RPM speedo formula

[tabahr]

I question the accuracy of my speedo. It seems to read fast, but i'm not sure. When I test it using the following method, it tests ok.

At 70 MPH in your 1:1 ratio gear (4th with manual or 3rd with auto) your Tachometer should read out your rearend ratio(3.42 or 3,420 RPM).

This has always worked in all my cars, so I think it may be a good test, and if it is my speedo is fine. Has anyone else heard of this? How does this test factor in the height of the tire?

Tracy

[M3EATER - GG '00 WS6 T/A]

Re: RPM speedo formula

Originally posted by tabahr
I How does this test factor in the height of the tire?

Tracy

It doesn't

Go out on the expressway where they have those little green mile posts. Do an indicated 60 MPH (on your speedo) as you go from sign to sign...... keep your speed as constant as possible.

Time how many seconds it takes to go between 2 signs.

OK....

X = your timed run in seconds

Ok, now .... 3600 seconds = 1 hour (right?)

Soooo........

3600 / X seconds = your actual MPH

EXAMPLE:

It took you 65 seconds to run the mile.

3600 / 65 = 55 mph


Now, what do you do with this? Convert it into a percentage error.

Error = The actual speed (from above) / 60 mph (or what ever you were indicating on the speedo).

EXAMPLE:

Error = 55 / 60 = .91

Sooo...... your speedo error is 9% below the indicated speed (in the range around 60 .... if you go way above or way below - that may change some).

[Injuneer]

The "test" isn't very reliable because:

-it is totally dependant on tire height.
-it assumes your tach is accurate (they aren't)

But, after saying that.... IF

-you have tires roughly the same diameter as the stires that came stock on your car (25.66" for 245/50-16 and 275/40-17).

-you use a scanner to read the rpm directly from the PCM, rather than looking at the tach

-you drive exactly 75MPH

... the "test" will work.

But, as Joe pointed out, use the mile markers on the freeway. Try and hold a steady indicated 60MPH for as long as you can... the more miles you test it, the less chance your results will be affected by things like a misplaced mile marker, or your ability to hold the speed exactly. Say you drive 5 miles, you get 305 seconds, you divide by 5 to get 61 seconds per mile, then use Joe's formula.

[94 formula]

I tired this in 4th gear in my 94 formula and at 70mph I was only at around 2900rpms what does this mean?

[Injuneer]

Originally posted by 94 formula
I tired this in 4th gear in my 94 formula and at 70mph I was only at around 2900rpms what does this mean?

I think it means you didn't read my post .

You have to drive at 75mph, not 70mph.

Did you use a scanner to read the RPM from the computer, rather than the tachometer, which is often off by a couple 100 RPM?

What size tires do you have?

What tranny do you have? What gear was it in?

I'm guessing that you have 3.23 gears. But you don't have to guess, or play games with the speedo and inaccurate tachometer. All you need to do is look at the build code tag to see what the "GU_" code is:

GU2 = 2.73
GU5 = 3.23
GU6 = 3.42

If you are more worried about the accuracy of your speedometer, use a stop watch and the mile markers as suggested above.