4 Door 2.7 3.73 gear what the max tire size I can upgrade?

Take with a grain of salt, as this isn't from Bronco experience...
In the land of the Jeeps, 31-33" probably. The bigger you go in tires without changing the ratio, the harder it will be to achieve a confortable RPM at highway speed. It also affects your off the start power. A 3.73 ratio is too low for big tires. You can adapt somewhat if you have a manual transmission, but with an automatic, who knows. It will probably hunt a lot more.

Consider, the squatch with 35s comes with a 4.7 ratio. If you havent taken delivery yet, you might want to consider at least the 4.46 option.

I dont see a 4.46 option. I have the same build and want to put 35 inch tires and lift, but doing it myself.

With 3.73 and stock tires, at 65mph in 7th gear (1:1) your engine rpm should be 2537
With 3.73 and 35" tires at 65mph in 7th gear your engine rpm should be 2327.5

35" tires in 10th rpm is 1466.3 vs 1598.8 for the stock tires in 10th. (3.73 gear)

To give you a comparison, the Sasquatch OBX is running 1874rpm in10th at 65mph.

mobroncogal

I dont see a 4.46 option. I have the same build and want to put 35 inch tires and lift, but doing it myself.

You can get the 4.27 gears if you add the rear locker option on an OBX.
4.46 gears I believe are standard for the manual transmission (not available in the OBX).

And also, @Drew, my wife's 2017 JKU has 3.21 gearing (V6, auto), it handles 285's just fine. But it does not tow for shit. You'll notice more of a difference in gearing while towing (at least from my experience). I would say 285's would be a happy median, at the same time over on B6G we've seen people drop 315's on stock Base models. As long as you aren't towing, I can see running 35's as well. But like @FourWheelKyle and @mike8675309 are eluding to, you will notice a performance drop.

It's funny in Jeep land everyone says bump up to 4.10 gears for 35s and you'll be fine. 3.73s are plenty for 33s. In Bronco land they say you need to jump up to 4.7s to run 35s and 4.46s for 33s. I run 285/70-17s on my Gladiator with the 3.6 Pentaslug and 3.73 gears and lost zero performance or MPG.

Garemlin

It's funny in Jeep land everyone says bump up to 4.10 gears for 35s and you'll be fine. 3.73s are plenty for 33s. In Bronco land they say you need to jump up to 4.7s to run 35s and 4.46s for 33s. I run 285/70-17s on my Gladiator with the 3.6 Pentaslug and 3.73 gears and lost zero performance or MPG.

This 100%. I do not understand the "I can't run X size because my gears are too high". The bronco doesn't come with 2.73s they come with 3.73s. and our tire sizes from ~32" to ~35", not 29" to 40".
For my two cents I would run 3.92s (Ring and pinion available from Dana Spicer) for the entire lineup. 3.92s with lockers....mmmmmmmmmmmm goodness.

Work your gears and engine, not your transmission. Keep that in mind and choose accordingly.

here is some additional useful data.
https://www.automobile-catalog.com/curve/2021/2953085/ford_bronco_2-door_base_2_7l_ecoboost.html
You can see that when rpm drops below 1500 rpm on the 2.7 you are losing a lot of hp and torque. You'll want to make your tire choices and gear choices based on your use case. Hills and trail driving, things where you need power and torque, then you want to optimize for that. I didn't look at the differences related to crawl speed in the low range.

You can do the calculations yourself.
This site:
https://www.4lo.com/calc/gearratio.php
And this data for the gear ratios.
https://www.ford.com/cmslibs/conten...21/mediakit/21_Bronco_Prelim_Tech_Specs_1.pdf

One of my wheeling friends has a jeep on 35s but he has 3.73 gearing. I have 35s on 4.10 gearing. We did a "ghost ride" test, 4L 1st gear and there's a pretty large difference. (He was moving a lot faster. Probably 3-5mph). That lower gear ratio means his low speed/hill climbing/rock crawlability is hampered, but, he probably has a better time on the highway than I do. I can maybe just eek out 60mph.

If you are going to 35s (and asking about gearing), my assumption was you're gonna be driving on really rough terrain and need that low speed torque. If you're using it as a mall crawler and just want the look of big tires, you can get away with lower gearing. Just make sure whatever you do you update the speedometer.

Back in the day, wagoneers from the 70s had like a 2.something ratio. They had big bulky axles and did all the gear reduction in the transmission. Modern vehicles have started taking advantage of final drive gearing to reduce some load on the transmission. Gear reduction also helps absorb shock loads from wheeling (like, when you crest over a rock and suddenly go from drive to coast). Driveshafts today spin a lot faster. But we also have better tech for balancing and dealing with vibrations.

Moral of the story, your (automatic) transmission is generally engineered to go some miles per hour at some RPM in some gear. There's a little bit of play in there but if you go up tire sizes and dont correctly adjust axle ratio, its going to misbehave.

Here's a calculator to play with https://tiresize.com/gear-ratio-calculator/