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.

[72longhornTX]

I have been using this calculator for years now, you select your trans and put in any info needed if it's 4wd or has an after drive unit on there. Then your tire size and gear ratio. Towards the bottom it will give you the RPM at a given speed in each gear.

http://www.grimmjeeper.com/gears.html

Big thing is, if you want it to be good on the highway. Find a gear ratio with with the tires and trans you have to get you under 2500 RPM at 75 or 70. Which ever you prefer.