What truck/specs should I get?

A few questions for you guys on the type of truck you would recommend purchasing.

Trucking is a part time job for me, 25,000 miles a year. I'm hauling grain from a farm to elevator. 75-85000 lbs gross pulling a 40' hopper bottom trailer. Elevator is 50 miles away, almost all interstate, 70 mph, pretty flat drive, no hills except the on ramp is uphill. Driving back empty to farm. This is what the truck will do 90% of the time and where it will make it's money. The other 10% of the time will be driving from fields back to our farm where we will store the grain, also on flat ground and not in muddy conditions.

I'm currently driving a daycab 2006 FLD 112 Freightliner with 750,000 miles, Cummins ISM 350hp. It gets the job done but looking to upgrade. Some of my main complaints with this truck is how loud it is inside the cab and how bumpy it rides. Seems like some of the older electronics are starting to go out, wiring is going bad.

I want your opinion on specs I should be looking for.

I want to stick with a daycab.

My main question is about gear ratios with what I'm doing.
I'm thinking somewhere in the 3.36-3.55 gear ratio would be a good place in terms of fuel mileage and what I'm doing, but I don't have much experience in this area and finding some conflicting reports online. Would a 13 speed instead of a 10 speed allow me to go to a higher gear ratio(3.9 or 3.7) and still maintain good fuel mileage? Would I be ok dropping around 2.64 or 2.8 with a 10 speed? I realize wheel diameter matters in this equation too but I'm honestly a little lost here. At first I thought I'd want to drive with a full 80,000lb load at 70 mph and be around 1300 rpms but today I saw a video from Rawze where he was saying that really these newer trucks like to be ran at 1500-1700 rpms. If I'm in 10th gear is my fuel mileage going to change alot from 1400 to 1700 rpms?

Should hp be a big consideration? The 350 I have now gets it done. Alot of the trucks I've looked at have anywhere between 425-500hp.

My initial thought was get a truck with around a half million miles and maybe 10 years old but I've since changed that to looking at 2015's and newer. For a second I had a thought of buying a pre emission truck but it seems any of those in good shape are just as expensive.
I've been told that 2014 and newer is good for Detroit engines, 2015 and newer for Volvo and Macks, and everything else I'd want to try and be 2016 or newer.

If you only have an answer to one of my many questions I'd love to hear from you, if you can answer a few or all, even better.

Thanks for reading and for your insight.

 

I might be stupid enough! Seriously though I'm here for an education, what is the problem with having that low of a gear ratio? Would it be ok having that low if it wasn't an auto?
Thanks for the reply.

 

I'm sure the comfort and noise issues will be solved with any truck that is a 2014 or newer. But yes I've heard that Pete's ride is something else!

 

Thanks for the info on the gear ratios and the speeds! Maybe I should shift my search back to a pre emissions truck, there are alot of them that are cheap and not in the best condition...