davidc1

The quick question is: Theoretically, can any work on the transmission change the speedometer calibration?

The back story:

There are two ways to get the AP2 shorter gear ratios in your car. The first is use an AP2 transmission, the second is replace the secondary output shaft of an AP1 transmission with an AP2 secondary output shaft. This will make gears 1-4 the same shorter ratio as an AP2, although it also changes your 5-6th gears to a shorter ratio as well. This is what I had.

Car is an AP2, with an AP1 tranny, 2.2L engine, AEM ECU. I got tired of the higher rpms on the highway with the AP2 secondary output shaft in my AP1 tranny. I drive a lot of freeway, and 4500rpm at about 80mph was just too much.

So, we changed the output shaft back to an AP1, so my tranny was now all AP1 ratios again.

The weird thing is, immediately after changing it, I get on the freeway, and I'm pretty much at 4500rpm at an indicated 80mph just like before. No change. But, overall, the engine "feels" like it's revving lower. Checked my speedo against my GPS and against another car pacing, and it appears my speedo is reading low, not high, but low, by about 3-5mph. This would make it "seem" as if I'm still at 4500rpm at 80mph.

Now, I don't know exactly how well my speedo was calibrated before hand, but, I assume it was probably the usual 1-3mph too high like the factory usually makes it.

So, question:

Is there any reason, even theoretical, that something could have happened in the course of removing the tranny and replacing the secondary output shaft, that would cause my speedo to start reading to low suddenly.

djsilverjohnson

I just took a look at the FSM and the sensor is mounted on the main transmission case, before the secondary output shaft. If that's the case your RPM vs. "Indicated MPH" won't change when you change the secondary output shaft, but the actual MPH will change. That means the real MPH at 4500rpm would be higher with the AP1 secondary output shaft. That's consistent with your assessment that your speedo is 3-5mph low with the AP1 shaft, vs. 1-3 high with the AP2 shaft. The calculation can also be put off if your tire diameter is different from OEM.

Wouldn't putting the AP2 transmission back in with the AP1 secondary output shaft be a better arrangement? It would give you the same 1-4 ratios as the AP1 and lower the revs at highway cruising speeds in 5th and 6th gear. I've read somewhere that the AP2 transmission also has better synchros. I have an AP1, but I've driven AP2's and the lower gear ratios are too short for my liking (autocross related)

davidc1

So, just to make sure I got it straight, you're saying switching from AP1 to AP2 transmissions, or vice versa, would change the actual speed I"m traveling, but not my indicated speedometer/tachometer relationship? Extending this....are you saying if I put the AP2 tranny back in the car, my speedo would go back to "normal"?

BTW, I checked and the sensor is the same part number for AP1 and AP2.

Lastly, one of the reasons I went back to AP1 tranny is that the ratio's are better for the road race tracks I'm on in SoCal.

davidc1

Yes, it would. One big problem though....I didn't think of it!! lol. But that would be perfect!!!!

djsilverjohnson

I autocross my AP1 in SCCA "Street" class, so I can't do many modifications. I'm building a different car for autocross in modified class, so the S2K will become my weekend/fun/hooning/track-day car. When that happens, I'll be on the lookout for an AP2 transmission and I will be swapping in an AP1 secondary output shaft.

To answer your question in the previous post, the speed sensor is getting it's pulses from the main transmission gearset, so it can't tell what secondary output gear you're using, what final drive ratio you're using, or what tire size you have mounted. The ECU is assuming everything is stock. Because of that, it's saying your turning 4,500rpm at 80mph, regardless of what changes you make downstream of the main gearset.