Re: Difference between 3.73 and 4.10 gears?
First, about 10% torque multiplication and RPM difference for the 4.11. It will be noticeable. However, it's likely best to dig a little deeper to determine if worth the money. My hunch is, you'd probably like it and you shouldn't lose mpg significantly, and in town you may even gain due to the engine working a bit easier in stop & go, but don't expect miracles. If you like the car, and can get a set of used gears installed cheap, and then correct the speedo/speed sensor, then it could be fun. If you are shelling out bucks for retail stuff, I would likely think about it.
As far as 5.13s, you have to look at the significant change in both starting line ratio (SLR) and overdrive (OD). I am not sure which transmission you have, but let's say 5 speed manual.
Say you have 3.73 rear X 3.27 1st gear now, that would be a 12.19:1 SLR, 4.11s would be a 13.43:1 SLR and is getting pretty short, although likely welcome in a lower torque motor. However, that 5.13 gear would be 5.13 X 3.27 = 16.77 and that is "low hole" truck level. What happens is that the engine winds out so quickly that you aren't even really loading it and just losing momentum with an immediate shift when running WOT at launch. I would expect with 5.13s, unless you have a ridiculously large tire, you'd be starting in second everywhere. If it's not clear, this car would likely be significantly slower than the 4.11 car as a guess due to mismatch. It "might" be slightly faster in the 1/8 mile if it's an automatic, but probably not
For a big block, I typically try to stay at 12.5 or under SLR, however, I think your 4.11s could be beneficial for a lower torque engine like a V6 and not a bad idea if costs are down Keep in mind I used T5 1st gear numbers, no idea what is in your car and automatic numbers are different. Regardless, with SLR, diminishing returns still say to me that 5.13s are WAY too deep
The other part is OD compound ratio, the ratio you have for OD. I like to stay between 2.50-ish and 3.0-ish depending on how much torque the engine makes at cruise RPM. Let's use .67 for argument purposes for OD. 3.73 X .67 = 2.49:1 this is about as tall as anyone would want to go in a performance vehicle as it puts the cruise RPM relativelyy low. Again, depending on use, you likely could benefit from 4.11s as it would put you at 2.75:1 which likely on the highway puts you in a little more efficient range depending on speed and tire size. Comparing a 5.13, you end up at 5.13 X .67 and you end up at 3.43:1 which isn't horrible on paper, but you could almost shift to 4th and see what that would be like. It gets into the slow lane kind of highway performance and certainly will take a hit on mileage. Remember, of course, this OD ratio at .67 was an example too, no idea which tranny you have.
For laughs, say you were crusing at 1800 rpm now with 3.73s, you'd be over 2600 rpm, with 5.13s., that doesn't sound bad, but it goes up from there and your driveshaft is turning 3900 RPM, as you go up, you start getting into custom driveshaft range because how fast it is spinning on the highway
I will tell you this though, unless you have a real passion for THIS car, the 4.11 will pretty much make it just feel a bit better, it won't turn it into a V8 slayer. If you want to build a high performance Mustang over time, it is probably wiser to start with a V8 car. Now if you intend to turbo or supercharge, etc, I also would not start with the rear axle because as you built the car, you will gear for where the engine makes power, and the 3.73s could be exactly what you need.
Hope this helps
70 Sportsroof, 427 FE/489 cid, TKO-600, 31 spline 4.10, A/C. modified Mass-flo EFI/reprogrammed A9L/CnC ported Victor.