What year Tahoe is this?
Ive done 87/93 octane tunes with a switch on the 99-02 Trucks with a COS5 EFI Live tune. It uses the "nitrous retard" settings to pull timing for the 87 octane.
The other easy way is to just get a holley Terminator x max and you can do whatever your heart desires with that. E85, 87, 89, 91, or 93 with a switch.

 

One of those things that's possible, but not recommended. Most of the time will be fine but there are some scenarios where sustained load under low spark will push EGT to dangerous levels. Think melted exhaust valves, etc. Your mileage may vary.

 

I never knew that. So the low octane table should probably stay completely stock, and do all of your tuning to the high octane table only, letting the computer decide how much to retard it? Except for people running speed density: So when would the high octane table come into play? Why would the computer ever use the high octane table?

 

In Speed Density the ECM sees the failed MAF and thinks there is a fueling issue then throws the MAF code. It doesn't understand the end user forced SD... and looks to the low octane table as a fail safe. In SD the computer never looks at the high octane table. That's why it's important to copy the high octane table and paste those numbers into the low table so you have optimal timing. You lose that fail safe cushion of pulling timing during knock.

If you change the OS to the "SD Enhanced" version you get the high/low octane table functionality back. You also lose that pesky secondary VE table. I'm running the 1 Bar SD Enhanced OS in my truck (P01).

 

Lots of great info,thanks everyone