I own an 84 TA 4bbl. I was recently driving on a highway, downhill, and my speedo said 70mph. The car felt like it was doing more, and the other times I've went down this hill it got up to 85mph. All of a sudden, I saw the blinking red lights on an unmarked police car behind me. After he pulled me over, I asked him how fast I was going. He said 82mph. My speedo only said 70mph. I have 225/60 R15 tires, I know they are supposed to be 245/50 R15's. I don't think that could cause that much of a difference though. Also, the rear end was replaced well before I bought the car, but I always thought it was an OE replacement. Any suggestions on how to correct this? Thanks for any replies.

BTW: I didn't get a ticket, just a warning... lol. Also, the cop wasn't using radar, he was driving (he said I passed him and he was doing ~70mph himself), and he equalled my speed... if this means anything.

 

Sounds like someone changed the gear ratio on you (from say 3.08 to 2.73 or so).

Here's my low-buck method of calibrating:

First you need to figure out how much you're off by. You know how highways have mile markers on them? Well, your odometer is driven by the same stuff as the speedo. Start at a mile marker on a STRAIGHT stretch of road and make note of your EXACT mileage. Then go to the next full mile marker (or if you can find several miles of straight road, so much the better) and read the EXACT mileage on your odometer. Let's say your odometer only read .9 miles traveled. Well then, you're off by roughly 10%. You can then make corrections to the speedo calibration with some idea of which way to go and by how much.

Here's another way- run next to a freiend's car that has a known-accurate speedo and compare MPH readings.

So let's take your example of 70 indicated on your speedo but the cop said it was more like 82. A very rough way to calibrate but it means you're off by about 17%. (82-70)/70. Always divide the difference by what is actually indicated on YOUR speedo.

Any good tranny shop can swap your speedo calibration gears if you give them the (accurate) info they need to recalibrate it, like what's above. If you want to recalibrate it yourself using this example, then you would need a speedo DRIVEN gear with 17% fewer teeth than what's in there now or a speedo DRIVE gear with 17% more teeth than what's in there now.