Your stock speedometer is fairly accurate up to around 100 Mph. After that any speedo that can't be calibrated is not very accurate. It could be off by as much as 5 - 10 MPH.
This is especially true if you are running aftermarket rims / tires or are using an aftermarket Ring & Pinion set like 3.55, 3.73, 4.10's etc. Even if you get the correct speedo gear for your new ring and pinion set, your stock speedo may still be off, based on your rim or tire combo. This is true for all types of cars.
For accurate readings in "race applications" the stock speedometer (and all the other stock gauges) are virtually useless. You'll need to upgrade to VDO (or another brand) of gauges that allow you to calibrate them.
For example, you can calibrate a VDO race speedo by simply pressing a button, driving one mile, then pressing it again, the speedo will compensate for rear end gear ratios, speedo gearing, windup, etc. You must also calibrate a speedo often to compensate for many factors to insure accurate readings (in racing). The stock gauges are fine for street applications and general driving, but for road racing / drag applications and to ensure accurate measurements, we recommend you upgrade your stock gauges. Just to note, 99% off all folks out there will be fine with the stock gauges, we are talking about sponsored race applications (or folks that want accurate readings) only.
Just to note, 99% off all folks out there will be fine with the stock gauges, we are talking about sponsored race applications (or folks that want accurate readings) only.