[RyanM]

I am 28 so my max HR calculated using one of the many formulas is 193.1bpm

Zone 3 (70-80% max) is 135.17-154.48bpm
Zone 4 (80-90%) is 154.48-173.79
Zone 5 (90-100%) is 173.79-193.1

What zone should I stay in for majority of my ride to increase endurance and get fitter? Zone 4 or Zone 3?

 

[Undecided]

I am 28 so my max HR calculated using one of the many formulas is 193.1bpm

Zone 3 (70-80% max) is 135.17-154.48bpm
Zone 4 (80-90%) is 154.48-173.79
Zone 5 (90-100%) is 173.79-193.1

What zone should I stay in for majority of my ride to increase endurance and get fitter? Zone 4 or Zone 3?

I think most coaches would say that there's no reason to rely on a prediction of maximum HR. Why not measure it? I think many coaches would say that maximum HR isn't the most useful reference point to set zones, too.

I'm assuming you're new to cycling, so long, steady rides in zone 3 would probably play an important role in building your "base" of endurance and fitness. Doing significant efforts in zone 4, both throughout shorter rides and within longer rides that are otherwise in zone 3, would also be valuable for both. Of course "endurance" and "fitness" are very fluid concepts and you can spent all the time increasing your ability to "endure" riding at some moderate amount of power without significantly increasing your ability to "endure" riding at a higher amount of power.

 

[Kerry Irons]

Wrong basis

I am 28 so my max HR calculated using one of the many formulas is 193.1bpm

Zone 3 (70-80% max) is 135.17-154.48bpm
Zone 4 (80-90%) is 154.48-173.79
Zone 5 (90-100%) is 173.79-193.1

What zone should I stay in for majority of my ride to increase endurance and get fitter? Zone 4 or Zone 3?

First of all, the formulas for max HR are essentially useless for what you want. Instead, base your training on your maximum sustainable HR (essentially your Lactate Threshold or LT) - what you can average in the last 20 minutes of a 30 minute all-out effort (time trial). From there, here are some zones you can calculate:

Active Recovery - Zone 1 <80% LT
Endurance - Zone 2 80-89% LT
Tempo - Zone 3 90-93% LT
Subthreshold - Zone 4 94-99% LT
Suprathreshold - Zone 5a 100-102% LT
Aerobic Capacity - Zone 5b 103-105% LT
Anaerobic Capacity - Zone 5c >105% LT

Using this procedure, zones 2 and 3 are what you want, but you need to go into the higher intensity as well (one or two days per week, do intervals) to build speed.

BTW, I know you did those numbers on a spreadsheet, but there is no such thing as a 5 significant figure heart rate. Some would argue that when you are exercising vigorously, there's maybe 2.5 significant figures.