I have a 96 grand marquis ls with the digital dash and I would like to swap out the 2.73s for 3.55s how do I reprogram the speedometer?

 

Hello, KNaujokas, and welcome to the Mercury Forims --
A "simple" gear ratio change is not quite as "simple" as it may appear, for a number of reasons:
1. The stock 2.73 : 1 ratio gives 80 mph @ 2,000 rpm. -- and yields 24-25 mpgs. at a steady cruise. Observed. Documented.
Around-town mileage of 14-16 mpgs. Observed. Documented.
2. The 3.55 : 1 ratio will yield approximately 20 mpgs - Hiway; 12 mpgs - Town. A BIG difference in your WALLET, and a BIG difference in engine wear and maintenance.
3. Changing the gear ratio does NOT make the differential assembly STRONGER -- to strengthen the Differential Housing, a cast-aluminum differential cover is necessary. Cost for The 95 MERC = Diff Cover, LubeLocker Gasket, Royal Purple 75W90 Gear Oil = About 150 dollars.
4. Reasons for changing gear ratios are to add pulling power for towing, and to reduce the 1/4th-mile strip ETs.
The 1996 FOMOCO 5,000-lb. Towing Package for your Merc has a 3.23 :1 Ratio. The 3.55s are a bit too much for towing.
Now, if ya wanna go RACING, it's a whole nother story -- because it will take A LOT more than a gear swap before you bring home any trophies. Besides, Bracket Racing is more about CONSISTENCY than actual top speed.
In my experience, the gear swap hassles were overcome by installing a B&M Shift Improver Kit -- and a cast aluminum diff cover.
The cast aluminum diff cover has Jack-Bolts that allow for bearing pre-load, which keeps the rear-end from grenading itself under high load.
These mods yielded a lower strip time AND a strengthened differential -- BOTH for less than 250 dollars.
Another thing: If ya don't change the gear ratio, there's no need to monkey with the speedometer.

8.8" Differential Cover, LubeLocker Gasket, Royal Purple 75W90 Gear Oil

 

If you absolutely MUST monkey with the speedometer -- There is no "reprogramming" -- changing speedometer gear is required.
1990 -1998 Automatic Overdrive Transmissions --
The 3.55 rear-end gear change requires a 21-TOOTH Speedometer gear.

Now, THAT'S a nice rear end !!