35 inch tire with 3.73 gear

[Madhorn]

I am currently running a 5.3 with an NV 4500 OD trans into a 4.56 gear on 35 inch tries. In OD at 2k rpm my mph is 58. I am considering a gear swap to 3.73. Anyone running anything similar? My calculations say that at 2k rpm with the 3.73 my mph would close to 70.

 

[roundhouse]

We are running a LS 5.3 an Nv-3500 with 35 inch tires and a 3.73

I’m not sure exactly what the rpm is at 70 but we’ve pondered switching to 4.11 or 4.56 but Probabaly won’t because of the expense

it doesn’t have the power to pull fifth gear unless the road is totally flat

The least little hill and you gotta drop back to fourth

So your calcs May be correct but I dont think the engine will pull it

And if you measure your tires you will find they are not 35 even though that’s what is stamped in the sidewall

Measure from the center of the hub to the pavement and you will Probabaly get 33.5 or something close

But if you wanna swap you shouldn’t have any trouble finding someone who would swap their 3.73 axles for your 4.56 axles

 

[Frankenchevy]

Not sure what axle you’re running. You may need to do the whole carrier to go from 4.56 to 3.73. I have 3.73s in my d60/14ff, but with a th400 and 33s, so I won’t be much help on the cruise rpm. Maybe 4.11s would be a happy medium.

 

[shiftpro]

If you're looking for fuel economy you won't find it with taller gears. Lower rpm doesn't automatically = better economy.

 

[4WDKC]

I have 305/315 70 r17 which come out to 34.5" with 410s, its not a rocket off the line but its better than 33s and 342s

 

[Sublime]

If you're looking for fuel economy you won't find it with taller gears. Lower rpm doesn't automatically = better economy.

Totally agree, I went from 3.07's to 4.11 and improved MPG. MPG was not the goal either but a welcome side effect.

 

[bucket]

If you're looking for fuel economy you won't find it with taller gears. Lower rpm doesn't automatically = better economy.

Totally agree, I went from 3.07's to 4.11 and improved MPG. MPG was not the goal either but a welcome side effect.

You guys are spot on. The best mpg will be achieved when the cruise rpm is at the lower end of the sweet spot of the power band. If the cruise rpm is below the power band, the engine will lug, mpg will decrease and the truck will be a slug.

 

[Madhorn]

That’s all great info. Think I will stick with what I have for now, run it till spring and see what the real mph and drive ability really is. Only have about 1000 miles on it at the moment and still needs exhaust and AC. Thanks for the input.

 

[yevgenievich]

I have 4.11 with 37" in one truck and it needs 4.56 to even use 4th gear. Other is stock 31" with 3.73 and it could already use 4.11 for 60mph roads. 3.73 gear work good at 75-85mph. 700r4 on first example, 4l80e on second

 

[shiftpro]

I have 4.11 with 37" in one truck and it needs 4.56 to even use 4th gear. Other is stock 31" with 3.73 and it could already use 4.11 for 60mph roads. 3.73 gear work good at 75-85mph. 700r4 on first example, 4l80e on second

Yeah be careful with the 700, they don't like to be 'lugged'. I know that sounds strange but for example, a BB with tons of torque will pull higher gears (and use a boatload of fuel) but put this torque at low rpm behind a 700 and it overheats so fast. 35" tires and 700 = 4.88 minumum gears. A true 35" tire or a 36 is even better to use 526 gearing.

 

[shiftpro]

And if you measure your tires you will find they are not 35 even though that’s what is stamped in the sidewall

Yes this is true but at highway speeds the tire stands up increasing the diameter. Not a much as a drag slick by all means, but enough I think to count.
Also, think of the difference in tire squish between 35 lbs of air vs 45. Not much, but just enough to notice. Yes the tire is a little taller with more psi.

 

[yevgenievich]

The 700 is getting changed to 400 once I find a good core as even without lugging shifting to 3rd with large wheels and any power is not really healthy for it

Yeah be careful with the 700, they don't like to be 'lugged'. I know that sounds strange but for example, a BB with tons of torque will pull higher gears (and use a boatload of fuel) but put this torque at low rpm behind a 700 and it overheats so fast. 35" tires and 700 = 4.88 minumum gears. A true 35" tire or a 36 is even better to use 526 gearing.

 

[shiftpro]

The 700 is getting changed to 400 once I find a good core as even without lugging shifting to 3rd with large wheels and any power is not really healthy for it

Exactly, down hill with the wind at your back...
Regarding your upcoming trans swap, what are doing about the tcase?

 

[shiftpro]

That’s all great info. Think I will stick with what I have for now, run it till spring and see what the real mph and drive ability really is. Only have about 1000 miles on it at the moment and still needs exhaust and AC. Thanks for the input.

We didn't forget about you.
And you have the best advice, drive it, love it, it's all good. I like even higher rpm @ 100Ks (60 mph) with a small block. With a BB 2000 rpm @ 60 mph.
Then when you tromp it something happens.

 

[RoryH19]

Timely discussion as I'm trying to determine which gears to use in my K5.
It now has a 6.0/4L65e/NP241 with the original 3.08 gears.
I'm running 33's and don't plan on changing the tire size.
I was thinking 3.73 but should I go with 4.11's instead?
Assume I will need to change the front as well to match?