At the very least, the computer uses input from the speed sensors (consists of a tone ring and sensor) located at each wheel along with parameters for a given gear set and stock tire size to indicate speed. There is no longer a mechanical connection between the speedometer and any of the systems that were at one time used to measure vehicle speed.
The tone ring is mounted to the hub. Tire size effects how fast the tone ring spins. Smaller tires it spins faster, larger tires it spins slower.
However fast the tone ring is spinning, the sensor sends that information to the computer which uses that information along with the programmed gear ratio and tire size (and possibly other parameters) to provide an indicated speed on the speedo.
Speed and distance depend on tire diameter and tone ring frequency. It matters not what gears are installed.
The OP could have had his answer days ago if he had jacked a tire and spun it 2 revolutions.
Assuming that the jeep is 100% stock...with revs of X and Speedo reading of Y in 6th gear, that means axle gears are the following ratio....regardless of whether I'm running 44" tires or running 26" bicycle tires.
But that's not how it went down.
07 JKU - 6spd - LSD rear
Doing it your way, to solve for axle ratio, you need to know:
It is a simple question to answer.
It just requires a tiny effort on your part. (Measure your tire diameter, or jack a tire and spin it 2 revolutions.)
RPM in photo
MPH in photo
What's the answer?
I think we're getting lost in the weeds. I don't have some crazy custom rig. It's a freaking bone stock Sahara. A stock JK Sahara in 2007 had like 2 or 3 possible ratios in the axles. My revs seem a bit high, so I already know it's one of the shorter ones. Does nobody else ever look at their tach on the highway? With a group of gearheads that pay attention to their rigs this would take like 3 minutes around a lunch table, nobody would need to check tires or anything else. A simple process of elimination based upon what is greater than and less than the other 1 or 2 available options.
07 JKU - 6spd - LSD rear
All the computer needs to know is distance per tire revolution ( for the miles part), and how long it takes to make one revolution (for the per hour part). It gets this data from the ABS tone rings.
If what you say is true, the mph would be wrong when coasting downhill with the engine at idle; when the automatic's torque converter is slipping; when either transmission is in a gear other than top gear, (if top gear is what is assumed); etc.
As mentioned in a previous post, people who change gear ratios and don't reprogram, don't see a different error in the speedometer.
A 30" tire travels about 7.85 feet per revolution.
That's about 672.6 revolutions per mile.
If the tire rotates at a rate of 672.6 times per minute, the speedometer will show 60 mph.
I don't see any gear ratio or engine rpm data in that.
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