Odd cruise control behavior, stuck in 4th gear

Hey all,
I just did a 3,400 mile road trip from CO to PA and back. I experienced some very odd behavior with my cruise control. If it had to downshift to accelerate up a hill, it would eventually just stay stuck in 4th gear regardless of engine load. Below are some examples to reproduce this.

Re-create the issue

• Cruise at 75 mph in 6th gear
• Engage cruise control
• Drive up the first hill, it will downshift, then go back into 6th
• Drive up the second hill, it will downshift to 4th and remain in 4th gear

Work around the issue.

• Enter sport mode and set to 6th gear
• Repeat steps above. Shifting returns to 6th gear when cruising on a level surface.

I noticed this quite a bit in Kansas and Iowa. It seems very odd, and I've never had a Toyota do this (my previous vehicle was a 4Runner). I have a 2016 SR5 Crewmax with the tow package and hence a lower gear differential. When driving without cruise, I can easily maintain 75 mph without applying much throttle. Any ideas what may be going on?

Thanks,
Todd

 

When driving in hilly terrain I usually kill the cruise to take advantage of gravity going downhill and momentum going uphill. I hate the "kick it in the a$$" response when a vehicle's cruise control thinks it has to get back up to speed as fast as possible. Therefore, I have probably never given my truck the chance to act like this.

 

Thanks @graysquirrel I run 87 octane, and I find that seems to run well. What's surprising about the elevation is after I figured out the shifter trick, I was in 6th gear going uphill slightly. From Iowa City to Denver the altitude increases from 1k to 5280 feet. I was very slowly going uphill. My truck did so in 6th gear without any issues. As soon as I put it back in D, after the next hill it would stay stuck in 4th gear. Put it in S and set the max to 6th, and it would shift back into 6th after climbing the hill.

 

Yup, I think it's a pretty common issue. Mine does that, I usually just do S6 and leave it there which seems to convince the trans that it actually has 6 gears instead of 3 or 4.