re programming ECM after a axle gear ratio change

[amauri]

I'm just curious and trying to learn something.

I've searched and found lots of posts about the pros and cons of one gear ratio vs. another, not looking for opinions on what ratio is best for whatever application.

What I am looking for (and could not find) is info about the engine and/or transmission control modules, how they would react if the axle ratio is changed?

Suppose we change from a 3.42 gear to a 3.73, wouldn't the computers need to be re-programmed to work with the different RPM's to MPH and transmission shift points?

Obviously, the speedometer and odometer would no longer be accurate.

 

[Newsa]

I'm under the impression that the speedometer/odometer is based on the individual speed sensors (2 on each axle=4 total). I would say "each wheel", but a dually has 6 wheels/rims and only 4 speed sensors. Unless you say that rims are not wheels, which i said once and was corrected.

The only way to make the speedometer/odometer inaccurate would be different size rims.

I had an isuzu amigo that measured vehicle speed from the transmission output to drive shaft. But our trucks do it differently

 

[amauri]

I'm under the impression that the speedometer/odometer is based on the individual speed sensors (2 on each axle=4 total).

Makes sense.
So in that case the ECM knows exactly how many RPMs the wheels should be turning for any transmission gear engaged at what ever RPMs the engine is turning. If we change the axle, wheel RPMs would no longer match what the ECM is expecting.

This is why I think that after changing the axle gear ratio, the ECM needs to be programmed to maintain optimum torque output for each gear to RPM ratio.

I hope I'm making sense, don't know how to explain any better than that.

 

[Newsa]

https://www.amazon.com/Hypertech-75...er+calibrator&qid=1554595916&s=gateway&sr=8-2

I don't know if this changes the readings the ecm/pcm/transmission uses for shift points. Or if it just changes the speedometer reading on the dash and EVIC

 

[pwr2tow]

The mph speedometer and odometer will still read accurate, No, you do not need to reprogram the ECM. All speed, ABS, TCS, etc measurements are taken at the wheel hub for the purpose of the ECM. The wheel still moves the same distance per revolution as it did before.
Shift points are load based so you control at what rpm's the transmission shifts with your right foot. More throttle the higher the rpm in shift point. Light throttle the transmission will shift to keep rpm's around 1100, the same with the gears you have now.

 

[TCDiesel]

You need to change the axle ratio the rear ABS/ESC sensor is located in the rear axle Housing.
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[AH64ID]

Something I have wondered as well since I'm considering 4.10's on my '18.... thou I am going to give it a towing season first to see how it does.

The speedo shouldn't be an issue, but I am unsure about trans programming. I've heard gearing is part of trans tuning, but everything I have seen in the programs is trans output rpm based, so gearing will have no effect.

 

[EricViviano]

There where some videos I watched in regards to the new trucks and their garbage 3.42 axle ratio as it relates to the automatic transmission. There where some changes needed so the automatic would function correctly. You will have to look into that more. I would have gladly ordered my 18 with 4.10's, but there is no choice of gear ratio any more, even with the G56. The small loss in fuel economy is well worth the gained low end.

Also the jump from 3.42 to 3.73 is a waste of time/money, but you said you didn't want to get into that, so I'll leave it alone.

 

[EDankievitch]

If I was ordering a new truck I'd request the 3.73 rear gear set. Having had both the 3.42 currently and the 3.73 in my previous two trucks I miss the 3.73's. If you stay stock the 3.42 is "ok" but once you start up sizing tires, and you don't have to go up by much. The higher gearing starts to lug the engine. My fuel Mpg's towing were better with the 3.73. The engine has more than enough torque to make up for the taller gears but you notice it in the Mpg's. That said I wouldn't spend the money to change from 3.42 to 3.73. I can't speak for going to a set of 4.11's.
Just my 2 cents worth...

 

[pwr2tow]

I wouldn't mess with that hypertech programmer, no need for it, you'll also lose your warranty messing with the ecm.

Your 68rfe is a learning transmission, even if it wasn't you still don't need a programmer for anything just changing gear ratio.

I know, I'm speaking from changing my 2016 2500 68rfe from 3.42 to 4.10 ratio.

Makes sense.
So in that case the ECM knows exactly how many RPMs the wheels should be turning for any transmission gear engaged at what ever RPMs the engine is turning. If we change the axle, wheel RPMs would no longer match what the ECM is expecting.

This is why I think that after changing the axle gear ratio, the ECM needs to be programmed to maintain optimum torque output for each gear to RPM ratio.

I hope I'm making sense, don't know how to explain any better than that.

https://www.amazon.com/Hypertech-75...er+calibrator&qid=1554595916&s=gateway&sr=8-2

I don't know if this changes the readings the ecm/pcm/transmission uses for shift points. Or if it just changes the speedometer reading on the dash and EVIC

 

[amauri]

An update for those interested.

After spending a lot of time searching and asking questions, I finally got the correct answer.
The final verdict is that yes, you do need to re-configure the BCM to the correct axle ratio parameter under the Vehicle Configuration #4 (has noting to do with speedo accuracy).

I used AlfaOBD to make the change today.

Vehicle Config 4:
Transfer Case High Ratio: 0.00
Axle Ratio: 3.42
Transfer Case Low Range Reduction Ratio: 65.54
Dynamic Tire Circumference:
.
.
.
.
.
.

 

[Cummins12V98]

IMHO a complete waste of money, simply tow in 5th at the same RPM'as as a 4.10 equipped truck in 6th.

 

[amauri]

This is a technical discussion, not a financial one.

Thx, but I don't need advice on how to spend my money.

 

[Donnie Straps]

I didnt change mine at first because I was told there was no need to do so. I finally did after I got the Alfa app for other uses and it didnt seem to change anything like shift points as far as I can tell. Truck sure pulls better and feels much stronger with 4.q0's. Doesnt constantly hunt for a gear while towing and taking hills where I live.

 

[Randomrunner12]

So since this is all based on the Auto I don't suppose I'd have to change anything if I went taller gears in my G56? I have the 3.42's in my truck.

 

[AH64ID]

So since this is all based on the Auto I don't suppose I'd have to change anything if I went taller gears in my G56? I have the 3.42's in my truck.

Why taller than 3.42?

But I don’t think the changes are for the auto only, as they are likely for things other than the transmission.

I know the 3rd gens would calculate what gear the manual trucks are in, so that seems like the ration might matter to the manual too.

 

[Randomrunner12]

Was just curious, thank you.

 

[Ozymandias]

Why taller than 3.42?

But I don’t think the changes are for the auto only, as they are likely for things other than the transmission.

I know the 3rd gens would calculate what gear the manual trucks are in, so that seems like the ration might matter to the manual too.

Yep, i think so too.
The ECM needs to know when which gear is in to set the TQM correctly on the spot.

 

[Cummins12V98]

This is a technical discussion, not a financial one.

Thx, but I don't need advice on how to spend my money.

Gave my "OPINION" don't like it carry on. I did have an answer as to why and I have owned both.

I was told by Dave Smith service Manager nothing had to be recalibrated after gear swap. I was offered a swap to do BOTH axles from 3.42 to 4.10 by him for $1,500 and decided not to after seeing how well the stock 3.42's performed towing in 5th.

 

[Cummins12V98]

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