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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain. |
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12-03-2014, 11:48 AM | #1 |
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Final drives and onboard calibrations
If you were to install an aftermarket final drive in your car, say a 4.5, what onboard measurements, if any would be thrown of? Milage? Gas milage? Anything? It seems to me that when we change the relationship between driveshaft revolutions as related to the revolutions of the wheels something will be amiss. Or am I not understanding something?
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12-03-2014, 11:52 AM | #2 |
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Gas mileage.
Changing wheel specifications will have more of an affect on the distance traveled/revolution.
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12-03-2014, 11:59 AM | #3 |
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But why doesn't the final drive have the same effect as a tire swap? You are essentially putting smaller diameter tires on aren't you? By that, I mean the effect is the same so shouldn't the consequences be the same?
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12-03-2014, 12:07 PM | #4 |
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With the final drive having more teeth per inch, your engine has to spin faster to get your wheels to reach any given speed.
Your wheels are not spinning faster, your engine is. That is why it affects your gas mileage and not your speedometer. When you change tire diameter, the speed your tires are rotating changes. Larger diameter wheels spin slower and smaller diameter wheels spin faster, which is why they have an affect on your speedometer.
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12-03-2014, 12:08 PM | #5 | |
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12-03-2014, 12:19 PM | #6 |
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The gear indicator might also be a little wonky since it calculated the gear you are in based on RPM and speed.
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12-03-2014, 12:43 PM | #7 | |
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12-03-2014, 12:49 PM | #8 |
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Which part of it doesn't make sense?
The affect of increasing the teeth on the final drive? Or the tire diameter affect on the speedometer?
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12-03-2014, 01:58 PM | #9 |
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12-03-2014, 01:59 PM | #10 |
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12-03-2014, 02:15 PM | #11 |
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12-03-2014, 03:46 PM | #12 |
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Speed is determined by the wheel speed (ABS) sensors, not by the speed of the output shaft in the transmission (like lots of cars).
If the ECU uses output shaft speed to determine speed, then a final drive swap will affect speed display. Since these cars use the wheel speed sensors, it's not an issue. Now changing tire diameter does since it's after the sensors. As for correcting it, as far as I know it's not an option with OFT or EcuTek at this point.
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12-03-2014, 04:31 PM | #13 | |
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12-03-2014, 04:52 PM | #14 |
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I have a 4.556 final drive in my auto. I also have a Solo DL that can display speed using GPS measurements (I don't leave it connected to the OBDII port all the time). The speedometer in my car matches the Solo DL +/- 1MPH. YMMV.
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