LS3 (376/525) MPG - Real world #'s?

[DidYouTryRebooting]

Hey everyone,

I'd like to know what mpg folks are pulling with the GM Connect and Cruise 525 (LS3 w/ ASA Cam).

I have around 4,000 miles on the block, T-56, 3.23 gears on a 67 Firebird and I'm only averaging 9-13 mpg. Car has stock tune from GM computer that comes with the package.

I know the car is expected to run rich with the overlap but I'm curious about real world baselines for comparison. There are few historical threads on the internet with numbers, and a couple do mention 10-12 mpg, but this is a far cry from the 15-28 people report with a stock LS3 or other variants.

And at a recent Saturday morning car gathering someone with a C3 Vette swore he was pulling 31 on the freeway with the same engine/trans combo.

Appreciate any responses!

[Mkelcy]

What's the 6th gear ratio in the T56?

[streetk14]

I’ve never really worried about fuel economy on my projects, but that’s probably what I’d expect doing around town type driving. I had a cam/header LS3 C6 Corvette (narrow body) that could do high 20’s on a steady, flat freeway drive where I was driving as conservative as possible. Stock C6s are known to be able to hit 30 mpg in the same kind of conditions. Their around town mileage even when stock is significantly worse.

For what it’s worth, a stock LS3 Grand Sport is rated at 16/26 mpg. I had one of these too, and I don’t think my average was any better than 12 mpg the way I drove it. It had headers and a Magnuson TVS2300 blower on it, so not stock.

I haven’t checked to see what my LS427/570 does in my Firebird, but I bet it isn’t good. A C6Z is rated at 15/24 with a stock cam and 0.50 6th gear. Those Zs are light and slippery, and with a cam and 0.63 6th gear, I don’t think I have a hope of hitting those numbers.

[DidYouTryRebooting]

What's the 6th gear ratio in the T56?

It's a T-56 Super Magnum so .63 in 6th. There isn't a much difference between city and highway for the gas mileage so far.

[gjestico]

That is a fairly big cam with a narrow lobe center (226/236 110) . Its not going to like puttering along at 1500 rpm I suspect; which is where you need to be for mileage. You might actually consider a smaller (Gawd!) more modern cam . Id be interested to see what Cam Motion would advise you.

[Mkelcy]

That is a fairly big cam with a narrow lobe center (226/236 110) . Its not going to like puttering along at 1500 rpm I suspect; which is where you need to be for mileage. You might actually consider a smaller (Gawd!) more modern cam . Id be interested to see what Cam Motion would advise you.

This^. I've run T56s behind pretty warmed over 383 SBCs and was always disappointed with the mileage. The engine just wasn't running in the sweet spot when cruising. Now I've got a 0.5 OD T56 behind my LS376/480 (think its the same thing as the 495 GMPP LS3) with 3.71 gears and got about 23 MPG on a cross country trip with NO effort to conserve fuel (think full throttle 4th and 5th gear passing on two lane roads). You may need less cam or more (higher numeric) rear gears.

[icemanrd19]

No mention of rear tire setup or what rpms you’re turning in 6th gear. My old ls3/480 setup would get low 20s with a 315/30/18 crushing 2200- 2300rpms or so around 70-75. Current setup gets 4mpg-22mpg depending on my right foot lol

- - - Updated - - -

Also get the car tuned and move in from that generic tune.

[DidYouTryRebooting]

No mention of rear tire setup or what rpms you’re turning in 6th gear. My old ls3/480 setup would get low 20s with a 315/30/18 crushing 2200- 2300rpms or so around 70-75. Current setup gets 4mpg-22mpg depending on my right foot lol

- - - Updated - - -

Also get the car tuned and move in from that generic tune.

Your mpg with the ls3/480 seems inline with other folks have shared but there are few willing to share their experience with the 525. I've had one person confirm they would get around 10mpg before they swapped to an LS7 cam, but that's all I have so far.

Rear wheels are 20x18.5 with 255/40/20 Pilot's. With the 3.23's, .63 6th of the T-56, RPM's at 75 are just a tick above 1800.

The stock tune is fat and happy but seems more thirsty than I would have expected. I'm hoping to find someone willing to share experience tuning this engine and a few more data points on the mpg.

I'll look around the Vegas area for a decent tuner but I might also explore swapping the cam if I continue taking the car on road trips.

Best!

[DidYouTryRebooting]

You may need less cam or more (higher numeric) rear gears.

Appreciate the response!

I feel a bit ignorant asking you to explain the the change in the rear gears but you're the second person to mention I might consider swapping to 3.73 or 4.11's. How would this help the mpg or just in general?

[streetk14]

I don’t think the 28” tall tires and gearing are helping. I can’t imagine that cam is too happy at 1800 rpm, but I’m not sure going to a 3.73 gear is going to increase mileage.

[hp jnkey]

Here's my story.

My LS3 is the entry level 430hp crate engine with 4L75e, 3.55 gear and 315/30/18 tire.
Stock tune, stock ecu from PSI, 69 Camaro.

I cruised up to Temecula last year, maybe 100 mile round trip, speed
was around 70
I was purposely checking gas mileage so the drive was very tame.
With cruise control on I managed a very disappointing 12-13 mpg.
I was expecting at least 20-21 on freeway runs and 15 around town.

My C6 with A6 auto was getting 25-26 on that same run.

A couple months later I ran into a guy with 69 Camaro at Cars/Coffee with same engine but he
was running 6 sp stick. Said he was getting 24 mpg on a freeway run up to
Corona and back. You pass thru Temecula on the way to Corona, so his trip was
similar to mine, just further.

He gave me the name of the tuner.
Tuners name is Antoine, he contracts to Turn Key Engines in Oceanside.
Antoine told me he has 12 years experience tuning.
Will get him to look at my car one day.

Same car with the old 327 with TH350 and 3.91 gears screamed down the highway and it was getting 10mpg

I considered the 495 hp crate engine but it has the GM hot cam which I read is difficult to tune
so I stayed away.

In comparison I have a 71 c10 shortbox with 5.3 and 4L60, 3.73 gears, it's getting 16mpg, as expected
and that's city and highway mileage.

[Mkelcy]

Appreciate the response!

I feel a bit ignorant asking you to explain the the change in the rear gears but you're the second person to mention I might consider swapping to 3.73 or 4.11's. How would this help the mpg or just in general?

If the engine is lugging - not operating in an efficient RPM range - when cruising, you need to get it into that range. Spinning the engine faster may actually improve mileage. While extreme, try running two fuel efficiency tests (top off, drive 50 plus miles, top off, calculate mileage, drive back, top off, calculate mileage) cruising in 5th one way and 6th the other. If they're close, you're likely lugging in 6th.

By the way, my gearing in 6th is almost identical to yours, so I'm looking hard at the cam as the culprit.

[gjestico]

I tend to avoid OEM Ecus on swaps unless there is a good reason, EG auto trans control. The OEM system of mixture control is very emissions based and always trying to target stoich.
I realize its abit late for your situation, But aftermarket ECUs often have the option of an integrated wideband O2 to allow you to run different target AFRs depending on where the engine is running. You can set a target AFR of 16:1 for example in the cruise rpm/load area you would be running in on the highway. If you get the spark advance right you probably could net some small amount of MPG.

[Jimbo1367]

Change the cam and tune. Throw in more geAr if u want more acceleration. Definitely needs a tune though