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?

Gas mileage.

Changing wheel specifications will have more of an affect on the distance traveled/revolution.

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?

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.

Understood. You're the man.

The gear indicator might also be a little wonky since it calculated the gear you are in based on RPM and speed.

This doesn't make any sense.

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?

It is... You think open flash tablet or ecutek could fix that?

I think it does...

Not sure.

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.

I dont think it makes sense either, think about it this way, you have a latter with the rungs spaced 5 feet apart. Its going to take more work to go the distance* than if the rungs were spaced 2 feet apart. QED, the engine needs less work to move one tooth from the higher FD than from stock. This would explain why you tend to accelerate faster with a higher FD.

*song from disney's hercules which has no effect on this conversation at all

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.