Tire size and fuel economy

[Fairlane514]

My 84 2wd short bed has 255/60/15 on 8 inch wheels all the way around. I have 3.73 gears with a 700R4 and a mild vortec 350 carbureted.  Wondering if going to a taller tire will help with gas mileage.  Thinking of a 30 or 31 inch tall tire....depending on fitment.

Also, I have lowered the bed with 2 inch shackles.

[Irish_Alley]

essentially every inch of tire you gain will let you lose 60+/- rpms. so if you dont need tires you will be  wasting money but you have a 29" (1966 rpms @ 65)right now, so if you step it up to a 31 if you can fit a 31 on your truck you will be at  1839@ 65. i think the more important thing is rolling resistance. most tire places will tell you about how well their tire scores. then you have air pressure, the higher the less resistance BUT tires steer with the outside tread so if you over inflate your tire you will be raising the outside off the ground. not completely but enough to effect the handling, so (max) pressure is good. you will be surprised at how much your driving habits will effect your millage ie. how hard you take off and what speed you will not go beyond.

[Fairlane514]

Thanks Irish Alley for your reply, it makes sense.  Right now I have 255/60/15 which is 27.05 inches tall so the difference would be much greater like 3-4 inches if I go to a 30 or 31 inch tire. 

The truck does mostly in town driving and not a lot of highway right now, but that could change.

[Irish_Alley]

lol dont know where i got 29 from

[HAULIN IT]

the difference would be much greater like 3-4 inches if I go to a 30 or 31 inch tire. 

But remember, when your doing your figuring...it's only 1/2 of that. You are only using the bottom half of the tire. To accurately figure the RPM, MPH, ect. you need to measure from the centerline of the axle to the road surface. Your 27" tire is not going to be 13 1/2". 
Lorne

[Stewart G Griffin]

It might, but i'm thinking wheel (tire and rim combo) weight and actual tire type/brand/model make more of a difference in mpg.   Since you have a 700R trans, then you are already doing pretty good cruise ratio-wise even with a 3.73 rear.

[BSR63]

Heres a laymans lesson learned for you. I purchased a 76 2wd. The truck was headed to be a lowrider. It had 14 by 10s and some low profile 50 series tires. On the highway the speedo would show 60-70 and I thought that was close. Not so, I was being followed (break down concerns) was told I was about 50mph. I found 4, 15inch steel wheels and went with a 235/70. Gas milege must have doubled.
I googled for a tire calculator and found one that you load whatever tire size and compare to others. I went with a 18X8 and a 235 50 tire. Practically the same height as 15 235 70. If you head down the route for a 35" tire on a smaller rim, I am thinking it will be the resistance gas guzzling 4X4 type stuff. Good luck.