Quote:
Originally Posted by tuffr2
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.
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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.