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.

Its physics.

W=f*d

Where f is Force and D is Distance.

Where the wheel spin is the W.

More teeth = less force but more distance traveled to produce the same amount of Work.

To go the same speed, your engine needs to spin faster with a gear that has more teeth......

I really don't understand the confusion. Your wheels are not bolted on your crankshaft.....

I was talking about from tooth to tooth of the 4.667 FD vs the stock 4.1 FD, not total work done. W not Wnet lol. work is more complicated than just F*D :lol: :cry:

Its all miniscule though in retrospect.

4.1-4.667 = .567
.567/4.100= .138
so a 13.8% increase over stock. So every RPM you cruise at, add 13.8% to it. Im assuming subtract 13.8% from the normal MPG, given all conditions are the same.

But it's a 13.8% increase in thrust at a given speed, which means faster acceleration. Also depending on the course/track it could put you in a better rpm range, like say instead of constantly shifting between 1st and 2nd, you can stay in second and still be where you want in terms of rpms. That increase can make a big difference on a track. As far as cost mpg wise it's not as simple as taking off 13.8% and there's too many variables to really say with any certainty. As far as top speed that's a really simple problem assuming you have enough power to top out sixth gear.

Gear display on the dash can be out. I know it was with the 5.1. It can be fixed via a tune. Everything else is fine.

BTW, driving from California to the SCCA Nationals in NE with a full set of race tires and wheels in the car as well as tools, jacks, luggage, popup, etc...I got at least 30MPG on the highway at 70MPH on cruse control, supercharged with the 4.556 final drive.

Neither, other than what wparsons had stated, the following part is completely false. 1 revolution is 1 revolution, the speedometer won't change, but the actual distance travelled and consequently the actual speed is different due to the circumference.

I assume he meant that at a given speed, small wheels will be rotating faster, so it sounds like you're both on the same page.

Yeah OK.
Totally makes sense, he was absolutely right.

i have a 4.56 i thought i would have affected my speedo but it didnt, still accurate to my gps. so im assuming speed isnt taken from the driveshaft. fuel econ isnt a noticeable difference, 6th gear at 70 is like a 600 rpm difference i think.

I have a 4.67. GPS is dead-on against the speedo at a +1MPH delta at 60MPH with stock tires at 32F/35R PSI (61MPH indicated / 60MPH GPS). That number didn't change with the FD, only after changing wheels/tires.

FWIW, there is a table for gear determination thresholds in the map; I'm assuming that's specifically dealing with the gear indicator on the dash for manual cars, because it doesn't seem to do anything on my A/T.

Theoretically you could just calculate the percent increase between your new final drive ratio and the stock 4.1, then increase the values by that percentage. Someone with a manual should test that out to see if it actually works that way. :)

It does.