smoky 94 wagon

i have this problem on my 94 taurus wagon. it is a 3.0l v6 and it is smoky. i bought it off a guy and have been checking my economy, and it is only 14 mpg! i think my speed sensor is dying because when i hit around 45 it will start to shake back and forth in the +- 1 mph range until i hit about 50 and it stops. it is also off by 5mph at about 70. so when i am going 70 according to my speedo i am actually going about 75. Would this be a major problem in not reading it right? also my car always puts out white smoke no matter what conditions, and when it is cold out i can blow out so much smoke you would think that the car is on fire. what would cause this? and if i brought this to a mechanic about how much would it cost me to get him to diagnose it? any ideas would be much appriciated!

Update:

I know now that it is a cracked head gasket, how much would i expect to pay on a fix like this. I just found this out and have been driving it under these conditions for about 200 miles mostly in 2-3 mile increments then a long cool down (back and forth between work school and home), how much damage could i have done to the car in this time. it does have a slight knock to the engine so i dont know if i destroyed one of the pistons, i am getting a mechanics oppinion on it today, so any thoughts would be most appriciated before i go in and make an ass out of my self. also is their any way of pinpointing the crack without taking apart the engine? Most appriciated!!!!

The engine knock could be caused by bad bearings. Have you checked the oil and coolant levels? As to where the head gasket gave way, removing the heads is the only way to tell exactly where the failure occurred. The heads have to be removed to fix it.
Costs, that depends on what's wrong and what is involved in repairing the problem. Labor rates and warranties vary from shop to shop.

I'm surprised, the 3.0 is the reliable motor, wonder if the previous owner had overheated the motor?? If you get coolant into the crankcase, chances are the bearings are going to be ruined, you're going to be talking some major $$$$.. I'm guessing just the headgasket job would be maybe $800+ dollars depending on the severity of damage, heads warped, etc.. throw in the bottom end I couldn't tell you how much.. no idea

yup, that is what i thought that the 3.0 is the reliable motor, but the previous owner did overheat it apparentally, i just got back from the mechanic and he said that it would be $1,500 to fix the damn thing!!! luckly i found the antifreeze low before any major damage was done but it needs all new gaskets and seals, and a full rebuild. Which really sucks! :banghead: If any of yall want to help me pay for this i will take donations :naughty: .