Diff ratio help

ah, i get it. so basically, you just want to build a freeway flyer that hauls balls and clips 1800 rpm so it doesn't feel like it's revving it's tits off, and gets "reasonable" mileage.

reasonable in quotes in terms of acceptable for a 650hp motor.

so not knowing what speeds you travel at, i can't give you more than a guess based on the tire size and OD and say that i wouldn't run anything less than a 3.70 gear. but having some experience with the 5.7 G3 in my ram, i can tell you it was most happy in the 1800~2000 range; which sat me between 65~75mph, and anything over that mileage decreased rapidly, anything under that i had to get on the pedal to make the power available.

i understand that's not an apples to apples comparison, but it might give you at least sort of a starting point. the benefit here is that you've got gobs of power so you're not going to be wanting in that department no matter what gear ratio you pick.

here's a link to a gearing calculator that may be of some assistance:

[URL]https://spicerparts.com/calculators/transmission-ratio-rpm-calculator[/URL]
Yes, you got it! Freeway flyer might be more like freeway cruiser, but yeah, 'reasonable' mileage is the aim for longer drives. Cruising speed in Australia is usually 60 miles an hour (posted limit) on highways, and 66mph on freeways (with people usually going 70mph without concern from most coppers). So 2000 to no more than 2200 hundred in sixth at 65mph is probably ideal. Rhat would see me happy for about 1000 of the 1200 kilometre three-monthly trip, while I don't mind getting 3mpg overtaking idiots for the remaing 200 kilometres.
Fuel economy is relative - mainly to the mood I'm in and the wife being absent or present. But I want some economy when I want it and I have a lot more gears in my hand if I need power. So, the more reasonable and experienced advice I get it based on understand what I want, it sounds more and more like a 3.9 diff ratio is rhe way to go - probably best compromise all-round, it seems.