If I swap diff ratios, how do I correct the speedometer

[Robert K]

I'm toying with the idea of replacing my 3.91 diff with a 3.46 ratio from a 5 speed. The thought is to lower the RPM's and hopefully increase fuel mileage. On a good day, with a tailwind, on a downhill slope, I can eek out 24 mpg in my 1991 535i. My uncle's old 735 used to get in the high 20's, but the one time I drove it, I noticed the revs were much lower.

I know there is a speed sensor in the tranny. Is that where the RPM information comes from for the speedometer? If I was to swap the diff ratios, what would I have to do to correct the speedometer. Surely it can't be that tough. Any info would be appreciated.

Grace and peace,

Robert K
1991 535i

[winfred]

i've never had a problem with diff swaps, the tone ring reads the rotation of the axle not the driveshaft, the problem comes when you change the wheel/tire size