driving style, the road conditions (gradients etc.) A taller rear end means less revs for the same speed, but more engine load when accelerating or driving uphill. This can mean either later upshifts or more accelerator to compensate which will, to a degree, negate some of the benefit. However if you are mainly doing freeway cruising under fairly low engine loads then you will gain some improvement. Just bear in mind that a 10% reduction in revs at the same road speed doesn't neccesarily equate to a 10% improvement in fuel economy.

[John B.]

My 5 speed with the 3.46LSD got 25-27mpg on the highway before installing the EAT chip. I haven't gone on any trips since so no idea if its improved or still the same. You could always pull the EAT chip & run cheaper gas to save money.

[winfred]

my eat chip bumped mpg up 1-2, i haven't really noticed a difference on intown mileage after changing from 3.91 to 4.27 (no road trip yet), my interstate mileage changes oddly and haven't been able to figure out why, ill make one 2500 mile round trip and avarage 25 with a best one tank run of 27.5 (all time best)and the next trip over the same roads with about the same amount of crap in the car and avarage 22 with a best of 23.5, all on a recent tune up and fresh 02 sensor. mabey they are ****ing with the formula of the gas or something

[gale]

"The only real question is whether it's a straight swap or not."

525i/535i will swap. The only issue is whether the axle flanges are the same size -- not to worry, they pry out easily with a screwdriver & swap directly from one diff to the other. You'll want to pry the flanges out anyway to replace the seals while you have the opportunity. There are 2 different input flange sizes also, usually all automatics have the small hub & all manuals have the large hub, but again they can be swapped also, just have to be careful to get the preload set right.

The speed sensor is universal, no need to change it but while the diff is out, remove the sensor & wipe off any ferritic fuzz that might have accumulated.

I have a 3.91 in mine now and I'm prepping a 3.64 for it for the same reasons as you.