for those that upgraded tire size

[HDMXDiesel0817]

For those of you that have replaced your stock 245 tires with a larger size, I'm curious to know if you have noticed a difference with your cruise control. Prior to putting my 285s on, the cruise held the speed exceptionally well going down nearly any grade. With these larger tires I've noticed that it doesn't hold it back as well. Have any of you noticed this difference as well? Even though I recalibrated the speedometer for the larger tire, it would appear that somewhere along the line something else should be recalibrated too. Am I wrong to think this?

This thread is just to see if anyone else has noticed this change, and possibly to find a solution to it without returning to the stock size. Thanks in advance.

[DmaxMaverick]

I noticed the change right away. My truck has had 285's on it for over 50K, and use a Predator to adjust for the change.

The reason is final drive ratio. By increasing the tire circumference, you are also raising the final drive ratio. This means that it takes more rolling speed to effect the engine speed, as compared to the original 245's. The 285's are an increase of approximately 8% over stock (in my case, anyway). You can expect about a 8% change of just about everything in the drivetrain, as the computer has no idea that you've changed tire size, even if you've compensated with a program that adjusted the speedo.

Adjusting the speedo calibration changes only that. It does not change the algorithm that calculates shift points, cruise operation, etc. The larger tire size will require the same amount of action at the wheel end, to invoke a reaction at the engine end as did for the stock tires. The difference, with larger tires, translates to more tire RPM (speed) than before, for the same action (or reaction). The perceived action is a delay, or increased parameters. The situation would be just the opposite if you changed to smaller tires.

I have noticed that the tranny program is dependent on engine speed and gear selection for shift points and tranny behavior. With a Predator, it didn't matter what I did to adjust for tire size, the tranny always reacted the same to RPM, regardless of what the speedo displayed.

In spite of all this gobble-d-gook, I'm pleased with mine.

[HDMXDiesel0817]

DmaxMaverick,

Thank you very much for your input and explanation. :D

I've only had the tires for about 5k, just about time for my free tire rotation, haha. I used the quad to adjust the speedo/odo. Thanks again for the explanation. No beef on the tires was just curious as to why it didn't hold it as well. Can't complain too much just have to use the brake pedal then... ;) hard pedal to find sometimes, the big one on the right is a fun one to play with.