When cruising on the interstate at about 75 mph, I am pushing about 4 grand on my tack ('78 L82, 4 speed). How can I identify the gear ratio in my rear end and the type of 4 speed in my car? What are typical rpm readings when running at about 75 mph?

'78 L82 w/4speed standard rear gear ratio was 3.70, with the option for a 3.36 ratio. With the rpms you're turning, I'd say you have the 3.70 gears. ALL 4 speeds have the same 4th gear ratio of 1 to 1, or direct drive. You could swap the rear gears for the 3.36 ratio and get your cruize rpms down to around 3000 @ 75mph or so, depending on tire size. 

Thanks for the info Joel. I kind of figured a 3.70 gear with the rpms I was pushing. It just makes it a little hard when cruising with a group on the interstate and I'm sure it doesn't help my gas consumption. I will have to consider my options; but since I just got her back on the road, I probably won't do anything this year. I just plan to enjoy her for a change instead of working on her.

'78 L82 4 speed here also and that rpm is probably about right. Don't hit the turnpike too often and only cruise at about 68 - 70 max and I thinks she tachs about 3500 at that.

I had the optional 3:08 in my Silver Anniv. L82 4-speed.  That would knock 600 rpm off your cruising rpm.

Thanks for the replies. I was curious if anyone else was in the same situation as I am in. I think for now I will just keep her at about 70 as Jeffm suggests for now.