GM engine swap fuel economy?

[BigRedChief]

I know there are at least a couple of people that have swapped in newer GM engines. What kind of MPG are you getting?

I've been super-impressed with my '01 Silverado's fuel economy. It's a 4.8L V8, 2WD but lifted a little and on 265/70-17 tires. I've seen 21-22 mpg max economy so far. If I could get 15-17mpg with the same engine and a 4WD drivetrain (4L60 + newer x-fer) in the Jeep it might be worth it since I need to rebuilt my transmission anyway ($1200 according to at least one shop).

[JeepinPete]

I'm pulling 16MPG highway or city. 5.3L/NV3500/NP241/35" BFG ATs/3.54 gears. 4.56 gears are going in soon (setting them up in the front axle this week), I'll report back as to what effect they have.

[JPSwapMohn]

Don't know if the LT1 is still considered a newer engine anymore, but I get 16-17 on average running LT1/4l60E/NP229/3.31 gears/32" tires. I will likely go to 4.10 gears eventually. It does well as-is, but think the lower gears would help.

[waynestiles]

Dumb question, but here goes, How would going from 3 something to 4:11 gears improve fuel economy? I always thought that going the other way improved economy at the cost of acceleration, and going from a lower gear ratio to a higher number improved acceleration at the expense of fuel burn.

I've read here that somewhere in there tire size and mass weight is a factor, to the extent that gear ratio can restrict tire sizes, but I'm not sure why or where the break points are.

[JPSwapMohn]

I do not know if it will absolutely help fuel economy, and it is not a dumb question. Basically, with the larger tires and the OD, I run in the 1550-1600 rpm range at 70mph on the freeway. Ideally I should be running at about 2000 rpm to maximize efficiency (as it was explained to me). The concept is that the engine is "lugging" instead of running along smoothly. I will say that I have not noticed this at all, but on a micro-scale I can understand it and understand that the computer works to compensate but there is room for efficiency gain in the middle. At this low rpm, the transmission is more likely to jump into and out of OD more frequently. 3rd gear is 1:1 and 4th is 0.70:1 (if I recall correctly).
I am not sure that I will get better fuel economy, but I do understand operating the engine in it's most efficient power band.

And hey, if it justifies getting better gears, then who am I to argue.. 8^)

[BigRedChief]

It's all about balance... You don't want gears so low that your cruise RPM is excessively high but you can get up to speed lightning quick (unless you're drag or mud racing, I guess). And you don't want gears so high that your cruise RPM is too low and the engine struggles to get the vehicle up to speed.

Going from the stock size tires on my Silverado (245-somethings) to the larger ones was a big step in the right direction for fuel economy. Since the truck has 3.73's (tow package) the gearing was so low that I was getting 17 city/19 hwy (gearing too low) and now I get 19 city/21 highway.

[FSJeeper]

The most significant gains you can get in fuel economy is to swap to a diesel.

[Lindel]

What FSJeeper said. The newer generation engines will get you an improvement in mileage, but it won't compare to the increase that you'll get by going to an oil burner.

As diesel swaps get more common, we might even see support coming from AA and Novak for adapters and the like.

[crispyboy]

It seems to me that changing the engine is only half the equation for getting better mpg. If I were doing a swap I would want transmission also.

[smearig]

It seems like the overdrive transmissions have more to do with the fuel efficiency than the engines do. I would think a 4.8 GM with an NV4500 would be about as good as it gets for gas V8 fuel economy in an FSJ.

[PlumCrazy]

Dumb question, but here goes, How would going from 3 something to 4:11 gears improve fuel economy? I always thought that going the other way improved economy at the cost of acceleration, and going from a lower gear ratio to a higher number improved acceleration at the expense of fuel burn.

I've read here that somewhere in there tire size and mass weight is a factor, to the extent that gear ratio can restrict tire sizes, but I'm not sure why or where the break points are.

It brings the RPM's up, so fuel economy will drop a little, but it will move more fluid in the tranny and extend the life of it. Most of those engines like above 1800 RPM at cruising speed.

One of the reasons I am setting up 3.55+ in the rear of my convertible with the overdrive tranny.

Wayne S

[BigRedChief]

So what's available for passenger drop t-cases that will mate to a 4L60E? And which one would be preferable? I started looking at what's out there and it seems like most of the Chevy stuff is driver-drop. It can't just be easy, can it...

I had another crazy idea... Two electric motors; one driving the front driveshaft, one driving the rear. A bank of batteries in the back. About a 2 liter engine under the hood with a generator setup just powerful enough to keep the electric motors running the Jeep down the road at 65-75mph and to charge the batteries at lower speeds. Even better would be a small diesel or gas turbine powering the generator. A guy can dream...

[JeepinPete]

Its easy, you just need to find a NP241 from a '88-'91 Chevy FS Blazer or Suburban.

[DieselSJ]

What FSJeeper said. The newer generation engines will get you an improvement in mileage, but it won't compare to the increase that you'll get by going to an oil burner.

As diesel swaps get more common, we might even see support coming from AA and Novak for adapters and the like.

From an adapter standpoint, the 6.2/6.5 is the same as a SBC and engine mounts from both sources have been successfully used in swaps. I would like to see them produce a 4BT mount though.