99 chev express speedometer reads wrong, double actual speed engine is not reving higher than actual speed would have it

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

sounds like a sensor has failed that shifts transmission...would start there first....have it scanned....good luck..

There are also wheel speed sensors, so that may be a cause as well.

CHANGE THE SPEED SENSOR AND LET ME KNOW IF THE FUEL GAUGE IS KEEP WRONG.

You're going to need to increase the tooth count on the driven gear on the speed sensor. probably with 3 additional teeth in the count.

your actual speed will be less. odo will read more than actual distance. fuel econemy will decrease... generaly a bad idea :) i think the differrence is about 12km/h at 100km/hr...so driving at what appears to be 100 will actually be 88 or at least close

I need to verify something about your vehicle. Are youan the original owner and did your speedometer suddenly start reading slower, and was reading normally before? Or did you just find out recently because you got a ticket or somebody clocked your speedometer by riding along side you? If you are not the original owner, I would check the tire height rating from the manufacturer versus the rating of the tire you have on the vehicle now. If the manufacturer installed a "G" series and you have a "H" series, for example, your speedometer will read slower than your actual speed as the "H" series is a taller tire. The higher the letter in the series, the taller the tire will be. (larger in diameter)
Regarding your random cylinder misfire, I would recommend replacing the spark plug wires. Also, the be sure to use the conveluet heat shields that surround the wires that are routed around the exhaust manifold. Be sure to route them correctly or the excessive heat will cause them to break down prematurely.

change the speedometer it is faulty

replace VSS. Vehicle Speed Sensor if it not working properly, all sensor fail soon or later. RPM majority of the time will be faulty wiring from your distributor. check make sure there is no oil leakage into you distributor, it will also create a short contact to the reading. You can try to clean the distributor sensor. if plan to replace, get one from junk yard. Or you can put after market rpm gage which they also make.

Do you have custom wheels and tires of a different size installed than the originals? This will mess up your speedo reading.