Gear ratios?

[The72]

Hi this is my first time posting, I've lurked through the threads and you are all very knowledgeable I feel like a dummy.

Hoping you guys/gals can help me out.

I'm trying to get my old truck on the road again. It has been sitting for about 10 years. I stopped driving it after high school. One thing I remember was that it was a gas hogg on the freeway, and I'd like to remedy that if I am going to drive it every now and then.

It is a 72 c15 that had a in-line 6 from factory as far as I know. We dropped a 305 into it off an 86 firebird including the transmission. Here is some info I found about the tranny

Transmission
Type: TurboHydramatic 700R4 4 speed automatic overdrive
Fluid Capacity 9.9 pints

Transmission Gear Ratios


1st 3.06:1
2nd 1.63:1
3rd 1.00:1
4th 0.70:1 (overdrive)

Would I have to find me another gear set up on the differential for a more economical cruiser?
Back then it did have shorter tires. It now has a taller tire much closer to a factory tire for 15" wheels. is it possible this taller tire would agree more with my set up and be less thirsty at 60ish mph?

Thanks a lot, if needed I'll post the exact tire size up in the morning. Thanks again.