I have an 86 ECM with 5 speed prom.
But I'm installing the 4 speed.

Besides the shift light, is this going to affect performance?
Whats the real difference, if the ECM thinks you have that 5th but you don't?

I went the other way... I had a 4 speed and swapped in a 5 speed Isuzu on my 86 V6. For me it made no difference whatsoever except that the ECM (PROM/CALPAK really) has no idea that there's another gear after fourth so it doesn't tell me to upshift into fifth . The shift light information isn't used for anything except to turn on the light on in manual cars (on autos it's a different story since it uses the signal to switch the TCC on and off). I don't believe there are any noteworthy differences that will adversely affect the performance or operation of the ECM or engine. It hasn't in mine.

The VSS gear however might need to be changed to keep the speedo correct. You can use this chart to help determine which color gear you should use to maintain speedo calibration. It's a little awkward to use but if you start by looking up your tire size on the lower table, then find the correct axle ratio and transmisison you are installing, then find the letter code for the "driven gear" you should be using (should be a "N" or an "O"), then look up either N or O in the upper table under the "driven gear" column and see what color gear that corresponds to. If you get lost, just shoot me a PM and I'll help you some more.

wow. but the speedo gear that's in in the 4 speed, is the stock speedo gear for that 4 speed
that came out of an 86 fiero. Shouldn't that be fine?

quote Originally posted by Bloozberry: The VSS gear however might need to be changed to keep the speedo correct. You can use this chart to help determine which color gear you should use to maintain speedo calibration. It's a little awkward to use but if you start by looking up your tire size on the lower table, then find the correct axle ratio and transmisison you are installing, then find the letter code for the "driven gear" you should be using (should be a "N" or an "O"), then look up either N or O in the upper table under the "driven gear" column and see what color gear that corresponds to. If you get lost, just shoot me a PM and I'll help you some more.

Question, there's something I'm missing here. I don't understand how it's possible to only change the driven gear.

How can the driven gears of different diameters all properly mesh with a driving gear inside the transmission that doesn't change?

quote Originally posted by Seanpaul: wow. but the speedo gear that's in in the 4 speed, is the stock speedo gear for that 4 speed that came out of an 86 fiero. Shouldn't that be fine?

It should be... but the tire sizes may have been different from the 86 four speed to the 86 five speed. You'll note that there's only one tooth difference between the gears so it's not like it matters much either way. I just thought you might want to check it out and swap the gears if there was a difference.

quote Orignally posted by pmbrunelle: How can the driven gears of different diameters all properly mesh with a driving gear inside the transmission that doesn't change?

I haven't checked this out myself, but I would imagine that the gears aren't the exact same diameter, but with only one more tooth out of 30, the clearances are just sloppy enough to take up some of the slack in one configuration and be looser in the other. My guess.

So basically when I'm in my 4th gear, with a 5 speed ECM it will just tell me i need to shift?
Think I'll just take that stupid bulb out. lol.