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Old 08-20-2018, 10:29 PM   #15
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Most modern vehicles calculate vehicle speed based off ABS sensors in the wheel hubs - gearing is irrelevant....change gears all you want, as long as you keep the tires the same size the speedo stays correct.
Thats not correct for GM, I would have to open a dodge and ford tune to double check. I correct speedometers all the time in GM and dodge, the gear ratio is in the tune and effects the speedometer.
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Old 08-21-2018, 04:25 AM   #16
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I always thought, not that I know much, that the speedometer is based off the gearing. Never heard, ever, it was based off the braking system.

Using the hub makes more sense though.

In 1987 I fixed a squeaky cable. It was connected to the transmission and to the back of the speedometer in the dash.

Something else to Google...ugh.
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Old 08-21-2018, 04:58 AM   #17
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I've read that they signal a low tire from the ABS sensors, but not vehicle speed.
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Old 08-21-2018, 09:00 AM   #18
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I always thought, not that I know much, that the speedometer is based off the gearing. Never heard, ever, it was based off the braking system.
I haven't worried about it lately, but back when I had an F-250 I had to get really familiar with how it worked so I could experiment with different tire sizes while maintining an accurate speedo. It was not based off the braking system, but it used the ABS module (computer chip) to count tire revolutions per mile. Then the main computer (powertrain control module or PCM) used the number of tire revs to compute distance, and a clock in the system to compute time/distance to determine MPH. Then the PCM updated the speedo, odo, and trip meter.

So tire size mattered to have accurate speedo (and odo and trip meter). If you changed tire size, even by a tiny bit, you had to update the ABS module with the new revs/mile number, else put up with an inaccurate speedo, odo and trip meter. I'm a stickler for accurate MPG computation, and that requires an accurate trip meter.

I suspect axle ratio didn't matter. It may be required in the ABS module for other reasons, but not for speed calculation. The ABS module counted the actual tire (or axle or hub) revolutions to get distance. A clock in the PCM knew the time, so it used time/distance to compute MPH. Axle ratio determines engine RPM and tire revs determines distance.

The dealer (and good speed shops) have an electronic tool they use to update the ABS module. Now they can also use a laptop or tablet computer with the right software to update the ABS module. On my F-250, you input the actual revs/mile per the tire specs in the ABS module, but on newer Fords the dealer inputs the tire size, and the ABS converts that to revs/mile. And the dealer's ABS update software will accept only tire sizes that are available on that vehicle. So I don't know how the speed shops update the speedo to get accurate MPH with huge mudders on lifted trucks. Apparently they use ABS update software that is different than what the dealer uses. Perhaps minytrker will expound on that.
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Old 08-21-2018, 09:30 AM   #19
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They get the low tire signal from a pressure sensor inside the tire, mounted to the valve stem. Tire shops are known for being careless and destroying them. For sure, Dodge uses a speed sensor in the transmission. I know because I had to have mine replaced.
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Old 08-21-2018, 10:44 AM   #20
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Interesting.
Not how Nissan (Titan, Xterra, Frontier, Pathfinder, Armada etc) works at all. Speed is absolutely measured by the ABS sensors on the wheel hubs. The computer has no idea what the gear ratio is, and it doesn't need to. It does need to know tire size obviously.

Domestics are still pretty old school in that regard I guess. They have ABS sensors on each wheel too... don't know why they would feel the need for an extra sensor on the trans. No longer the cable operated days...
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Old 08-21-2018, 10:58 AM   #21
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I've read that a low tire will turn slower and set the low tire light, on some cars. Google it.

I also know for a fact that KIA Soul doesn't use the ABS sensors for the speedometer. I swapped to 15" tires when the 18" ones wore out.
The speedometer was way off.
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Old 08-21-2018, 11:21 AM   #22
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I will not miss the squeaky speedometer cable on days below 30 degrees. In summer no squeaking, in winter I had squeaking. It was kind of season sensor I guess. Squeaking was definitely winter.

I will not ask about the throttle in todays trucks/cars. I am guessing no more throttle linkage and computer looks at position of gas pedal and adjust throttle according to that position.
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Old 08-21-2018, 05:01 PM   #23
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When I changed the gears in mine the abs lights would flash and bing. That's why I said I had to program it with a hypertech
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