Lots of time I'll see a photo of a dead deer with his ears straight out horizontal. I want to use that to get the inside diameter of the antlers but haven't ever read this dimension. Anyone know?

I'd say it varies based on deer, but part of the reason why people use ears to judge is because they are a pretty good guide for AR compliance. My general thought is ears are 4" and the head is 6", so you're at 14" ear to ear which meets AR

Straight out, you're probably in the 15-17" range depending on the deer. In the alert position they are typically 12-13".

This is the alert position, ears are up, not straight out.

Ah, I consider "alert" as horizontal versus relaxed and back

Yeah, by the alert position, they mean as shown in the drawing posted by QuitShootinYoungBucks.

It also depends on location. The farther west you go, the bigger the ears get. It really fooled me the first few years I hunted west Texas.

Bless them folks in Mississippi, they can't even create an accurate illustration and easy to follow description of inside spread. Not sure their method of determining main beam length is easy to follow either.

It depends on lots of factors...including where you are hunting, age of the deer, overall body size of the deer in that area, etc. Even then you can have large variances in ear width in the same deer on one property. Ear length varies from 5" to 8" in Texas and larger in the midwest states. I have watched 2 bucks the same age class standing close together....one looked wider than the other but studying them little closer I realized that one had much larger ear size than the other one...they both were almost the same width when they got side x side. I have seen deer in Texas range from 13" ear width to around 18" tip to tip ear width in the alert to laid out ear position. I have them here range from 13" to about 16". The best thing to do is to guess how wide the ear tips appear to be then if you kill the deer measure them him on the ground. Lay the are flat out to upright alert positions to see. I also measure mounts from the ranch I am hunting or my own mounts to see how wide they are tip to tip.

Same ranch and same feeder location. Which buck is wider?

At a glance, the 1st deer would appear to be wider, but I suspect the 2nd buck has him by a smidge. I'd bet they're both very close to each other, maybe an ~inch difference and roughly 14-15"?

First buck dressed 110 lbs and second one dressed 140 lbs.

Based on the shape of the main beams, I would say the second buck is wider. Those on the first buck go up and then out. The main beams on the second buck travel out before rising which allows for a wider spread.

I am probably wrong, but I will still go with the top one. If this was East Texas both deer would get a pass, as neither one would be over 13" enough for me to take a chance. This is the reason I hate AR's, I understand what they are trying to do but both of those deer are old enough.

I'd call second wider in STX's post.

I think both those bucks were shot and in an AR county...

14 5/8" and 15 3/4"

Like SuperLongScreenName said, that pic is the alert position.
We measured a 5year old buck's ears about 4 years ago with the ears straight out and he was 17" wide, ear tip to ear tip. The following season, i had a buck running a doe who's ears were straight out for a lot of his chase and he was an inch outside his ears on both sides. Good solid 5 year old (aged off of teeth) 10 point. Smoked him and had the worse case of ground shrinkage to date. Outside of ear to outside of ear with them straight out were 13" wide.

I feel like a copy cat always agreeing with Stx but he explains things well. The bucks we kill in NWTx and most of my mounts will vary but most of them average about 6" for the ear itself.

13 6/8"(110 lbs) and 14 2/8"(140 lbs) inside on those two bucks.

Sure makes me glad I hunt on MLD level 3 or whatever it's called today.