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Gearing to put behind a mildly built LQ4/4l65

Gtlackey

Gtlackey

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We are building a slightly built LQ4 in my son's 1974 GMC Jimmy as mentioned above and looking re-gear the differentials from the original 3.08 to a posi type system. He is building a daily driver for a teenager's wallet. Expecting in the range of 400-430hp at crank. Cam normal operating range ~1500-5500 on stock converter and 4l65. We are putting a NP205 behind it, but not looking for much off-road or 4wd use. Only occasional for camping.
Here are a couple of decisions we are trying to make and would like some input from you guys.
1. Which carrier for a low stress posi usage? He is looking for bang for the buck. Eaton, Yukon, GM, etc?

2. we are looking at either 3.42 or 3.73 gearing? (Currently leaning to 3.42 to keep gas mileage and highway mileage reasonable.)


we appreciate any input.
 
Yeah tire size is the needed variable here. 33’s with 3.73’s isn’t a bad combo with a 4L65 for power and economy. If 35’s are in the plan I’d go for 4.10’s minimum to 4.56 if you want to have more power on tap.

For the rear diff I think an Eaton tru-trac would be the best limited slip to put in.
 
Use
www.grimmjeeper.com
Put in your numbers. And try for a highway cruising rpm of about 2500. Give or take a couple hundred is fine.
But remember that just because your cruising speed is a lower rpm, doesn't mean it's more efficient.
You get a brick trying to push air at 1800 rpm with a big block, you may be fine. But an LS based engine doesn't have the torque there, so you'd have to push the pedal a little harder to maintain that.
A warmed over lq4 is going to be happier at the 2300-2600 area.
 
shady said:
Use
www.grimmjeeper.com
Put in your numbers. And try for a highway cruising rpm of about 2500. Give or take a couple hundred is fine.
But remember that just because your cruising speed is a lower rpm, doesn't mean it's more efficient.
You get a brick trying to push air at 1800 rpm with a big block, you may be fine. But an LS based engine doesn't have the torque there, so you'd have to push the pedal a little harder to maintain that.
A warmed over lq4 is going to be happier at the 2300-2600 area.
Click to expand...
great resource. Thank you!
 
My bored lq4 with a mild cam was getting 15-18 on the highway in my 2wd 5800# burb with 4.56 and 31.5" tires.
So it's kinda possible to get decent numbers with em. That was long cruising.
Not DD numbers, lol. Probably would have been more like 10 there:thinking:
 
to me transmission is far more important than hp/tq or motor selection when determining which gear ratio. To me, a 4spd auto, assuming you want to use overdrive, is always generally going to be:

<33’s - 3.73
33’s - 3.73/ 4.10 (preferred)
35’s - 4.10/ 4.56 (preferred)
37’s - 4.56/ 4.88 (preferred)
38’s+ - 5.13’s

Once you get into 8-10 spd transmissions, the ratios are so close that you cant follow that general logic. For example, most new trucks come with 3.23/3.42/3.55 with 10spds and 32/33” tires stock because of the gearing.
 
My 6spd tundra came with 4.30’s factory on 31’s and I’m running 5.29’s with 34’s and about to drop down to 32’s. Trans is geared taller than GM but just a point of reference. I much prefer having extra gearing as I feel vehicles are under geared to maximize economy//emissions and not power.

Don’t be scared to gear. Every vehicle I’ve geared from sports cars to trucks has either kept the same or slightly gained mileage.
 
Gtlackey said:
Yeah, sorry forgot that. 2" lift on 33" tires
Click to expand...
Agreed with all above on not being afraid of deeper side of gearing - a lighter use of the skinny pedal in DD situations will be more effective than limited time frame spent at constant highway speeds.

Like mentioned earlier those motors don’t have the abundance of very low end torque in such a low power band - don’t make that transmission constantly hunt for gearing with too low of a final drive ratio.
Newer truck stats use slicker aerodynamics and lesser ratios only to get a non real world snapshot of a misleading MPG numbers on a flat closed course under perfect conditions and limited timeframes - not realistic!

Proper gearing will allow less pedal effort to keep that brick of a truck moving at speed and will net ya better results.

As far as L/S’s go the Torsen type diffs are very user friendly and mild but the clutch friction disc and cone type type of L/S’s are gonna be cheaper.
Just stay away from anything that bares the name Gov-Loc - these diffs were made for the 12B too and may still be offered so beware.

BTW, other than forgetting to include the tire size ( totally understandable as we all get older and no doubt you intended to include it ) all the other info was damn nice to have allowing everyone to paint a mental picture so good advice could be given.
So many times someone will post up a question seeking advice or thoughts and give next to no details of what they are working with.
 
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Years ago i did a 8hr trip in a truck with a vac guage . The higher the # the better the economy. Empty with trailer we could do 4th od on 4.10 / 31" tires . Truck on trailer = 3rd no overdrive .

So as said gear it up and do it one time don’t be afraid.
 
agree with all the above. Gearing like the 3/4T that engine would have come in (probably) will work well in the K5 even though it's a fairly light truck because of the deeper overdrive gear of the 4L65 and the fact that a K5 on 33s at highway speed (60+) is borderline a tow/haul load to start with. If you're not going to be on the freeway much, the 4.10s are still nice because it'll let him use all 4 gears more often than a lower numeric diff ratio.
My parents ordered a '05 Yukon (short one) with the 5.3L and 4.10s and the combo always felt right. We did run 33s on it (295/75-16) and it worked well all around and always turned in 15-ish mpg. I wouldn't expect that milage with the K5 but it'll be about as good as you can make it.

The extra displacement and engine work will make it a lot of fun I bet. That's going to have a torque curve and overall output that I could only dream about from my built 383 with 1990 technology in my '82 K5. It was good for the time and got me through the end of high school, college and a couple years of life after that but I'd rather have had what he'll have.
 
I have a similar setup. 33's with 3.73's and I have excellent acceleration. Great on highway too. I used a Yukon Guragrip in my 14 sf rear-end. However I see nothing wrong with 4.10. Have fun.
 
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