Help with some final drive math

[hat_man]

That's the one. If I read this article right, it's a remote possibility for my truck. When I win the lotto that is. LoL

How to replace a M5OD-R1 with a M5OD-R1HD

[Ecky]

Originally Posted by hat_man

I just don't want to be too slow. The little towns I pass through lead directly into two lane blacktop with few if any safe passing zones. Slowing down other drivers also on their way to work could result in someone trying to pass in a sketchy area. I can't control other drivers but I don't want to make a bad situation worse if you know what I mean.

Any idea on how slow I might be from a dead stop with the 3.27? After reading all this great information, I am again leaning towards that. Unless it's still going to be a turtle.

I can probably improve acceleration by freeing up some usable hp with an e-fan and maybe an aluminum drive shaft. Building a hybrid M5OD-R1/HD transmission would definitely help but be pretty expensive.

Basically, in terms of power to weight, your 5th gear is like my 3rd gear (closer to my 3rd than 4th). If I want to accelerate like you can in 5th right now, I downshift to 3rd. If I want to accelerate like your 3rd gear, I downshift to 2nd. I just have 2 more gears on top of your 5th gear for economy, while effectively having not as many choices in the middle.

My car still scoots in 1st and 2nd. I can just leave it in 1st and 2nd much longer.

EDIT: So it would be somewhat akin to your new gearing being something like:
1.25st gear
2.5nd gear
3.75rd gear
5th gear
6.5th gear

My only concern with taller gearing personally would be whether you'd go past the point where you can no longer reasonably use 5th gear, but I don't think you'd be anywhere near that with a 3.27. Maybe with a 3 or less.

My 2 cents.

[hayden55]

I always take too slow as an opinion coming from owning a 2400lb civic with low compression with a CX trans that spun like 2100 in 5th at 70 lol

[hat_man]

2100 rpm in 5th at 70 mph is somewhere near both the 3.27 and the 3.45


3.27 = 2151 rpm (55 mph/4th gear)
1700 rpm (55 mph/5th gear)
2016 rpm (65 mph/5th gear)

3.45 = 2270 rpm (55 mph/4th gear)
1793 rpm (55 mph/5th gear)
2127 rpm (65 mph/5th gear)

My Ranger at 3100 lbs would be slow then? If I'm reading you and Ecky right it sounds like a bit of a balancing act.

[Ecky]

One perspective is that RPM is really just the number used to determine the relationship between engine torque and vehicle weight. If you have a heavier vehicle but the same engine, you'd need more RPM for it to drive the same. Conversely, if you put a bigger engine in your vehicle, you need to reduce RPM for it to feel the same, and for the engine to be similarly loaded. But, this is why you have a set of different gears to choose from. And you can't directly compare RPM of your truck to that of a Civic in a given gear.

Lowering RPM has a very similar effect on economy and feel to downsizing the engine.

EDIT: A great example is the 2018 Corvette. It weighs 3600# - more than your truck. Its rear axle ratio is 2.41. 8th gear is a very tall 0.65. It has big tires - 20 inch rims, in fact. In top gear it's able to cruise along at 1330rpm @ 65mph. It's a very fast car, but some have reported seeing upwards of 40mpg on the highway despite having a monstrous supercharged 6.2L engine. It can still accelerate well in lower gears, but it's nice top have that top gear and tall final drive for relaxed, efficient cruising.

[hat_man]

I see what you are saying Ecky, but a Corvette has more torque than my Ranger. Even with the really tall rear gear it has no problem getting up and moving from a dead stop. I'm assuming that it's lower gears are rather short and with 8 gears to go through, the transition from short lower gears to really tall FD must be mechanically easy.

I only have a couple of gears before I make the jump to 1:1 and then to FD. I really want the 3.27, but I guess it's fear of the unknown and the cost involved to find out if it was a bad idea. But the flipside to that argument is going with the 3.45 and finding out I would have been fine with the 3.27 and pining about having to pay for the same swap all over again.

Now my head hurts too.

[Ecky]

I'm good at making people's heads hurt.

Seems to me you can either:

1) Go with the safe 3.45, and worry you might have saved more fuel with the taller gear, or...

2) Go with the less safe 3.27, and risk feeling like your truck accelerates too slowly.


I expect the 3.27 will be fine, personally, but I also have very low expectations on how a vehicle should accelerate.

[19bonestock88]

You’re overthinking it... get the 3.27FD, and if it feels too slow, then go for some mods that free up power(alum drive shaft, clutch fan delete, tune, etc)... find some light weight wheels to swap on for an extra kick...

[Ecky]

Originally Posted by 19bonestock88

You’re overthinking it... get the 3.27FD, and if it feels too slow, then go for some mods that free up power(alum drive shaft, clutch fan delete, tune, etc)... find some light weight wheels to swap on for an extra kick...

I like this idea.

Even better, OP could downsize tires just a little next time they're up for replacement. That would help tremendously. Double whammy of increasing RPM, and greatly reducing unsprung/rotating mass.

[19bonestock88]

I got a suggested mod list...(got me dreaming cause I had two Rangers)

3.08FD
205/65 tires on 15x7”RPF1
Alum driveshaft
Clutch fan delete
87oct tune(for enabling e-fan)

And then do a 4/5 drop and clean up the aero... 45mpg anyone?