What ratio do I need?

[Pender1]

I've got a '84 with 10 bolt front and rear. 305ci with a SM465 and a NP208 on it. I'd like to swap to a 350ci eventually, but not yet. I'm planning on putting 31" tires on it. This will be mostly a weekend around town/light farm use truck, but I'll occasionally have to go 60mph for about 40 miles to get from one farm to the other. I was looking at going to 3.73 from the 3.08 that are in it now. Would that be the best choice, or should I go on up to 4.10's?

 

[skysurfer]

Of those two, I would go with the 3.73 considering the 1.00:1 final drive of the SM465.

 

[Pender1]

Of those two, I would go with the 3.73 considering the 1.00:1 final drive of the SM465.

Since you phrased it that way, is there a better ratio that I'm not considering?

 

[CSFJ]

What about the 3.42 ?

 

[CSFJ]

I also had an '84 with that same drive line. Had the 3.42 gearset with 31 in tires. Never had a problem. Had the 305 and replaced it with the 350 too.

 

[skysurfer]

Since you phrased it that way, is there a better ratio that I'm not considering?

No, I think you're on the right track. 3.73 and 31" tires will have you doing 60.5 mph at 2450 rpm.

 

[Pender1]

I also had an '84 with that same drive line. Had the 3.42 gearset with 31 in tires. Never had a problem. Had the 305 and replaced it with the 350 too.

Given that most of my driving will be <40mph, I don't think 3.42 gets me enough to make it worth the effort of regearing. I'd much rather have more low-end.

No, I think you're on the right track. 3.73 and 31" tires will have you doing 60.5 mph at 2450 rpm.

I'm going to show my noob status here a little, but what is the typical desired rpm range for highway?

 

[350runner]

I'm at 65mph @1500 rpm with 700r4, 3.08s and 31s knocking out 17.5 @ 70mph. To each his own tho. I think it's really personal preference and your drivetrain combo

Sent from the dust in front of you!

 

[skysurfer]

I prefer somewhere in the 2200-2500 range, but cam profile can change that.

I'm at 65mph @1500 rpm with 700r4, 3.08s and 31s knocking out 17.5 @ 70mph. To each his own tho. I think it's really personal preference and your drivetrain combo

That seems really low but if it works for you... I wouldn't try that combo in the OP's 4wd.

 

[Pender1]

Thanks for the info guys. I'm sure I'll be back with more questions as I dig into this truck some more.

 

[350runner]

That seems really low but if it works for you... I wouldn't try that combo in the OP's 4wd.

Yea it works for me, but like I say it's personal preference plus your drive train combo. That's really what matters I'm sure he would benefit from those 3.73s and like em. Heck I would if I wasn't going for mpgs for my daily or if I was going with bigger tires and rims.

Sent from the dust in front of you!

 

[MadOgre]

I would go 3.42s.

Low granny gear, 1:1 in 4th gear 31 inch tires No need for excessive gearing. 31s will wear to 29 1/2

so unless you want to run high rpms I would go 3.42s

 

[MadOgre]

I have 33s on 3.42s with a 700r4 first gear is 3.06:1 with no prob. I don't use OD because I don't really need to so I leave it in 3rd 1:1 and im about 2500 rpm at 65mph

So with tires 2 inches smaller at 31 and a granny low 1st of 6.58:1 why do you need steeper gears ? I can see going from 3.08s to 3.42s but 3.73s are just getting to be pointless unless you like screaming revs all over town.

 

[77 K20]

I'd only do 4.10's if you were planning on towing heavy on the highway... which it sounds like you won't be. Here is a gear ratio chart that shows gearing vs tire size and if the setup is more for power or gas mileage.

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[Pender1]

I'd only do 4.10's if you were planning on towing heavy on the highway... which it sounds like you won't be. Here is a gear ratio chart that shows gearing vs tire size and if the setup is more for power or gas mileage.

You must be registered for see images attach

Well according to that, 3.73's are the obvious choice over 3.42's. This isn't a daily driver or a long distance truck, so I'll probably do better with the power bump. Unless I step up to a 33" tire the 4.10's look like overkill.