I've recently become aware that my speedo reads high, after checking against dyno rollers and GPS speeds. I'm pretty sure I know why: the chip in my cluster says 3.89 on it, and the rear gears in the car are 4.10 (positive on this-I counted teeth.) I assumed the car had its original cluster when I bought it, and therefore had the original chip. But now it seems that either the cluster or chip was replaced with one from a 3.89-geared car. I 've since replaced the cluster, too, which works fine.
I'm looking for a 4.10 chip for the late (5 green light SI) cluster. (Any difference?)
But there's one lingering question. Why the hell would the chip affect the speedo reading at all? The speed input comes from the ring gear sensor, which doesn't care if the gear ratio is 2.0 or 6.0:1, as long as the wheels turn the same RPM. If driveshaft RPM was used to compute speed, I could understand the disparity, but that would also require tachometer and current gear information, and even then only work for a 5-speed car. WTF?
I'm looking for a 4.10 chip for the late (5 green light SI) cluster. (Any difference?)
But there's one lingering question. Why the hell would the chip affect the speedo reading at all? The speed input comes from the ring gear sensor, which doesn't care if the gear ratio is 2.0 or 6.0:1, as long as the wheels turn the same RPM. If driveshaft RPM was used to compute speed, I could understand the disparity, but that would also require tachometer and current gear information, and even then only work for a 5-speed car. WTF?
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