speedometer gear accuracy question.

[Gun Jam]

Right now I have a (I think) 7 tooth drive gear and a 21 tooth driven gear. It is accurate to 5mph at 80 MPH indicated. If I switch to a 6 tooth drive gear and a 18 tooth driven gear (This, in theory the same result as the 7 tooth / 21 tooth setup) would the gauge be as accurate? Does accuracy change with tooth count or will the two setup provide the same result in real life (not theory) thanks -Gun

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[REAVER117]

I haven't tried this, but 'in theory' the accuracy should decrease as the number of teeth decreases. I would guess that you wouldn't see any real world change, overall accuracy of the speedometer is probably more dependent on the actual speedo unit and the size of the rear tires.

[fakesnakes]

It should be just as accurate. 3 to 1 is 3 to 1 regardless of the number of teeth to get there. Your ratio should take into consideration your rear tire diameter.

[Norm Peterson]

It is a straight mechanical ratio, since gears do not permit 'slip' (if they do, they're toast and need to be replaced).

If you're off by about 5 mph out of 80, that's about one tooth on the driven (cable) gear. You'd need either a 20 or a 22 to fix the situation having the 7 drive gear, depending on whether you need the speedometer to speed up or slow down what it is showing. With a 6 drive gear, that'd be either 17 or 19.


or get an electronic speedometer that you calibrate yourself


Norm

[Gun Jam]

Thanks guys.

Im asking because im fixing to replace the tail shaft bushing and it seems like it would be a good idea to switch the gears to something more on the mid range. Right now its way on the outer limits as in they dont make a 22 tooth driven gear.

Do you guys have the chart that says what ratio you need for a given rearend gear....

thanks

-Gun

[fakesnakes]

This one should work:

http://www.428cobrajet.org/speedo-calc.html

[Norm Peterson]

Close enough for most uses. But it probably under-estimates the ratio by about 3.5% (this applies to most tires). If it calculated 2.627 for you and suggested 6:16, you'd probably be a little more accurate using 2.72 and choosing 7:19 instead.

FWIW, 6:16 gives you a little extra cushion against speeding that you don't "see", but racks up the odometer miles a little faster than actually happens.


Norm

[kalli]

if possible i prefer a speedo that shows _slightly_ more than actual. just as cushion.
like setting your alarm clock early

[Carlos Pineiro]

If you don't mind, I'd like to piggyback this thread with a related Q:

Is there a calculator similar to the 429 calc. to figure out my actual speed considering all the mods I've done to the rear end and wheels? It's fun driving to the store at 110 MPH, but eventually I need to come back to reality.

CP

[Norm Peterson]

I think most speedometers work off a calibration of 1000 cable revolutions per mile (or very close). The tire revs per mile formula that agrees more closely with the mfr provided data is approximately 20900 ÷ [TireOuterDiameter]. The rest is simple math involving the axle ratio and solving for the speedometer drive:driven gear ratio that puts the cable at 1000 revs/mile. And then finding the drive and driven gears that most closely match that.

The correction box that the linked site refers to for some other engine or powertrain has its own internal ratio that you'd need to know and include in your calculations.

It wouldn't be very hard to throw a spreadsheet together to do the math (not gonna happen tonight, but maybe tomorrow). But the gear availability and part numbers is something you'd need to spend some time with your friendly local parts counter guy to get.


Norm