Changing the the ratio of rears

What model # trans is in the truck? It could be a double over

 

I would leave it the way it is for that kinda work your doing. the 46 rears are almost bullet proof and not very cheap to get parts for when you do need to. I will bet it would cost around $5000 to re-ratio them and that is if they don't find anything wrong with them when they are opened up.

 

Ask the owner how fast he ran and what he got for mileage .. It appears to be geared to run 55 .. Year make and engine may help others to give you advice Ratio swap is 2to4000 from I've heard as it sits now it's geared to pull at slower speeds before I did anything I'd run it a few trips at different speeds and go from there

 

What size tires are on the truck now? If you've got lo pro 22.5's on there, buy used 24.5 rims and the next time you need tires get 11R24.5's. The effect on your RPM's going from LP22.5 to 11R24.5 is the same as changing 4.10 gears to 3.70...but you'll only be out the cost of the rims (which could be mostly recovered by selling your 22.5 rims) and the difference in price between the 11R24.5 tires and the LP22.5 tires you would have bought anyway. And if you aren't happy with the change, it is easy enough to swap back.

For what it's worth, I've got 4.17 rears and 11R24.5 tires and can run 70-75 mph all day. Engines matter, though, because Macks like the 1400-1800 range, and 75 mph is at the upper end of that. If it has a Cat motor or one of the others that prefer the 1100-1500 range, you may be better off with the gearing swap unless you are going to pull really heavy loads or stay relatively local.

 

Changing ring and pinion is very uch a big deal, that truck is speced for Heavy Haul.

 

Depending on the trans model, if you sat at 65mph max, it will be fine. Will pull a hill like a raped ape!
MPG wont be optimal, but you will get way over 5mpg with that setup, as the engine boost will be low most of the time.

Martin

 

That's a bet you'd lose. If your goal is to stay above 5, you'd be extremely pleased to run a truck with my truck's specs. While I've had more than a few tanks better than 6mpg, I have NEVER had one less than 5.