I am new to Stangnet, this is my first post. I just got 4.10 gears in my 1990 LX 5.0 and the speedometer is off by about 25 mph. What do I need to get to fix my speedo?
Before someone jumps all over you about this... You should do a search on a topic before posting. This one has been covered a thousand times, at least.
You do need a new speedo gear (technically called the "driven gear"). The gear you require depends on whether you have a 5-speed, or AOD.
If you have an AOD (1990 or later), the 23-tooth driven gear is as close as you can get. Your speedo will still be about 10 or 15% off. (Note: Your speedo is not currently 25mph off, it's off by a percentage. If it's off by 25mph at 75mph, you have a 33% error.) You'll need a speedometer drive reduction box (ask at a tranny shop) that goes on the end of the speedo cable, to get the speedo right on.
If you have a 5-speed (1990 or later), a 23-tooth driven gear will get you close, but you'll also need to change the speedometer DRIVE gear from an 8-tooth to a 7-tooth. This gear is on the tranny output shaft, and requires some work to replace, although the gear itself is relatively inexpensive (the AODs drive gear is machined onto the shaft, and requires major dissasembly to change the entire shaft). Even if you do this, your speedo will be 4 or 5% off. The 5-speed/4.10 combo is pretty hard to correct without getting a reduction box.
If you changed to tire sizes that are different from stock, that'll also throw off you speedometer readings, and may require different speedo gears to correct.
With 4.10's, and change in the drive gear is necessary - which means getting into the tranny - not complex, but a fairly big job. I believe a 6 tooth drive gear is also available, which would allow correction without a reduction box - if you have the T-5. But it's probably easier to simply get the proper reduction box with either tranny. Do a google search on speedo shops and you'll find info on who makes the reduction boxes. Most of the better boxes allow you to change the gears in the box to allow for future ratio/tire size changes.