Would gas mileage improve going from 4.56 to 4.10 gears 72 K10 with 35s

[fishmunger]

Have a mostly stock 72 K10, 350, auto with 35s and 4.56 gears. It takes a lot of RPMs to go down the highway. I'm wondering if it is worth going to 4.10 gears, if it would actually make much of a difference fuel economy wise. Reason I ask is, I am probably going to have both front and rear axles rebuilt, and that would be the time to swap the gears.

I'll be using the truck to frequently tow my 4000# boat (mostly highway), if that matters. I would think 4.10s would still be low enough to not be sluggish around town?

I don't have any plans on going to an overdrive trans.

Thanks for your opinions!

[Greasey Harley]

Maybe, but I wouldn't expect much gain in the mpg department.
It would probably drive better down the highway
4:10 gears would put you around 2755 rpm's@65 mph, as apposed to the 4:56 gears which have you around 3068 rpm's @ 65mph.
If you're getting 13mpg with either one of those combos, you're doing great.

https://www.crawlpedia.com/rpm_gear_calculator.htm

[tdangle]

Those numbers are assuming a manual transmission. If you have an automatic 3 speed add about 300 rpm or so for the torque converter slippage.

[fishmunger]

Thanks guys. So I'll be spinning about 3300 rpms to go 65 mph. Despite the high revs, you think the mpg gain would be negligible if I dropped gears?

It it any 'easier' longevity-wise on your drivetrain (rear end/trans) by using the lower gears in a lifted truck with big tires?

I figure the small block is not gonna care all that much whether it spins 3000 or 3300 down the highway?

[Ziegelsteinfaust]

Tires have alot to do with mpg, and probably more then a gear change. Given you have 35's they are likely offeoad vs AT tires.

If you have mudders or some other aggressive tread. Less rpm can actually mean less mpg. Due to more strain on the engine, and more fuel to run the same speed. We're a extra 500 or so rpm the engine puts down more power with less fuel, and can overcome tire resistance easier. Except maybe on your ears.

A friend i had 20 years ago built 283 in a K30 rclb, and compared to his friend with a 396 in a K20. They got the same highway mpg, but once off road his fuel supplies would last twice as long. That is what I was told. He had 5.13's gears with 35's.

Remember its a combo not just a part.

[burnin oil]

Before I spent the money on gears I would get a 4spd auto that has OD.

I have 4.10s with a nv4500 in my 72 k20. I keep toying with going to 5.14s since the motor is cammed pretty good. I am on 32s currently.

[kwmech]

Something is off with those numbers. on a 1-1 ratio you should be in the 27-2800 rpm range. Clearly not over 3 grand. Anyways a drop from 4.56 to 4.10 is only 10 percent which will net you 3000. Either your speedo is off or the tachometer

[fishmunger]

I'm at about 2600-2700 rpm at 55 mph, which fits what the calculator says factoring in a couple of hundred rpm for auto trans slippage?

And I've pulled both diff covers and verified my gears are 4.56

[b454rat]

Math shows 4.56 gears, 35” tires, trans 1-1, rpms a touch over 3k, 70mph. Going to 4.10s will be couple 100 less.

[fishmunger]

yeah you're right b454rat. but tdangle was saying you gotta go +300 to those numbers to compensate for auto trans converter slippage?

[mongocanfly]

the difference between 4.10s and 4.56s is very close to the difference between a new tire and a tire that's at the end of its life..
with 4.10s and 35s your 65mph rpm would be 2558
say you had 4.10s and your 35s were worn down to 33s your 65mph rpm would be 2713
diff of 155 rpm

not accounting for any slippage
with 35s-4.56@65=2845rpm
with 35s-4.10@65=2558rpm
diff=287rpm
so the rpm drop is minimal

this is the one I use... http://www.csgnetwork.com/multirpmcalc.html

but say you kept your 4.56s and your 35s but swapped to a 700r4 that has a .7od...then your 65mph rpm would drop to 1992
I know you may not want to change trans but that would be your best option for a significant rpm drop and keeping your low gears...
and probably cheaper than doing a 2 axle gear swap
and when you tow your boat you can always stay out of od ( if needed )

[tdangle]

Originally Posted by mongocanfly

the difference between 4.10s and 4.56s is very close to the difference between a new tire and a tire that's at the end of its life..
with 4.10s and 35s your 65mph rpm would be 2558
say you had 4.10s and your 35s were worn down to 33s your 65mph rpm would be 2713
diff of 155 rpm

not accounting for any slippage
with 35s-4.56@65=2845rpm
with 35s-4.10@65=2558rpm
diff=287rpm
so the rpm drop is minimal

this is the one I use... http://www.csgnetwork.com/multirpmcalc.html

but say you kept your 4.56s and your 35s but swapped to a 700r4 that has a .7od...then your 65mph rpm would drop to 1992
I know you may not want to change trans but that would be your best option for a significant rpm drop and keeping your low gears...
and probably cheaper than doing a 2 axle gear swap
and when you tow your boat you can always stay out of od ( if needed )

This is what I did. Had a TH350 and 4.10 gears. Swapped out TH350 to a 2004r (any OD trans would be great) and lowered freeway rpm down quite a bit. With the lockup torque converter 65 MPH is about 2000 or so. Much nicer