Rear Gears vs Fuel Mileage

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

RDF1

I'm Awesome
Joined
Apr 3, 2018
Messages
1,182
Reaction score
971
Location
MS
So I've been thinking alot and trying to figure out if this will work or not.

Quick down and dirty on the setup

91 C1500
2wd Regular cab Stepside (4100#)
5.3 LM7
Factory 706 heads
90# bosch injectors
ISKY 212/212 .561/.561 114 cam
S475/83 Turbo (W/A intercooled)
4L80E (2800 circle D stall)
3.73s
275/60/17s (30" tire)

93 octane

70mph @ 2250 rpms ~18% throttle
High 15s AFR and 40* timing

60mph @ 1950 rpms ~13% throttle
Low 16s AFR and 44* timing



Last few tanks it's been getting 19-20mpg.

I would really like to get it up closer to 25 if possible but I feel like with the 3.73s the engine is just turning too many rpms and it's got plenty of power to pull the hills in 4th with the TC locked so maybe less rpms = less fuel if it doesn't need much more throttle.

I can get another 8.5 rear with 3.08s for $200 and thinking about trying it. Should bring rpms down a good bit. If it doesn't, my buddy can use it for his project truck.

3.73
70mph - 2250 rpms

3.08
70mph - ~1800-1850 rpms

I feel like ~400 rpm drop would help Atleast 2-3mpg. Or I'm hoping.

Ideas?
Past experiences?
Suggestions?


I'm not worried about throttle response, off idle torque, bottom end grunt, low 60' at the track nothing like that. I put this truck together to ride in. And fixin to build a house but it's 48 Miles from work. Which is 24 miles further than what I'm driving now.
I don't drive any in a city it's all backroads, interstate and highway.
If the 3.08s work out I'll have to swap them into this rear that I have. It's already got a Eaton LSD and the tubes are welded to the housing and has Moser? Axles.

Good thing is I could trap in 2nd thru the 1/4 mile with 3.08s hahah.
7000rpm 2-3 up shift would be ~135mph.









Sent from my HiDiNg PlAcE
 

PlayingWithTBI

2022 Truck of the Year
Supporting Member
Joined
Jan 3, 2019
Messages
9,546
Reaction score
14,666
Location
Tonopah, AZ
With that cam and TC locked @ 70MPH I would think you'd be too far out of your power curve to pull hills comfortably (3.23 gears would get you closer to 2,000RPM @ 70MPH). If it's only gonna cost you some labor to find out, might as well try it...
 

RDF1

I'm Awesome
Joined
Apr 3, 2018
Messages
1,182
Reaction score
971
Location
MS
With that cam and TC locked @ 70MPH I would think you'd be too far out of your power curve to pull hills comfortably (3.23 gears would get you closer to 2,000RPM @ 70MPH). If it's only gonna cost you some labor to find out, might as well try it...
It chugs along good down to 1200rpms.
I had 3.42 in my other turbo truck that was heavier and it was 2000@70. It towed like a champ so that's why I was thinking going even higher gears in this lighter truck.

Sent from my HiDiNg PlAcE
 
Top