jakeb

iTrader: (9)

I am going to be swapping a ls1/t56 into a 85 BMW 5 series. I would like to get the best gas milage possible, mostly while on the xway in 6th gear. There are a fair amount of different rear ends available for the chassis I am swapping into.

What would be a good ratio to keep the ls1 in the sweet spot for good economy but not be so low in the rpm range that it is lugging along.

Bob@BruteSpeed

iTrader: (16)

3.42 to 3.55 would be a good gear for what you're looking for. Bob

dodson55

iTrader: (18)

depends on you cruising speed if you are going to be doing 70 or less i would go 3.73 or so if 75 or more something in the 3.42-3.55 range

V68Sweep

On a 700 mile trip I got 25mpg in 6th gear averaging 90mph with my 3.42

jakeb

iTrader: (9)

I normally cruise at around 75mph... I can get a 3.73 without any issue. A 3.42 or around there might be a bit harder to find.

A 3.25 is available without much digging, but that might be a bit low.

Thanks for the help guys I will look for something around a 3.42.

MidwestChassis

iTrader: (5)

Hard to give a good answer with out knowing tire height.

jakeb

iTrader: (9)

Tire will be a 225/45/17 or maybe a 225/40/17...depending on what fits. Most likely the 45. so 25" diameter or 24" with the 40

bww3588

iTrader: (49)

3.25 might be good with a 24 25 inch tire. Stock f body is 26 and some odd change.

MidwestChassis

iTrader: (5)

That is a very short tire and if you want to keep your MPG in check then the 3.25 would be a good choice. I would imagine even with the swap this is a pretty light car? That will play into it some as well.

jakeb

iTrader: (9)

Not sure what you would call light....its not 2000 light. But should be around 3300lbs with the ls1.

A.R. Shale Targa

iTrader: (2)

Agreed on the 3.25 being perfect for the shorter tire.
My M6/3.42 combo easily knocks down 30 mpgs freeway @ 70 mph since the
tach only strolls along @ 1500. C5 rears measure 26.5" tall FWIW

MidwestChassis

iTrader: (5)

Had to utilize Google, didn't remember the older ones being that heavy but the 5 series were heavy weren't they. 3300 is still a good weight to be working with in my opinion.