I've been playing around with the idea of dropping an LT1 with an M6 in my 90 240sx in the coming years. Lets not turn this into a debate of fitment because it has been done before yadda yadda.

With the significant weight loss of going to a much lighter shell - do you think the gas mileage would greatly increase? Keeps the stock 240sx rear that has a 4.06 ratio.

 

Any time you decrease weight you are going to help the gas mileage. The key here is "greatly increase". I don't think you will greatly increase the gas mileage.
On a side issue, why an LT1, why not an LS(X), which is already a lighter and smaller package that gets great gas mileage.

 

shouldent have a problem getting 25+ mpg......

 

Ls1 would be more expensive I thought...

 

I get that now in my car!!!! On the highway that is.

 

Yes, probably. But they are coming down. I've seen LS1 "pull outs"(complete used engines) for as little as $500.00, most are quite a bit more. And I'm sure the chassis mods. for this setup would be more also. Would be a great package though.

 

There is a group of fellas from another forum I frequent that are going to build kits to drop this in. Since they've done it already on a few 240s

 

I have a LT1 setup in a 88 Rx7 with a auto with the mazda 4.10 gears. My motor is a stock longblock with bolt-on's, Yank SS3600 converter. My car seems comparable in weight and gears to a 240.(I think )
I recently took a road trip and got 22mpg, mostly highway with a few stops for food & lots of stop & go traffic. This is with the converter locked in overdrive. On this trip I'd say the car's weight was somewhere near 3200 including, me, my co-pilot, and tools.
I've heard of people with setup's like mine with the T56 getting upper 20's on the highway.
With that said I've pulled down 27 on the highway in my camaro. From what I've seen the extra weight doesn't hold back the MPG too much on the highway.