Tach Speed equals MPH
 

I keep hearing about 'matching the tach with the speed of the car.' Can't seem to find a previous thread about this (maybe I am using the wrong search-word). This matching technique has been mentioned in the BMW M3 show on Speed Channel and also mentioned on the Motorcyclist show.

Could someone elaborate on this? My Porsche speedo is no longer stock as it now graduated to show a max speed of 200 MPH. The tach is factory, though!

I am not sure about your specific model, but it mine the owners manual has the RPM to speed chart in the back.. Hope this helps..

Sounds like a description of rev-matching while downshifting. Let's say you're coming to a corner that you need to exit in 3rd gear. You should downshift to third before the corner. You're in 4th at 3000 rpm, to smoothly downshift to 3rd, you should put the car in neutral, rev the engine up, then put it in 3rd and let the clutch out. When you get good at it, it takes no longer than a regular downshift, you're quickly tapping the gas right as you hit the clutch. The next step beyond this is "heel and toe" where you rev the engine up while braking at the same time. Do a search on heel and toe and you'll get a bunch of posts with better descriptions. Lots of books have been written on this stuff, so don't worry if it takes while to get it, but it really is oneof the most fun things to do when driving when you get it right.

The M3 show probably discussed this as the BMW SMG system does this all automatically. It's not a traditional automatic, it's a manual trans shifted by the car.

If you are looking for your actual speed given your gear ratio and rpm then these are formulas (the same one just switched around) that you can use to calculate.

Of course you would have to know your overall gear ratio, or the ratio in any gear that you wanted to do calculations for and the height of you tire in inches.
If you are using a 225 50 16 then you can use 24.9" If it is something else then you would need to figure it out.

225/25.4*50*2+16 would be the formula to use.

width in mm / 25.4 * aspect ratio * 2 + rim size

To calculate the overall gear ratio just multiple the gears ratio by the differential ratio. For instance my car is .89*3.44 = 3.06

Mph = rpm x tire diameter/Gear ratio x 336

Rpm = mph x gear ratio x 336/Tire diameter