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schuetz1619

2002 speedometer / differential combinations

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(1) My tii has a 320i 5-speed overdrive transmission and a 320i LSD diff. I understand that the 320i diff came in both 3.64:1 and 3.91:1 gearing. How can I tell which I have (preferably without having to pull it)?

(2) 2002 speedometers come with a calibration reference number on the back. Does anyone know what calibration number is intended for use with which differential gear ratio?

(3) I just had my speedometer calibrated by Speedometer Service in Portland according to the calibration number stamped onto the back of the unit. As measured with a GPS unit, it is accurate at 35 mph, but as you go faster, it reads increasingly incorrectly, on the high side. For example, at 65 mph it reads 70. Also, the odometer reads 6% too high. Is this what would happen if the diff / speedometer combination were wrong?

Gordon Paine

Eugene, Oregon

1971 Euro 2002tii 27000056

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If it is reading correct at 35, then you have the right speedo for your diff. The error at higher speeds is just a limitation of the speedo (perhaps older weaker springs, etc).

The error I had with the wrong speedo for the diff was in the order of 30% IIRC (a 3.9 diff on a 74Tii). I bought a new speedo of the correct calibration and it works perfectly now.

B

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List (below) taken from a post earlier this year.

3.45:1 diff -- W=1.24 speedo

3.64:1 diff -- W=1.297 speedo (W=1.30 is close enough)

3.90:1 diff -- W=1.39 speedo ('76 49-State 2002)

3.91:1 diff -- W=1.393 speedo (E21 320is LSD)

4.10:1 diff -- W=1.4 speedo (1.461 calculated)

4.11:1 diff -- W=1.4 speedo (1.465 calculated)

4.38:1 diff -- W=1.559 speedo (calculated)

HTH

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Another veriable in the equation is your wheel/tire size. The original tire diameter on your car was approx. 23.4 in. If you're running something different that will also enter into the equation.

Bob Napier

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If it's right at 35 and wrong at higher speeds, if I were you I'd be calling my vendor.

GL,

Ray

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I assume that GPS-calculated speed is fairly accurate. I have no particular reason for assuming that other than (1.) the consistency I've witnessed among 4 different GPS units; and (2.) some vague confidence in technology I don't understand. Is it reasonable to compare our speedometers to our GPS units and assume the GPS speed is more accurate?

Steve

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Another veriable in the equation is your wheel/tire size. The original tire diameter on your car was approx. 23.4 in. If you're running something different that will also enter into the equation.

I'm going with Bob on this one. At the relatively low speed of 35MPH the variation would be less visible until you increase speed and the error is exaggerated.

Remember the speedometer simply measures the number of rotations of the transmission output shaft. The rest is a matter of ratios ending in the ratio between one revolution of the tranny output shaft and the rotation of the rear tire. If you don't have the correct tire size for the ratios that the "system" is designed for then you have error that is increased as the speed increases.

One turn of the driveline typically translates to 23.4" of forward travel. If you have a different tire size the ratio changes so one turn of the driveline equals something else.

Todd

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