Change tire size and tranny not shifting properly

[Chidds]

Hi everyone,

I have a 2020 Rebel and was running my stock Duratrac's tires and got 90km out of them, I liked them and the performance I got so i wanted to stay with them, I found a set of 285/75/18 new on MP and put them on knowing they are a little larger than stock 33.2 inch to 34.8, they look great. The truck seems to run great and shifts well in normal driving but when I haul my sled trailer it is having a hard time staying in 8th gear. What is the best way to fix my problem, what is the best programmer out there to reprogram my Ecm to help it shift like stock again? What have people found works the best when changing tire size? Any help is greatly appreciated.

 

[Longbox]

Use tow/haul?

 

[HSKR R/T]

Hi everyone,

I have a 2020 Rebel and was running my stock Duratrac's tires and got 90km out of them, I liked them and the performance I got so i wanted to stay with them, I found a set of 285/75/18 new on MP and put them on knowing they are a little larger than stock 33.2 inch to 34.8, they look great. The truck seems to run great and shifts well in normal driving but when I haul my sled trailer it is having a hard time staying in 8th gear. What is the best way to fix my problem, what is the best programmer out there to reprogram my Ecm to help it shift like stock again? What have people found works the best when changing tire size? Any help is greatly appreciated.

Alfaobd (android) or Jscan(apple) a Bluetooth OBD2 scan tool(blinker MC+) and a SGW bypass.

TazerDT allows you to change tire sizes, plus has a bunch of other features not available any other way.

Pulsar or Pulsar XT also allow you to adjust for tire size, although this is most expensive option. It with the Pulsar you do get added performance.

Now this may not "fix" your problem of shifting in and out of 8 th when towing. That could just be because of the extra power it takes to turn those big tires. More weight and more wind resistance.

 

[Chidds]

Use tow/haul?

If I put the tow/haul on it will stay in 7th for sure and will rip through fuel and revs are up.
The sled trailer is not that big, just enough drag on the truck to only allow 8th gear down a hill or on flat highway.

 

[Ready Go]

"Now this may not "fix" your problem of shifting in and out of 8 th when towing. That could just be because of the extra power it takes to turn those big tires. More weight and more wind resistance."

Like HSKR R/T stated..plus, going from 33.2" to 34.8" killed your rear gearing, especially if you started with a 3.21. In 8th gear, that gearing loss will really make the engine struggle. I get that it looks cool now, but nothings free. Gearing is a bigger issue than most people give credit to, when picking tire sizes (and weight also). Gearing and tire weight are even more important with a 3.21 gear. Ignore "some" of this if your 3.92 gear, but it still drops overall a quite a bit with that 1.6" change.

 

[Chidds]

Alfaobd (android) or Jscan(apple) a Bluetooth OBD2 scan tool(blinker MC+) and a SGW bypass.

TazerDT allows you to change tire sizes, plus has a bunch of other features not available any other way.

Pulsar or Pulsar XT also allow you to adjust for tire size, although this is most expensive option. It with the Pulsar you do get added performance.

Now this may not "fix" your problem of shifting in and out of 8 th when towing. That could just be because of the extra power it takes to turn those big tires. More weight and more wind resistance.

Thank you very much for your suggestions, I will look into this tonight. Cheers

 

[Chidds]

I do have access to a snap on Zuse scanner but have not seen anything to change the tire size. Has anyone been able to change tire size with this scanner?

 

[HSKR R/T]

I do have access to a snap on Zuse scanner but have not seen anything to change the tire size. Has anyone been able to change tire size with this scanner?

If it has something would probably be under ABS menu. But would want a number for either rotations per miles or circumference value, usually in millimeters

 

[John Kloehr]

Tow/haul will help with any gear hunting but not with gear selection. You might get some drivability improvement with AlfaOBD to adjust tire size, at least to get the speedometer correct.

The tire size change is about 5%. Your speedometer is reading 5% low, and the same amount of torque is lost. Correcting for tire size will also likely change shift points for the better.

 

[Chidds]

"Now this may not "fix" your problem of shifting in and out of 8 th when towing. That could just be because of the extra power it takes to turn those big tires. More weight and more wind resistance."

Like HSKR R/T stated..plus, going from 33.2" to 34.8" killed your rear gearing, especially if you started with a 3.21. In 8th gear, that gearing loss will really make the engine struggle. I get that it looks cool now, but nothings free. Gearing is a bigger issue than most people give credit to, when picking tire sizes (and weight also). Gearing and tire weight are even more important with a 3.21 gear. Ignore "some" of this if your 3.92 gear, but it still drops overall a quite a bit with that 1.6" change.

I do agree with the regearing with this, but would rather put the money in a tuner if possible maybe add a few LBS and HP
Looking into a EOC tuner as we speak. This help?

 

[Chidds]

Tow/haul will help with any gear hunting but not with gear selection. You might get some drivability improvement with AlfaOBD to adjust tire size, at least to get the speedometer correct.

The tire size change is about 5%. Your speedometer is reading 5% low, and the same amount of torque is lost. Correcting for tire size will also likely change shift points for the better.

Thank you for your input on this, Cheers

 

[John Kloehr]

I do agree with the regearing with this, but would rather put the money in a tuner if possible maybe add a few LBS and HP
Looking into a EOC tuner as we speak. This help?

It is a lot of money for little gain (cost per horsepower). First you need to unlock the PCM, buy the tuner and any needed credits, buy a tune or pay for a custom tune (custom is usually better result). Need to pick someone who knows Hemi engines, it is not a Chevy. Unlocking the PCM voids your powertrain warranty, and the truck will likely fail smog forevermore.

You said you have a Rebel. I thought all Rebels (like mine) have 3.92 axles. I can spin the tires and everything is stock. I actually would not mind a little less torque so I would not have to feather the gas so much from a standing start.

On edit: Mine does have eTorque so I get an extra bit of torque off the line.