I just had my 4.10s installed and when I drive around the speedometer is WAY off like I was going around 35 and it read almost 50. When I was going around 20 it read around 40... What do I need to get to fix this problem?
A 23 tooth white speedometer gear should be all you need. I don't know the tooth count for the 90-98' T-5's though as the internal drive gear is a higher tooth count (8) than the 83-89' 7 tooth gear.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
For 1990 T5s though the 23 tooth gear is as big as you can go and is the correction for 3.55's, so not only would 3.73's be off but 4.10's will really be off. Your only true option without that adapter as mentioned earlier would be o swap out the 8 tooth drive gear for the 7 toothed one - THEN the 23 tooth would be correct. With just the 23 tooth driven gear setup, your speedometer should be about 13%-15% off still.
There is a 6-tooth drive gear that can be used - and then you should be able to get fairly accurate with a 19 or 21 tooth driven gear. Tranny probably has to come out to replace the drive gear - at the very least, the tail shaft housing has to come off the back of the tranny. The cost of that (unless you can do it yourself) will likely be more in labor than the cost of using the speedo box that jrichker described above. The 23 tooth alone still won't be accurate and it likely won't last very long. I'd follow jrichker's advice if I were in your shoes.
You seemed surprised by the result - didn't the gear installer tell you that the change was gonna throw the speedo off?
Yes, it will still be quite a bit off and the 23t gear usually will just shread or not last very long due to the extreme difference with the 8t drive gear.
Ive got 373 and AOD with 23 gear and Im right about 3 or 4 mph off So with the 410 youll be about 7or 8 mph off the only way is the get the adapter box that some one has already suggested
Got it through Summit Racing. I made my own custom dash. The speedo works off of any Ford Speed Sensor located on the tranny between the speedo cable and tranny... Just like the ones in any FI Mustang. Basically all you do is measure a 2 mile distance... You start the testing before you move, and stop it right at 2 miles and it will record it, and somehow determine your speed. Your speedometer is only as accurate as your measured two mile distance. I just had my fiance drive her new car, and stop right when it was two miles. It's seriously dead on. I love it. And no more gay speedo needle bounce, either. Here's a pic..