Ideal gearing with SM465

[MalibuSSwagon]

1986 C10 2WD, 235/75/15 tires, 305 V8

I tow around 3000 lbs regularly with this truck (car and trailer). My dad is telling me to install 3.73 rear gears in order to tow easily, right now I have the 2.73 stock highway rear gear and its a bit tough to tow with.

What gear ratio would work ideally with the SM465 gearing? About how high will my RPM's be on the highway, right now its overdrive range 1800 - 2100rpm on the highway.

[INSIDIOUS '86]

if you tow regularly 3:73 would do you a world of favor for that 305. you may see improvement in gas mileage despite higher cruze rpm due that you dont have to mash the throttle so much just to get going and therefore use less gas

[MalibuSSwagon]

Sounds good to me.

Second part to question, which 1/2 ton rear axles can I pull from a yard which would have this gearing stock? Year range, etc.

8.5 10 bolt rear on a C10, correct?

[MalibuSSwagon]

Did my research, got the axle codes I should be looking for.

Now what I'm wondering is if I can use a 4WD rear axle, swap out the 6 lug axle shafts for the 5 lug axles,and everything else will be the same, offset and axle width and all that.

Going to hit a couple of local yards after I get out of work in a few hours.

[Edahall]

Originally Posted by MalibuSSwagon

1986 C10 2WD, 235/75/15 tires, 305 V8

I tow around 3000 lbs regularly with this truck (car and trailer). My dad is telling me to install 3.73 rear gears in order to tow easily, right now I have the 2.73 stock highway rear gear and its a bit tough to tow with.

What gear ratio would work ideally with the SM465 gearing? About how high will my RPM's be on the highway, right now its overdrive range 1800 - 2100rpm on the highway.

3.73's and highway cruising at 65 mph will cause you to wish you have an overdrive. However, if towing is the primary use, then 3.73's would be much better suited.

[76stepsidechevy]

Originally Posted by Edahall

3.73's and highway cruising at 65 mph will cause you to wish you have an overdrive. However, if towing is the primary use, then 3.73's would be much better suited.

yep, my truck has a 465 and factory 3.73s and I don't know how anyone ever drove it on the highway. The truck was lifted when I got it but had 265/75/16s on it, Driving home I was doing about 55 maybe 60 at just over over 3 grand. The first thing I did was put a set of 33s on it which put me at 2500 at 65. I still have power off the line and theres always that granny gear if I need to rip a house down or something

[MalibuSSwagon]

With my stock 235/75/15, I'd be seeing 3000rpm at 65 mph, with the 3.73. Not so fun, and would wear out the old 305 pretty quick I'm sure.

Thinking I might pickup an NV3500 to offset that issue, as long as it can take the abuse of towing.

[Edahall]

Here's your RPM's for the various gears assuming 65 mph and 235/75/15 tires.

2.73 = 2063
3.08 = 2328
3.42 = 2585
3.73 = 2819

If you just use the truck for towing, then 3.73's would be ideal. But if you use the truck for cruising, I would consider a gear ratio less than 3.73's. Your engine will be screaming down the highway and your fuel economy will suffer. Also, keep in mind that 3.73's are weaker and often don't last so long towing.

[MalibuSSwagon]

So I've found a local selling a 3.73 posi rear axle from a 1/2 ton Suburban. He said he could do $250 for it. Now I figure this is a good deal, what do you guys think? This is a direct bolt up, correct?

[Pyrotechnic]

Look for straight 6 or 4.3 V6 trucks,. These all got numerically higher gears because they needed them. It seems like V8 trucks, both 2WD and 4x4 always came with a crappy highway gear to get better gas mileage unless something different was ordered.

If you are handy with a welder, you can also check out the rears from 88-98 trucks since they are only an inch wider on each side and will work fine if you weld on the spring pads and shock mounts from your old rear.

3.42 and a 28" tire is a good setup. It's pretty balanced as far as highway RPM and off the line power.

[Taher143]

Hi how are you i want to do custom gear ratio to transmission SM465 please advice me

[JBradley500]

Originally Posted by Pyrotechnic

Look for straight 6 or 4.3 V6 trucks,. These all got numerically higher gears because they needed them. It seems like V8 trucks, both 2WD and 4x4 always came with a crappy highway gear to get better gas mileage unless something different was ordered.

Is this always the case? The use for the 250ci c10 I just bought (see signature for details) would be best suited with 2.76 or 3.08 gears for my uses but I haven't had time to get it licensed or tear the rear cover off to see which gears it has.

On the optional equipment list for my truck there isn't anything about gear ratios.

[barry1982]

Im not positive about the 73-87 Pickup Diffs..... but I know 88-98 4 x 4 diffs are a little wider than 2WD Diffs. Tried to install a diff from a 4 x 4 into a 93 2WD, no go. Axles are longer in the 4 x4.