i have a big tripe planned this sommer going from calgary Alberta to Las Vegas in 24 hours steady,

I was wondering if i run a smaller tire diamiter , if it would give my better MPG. i know the speedo will be messed up but i have a tunner for that.

Any imput would be great

 

That of course would depend on how small you're talking about. Less rotational mass=better gas mileage. Big tires=worse gas mileage, and small tires could even mean worse gas mileage. A smaller tire will make your truck turn higher rpms at the same speed which could make your gas mileage suffer.

 

I would just up the air pressure in the tires to the maximum safe level, less rolling resistance, and if you got torsion bars you can lower them to bring the front of the truck down allowing less air under the truck thus making less drag... but lowering the front end will drive and alignment, so never mind. Up the air pressure.

 

Since you will be on the highway mostly, a smaller diameter tire will actually hurt your gas mileage. A smaller tire essentially lowers your rear end ratio (higher numerically). For example, if your rear end ratio is 3.73, the smaller tires will change your ratio to 3.80 (depending on what size tires you choose). There are good tire/rear end ratio formulas out there to help you with this. If you want to maximize highway mileage, get a taller tire and keep the speed around 65 or 70.

 

Yeah, ran around on my 255/50/16s for a few days and lost about 3-4 mpg.Mostly highway driving. As said above, just air them up some.

 

look at what GM put on the truck at the factory in most cases. relatively tall and skinny with on-road tread.

 

1600-1800 (max) RPM cruising speed and 38-40 psi in the tires will get you max MPG.

 

thanks guys this will help

 

heh, when I want to get the best mileage, I only get up to about 1500 or so and I pump my tires up to 45psi.

 

Lowering the truck is guaranteed to improve mileage. The closer you can get the front air damn to the ground the better. Smaller tires help get lower but the final drive ratio is more complicated. The engine will generally be the most efficient at converting fuel into torque when MAP is high. The best gears and tire combination for fuel economy is going to be right at the point where the top gear of the transmission will sustain 65mph over reasonably level ground without having to downshift to get more torque from the engine. It's going to be a fairly low engine RPM at 65mph.
At high vacuum the engine works less efficiently. At low vacuum (pressure closer to atmospheric pressure) fuel economy will be better from the higher cylinder pressures and better combustion.
29" tires and 3.23:1 gears with a 2/4" drop I'd bet would give you 2MPG better. That would be a 65MPH cruising RPM of about 1700 with a 4L60-E in overdrive. There isn't a lot of low-end torque in a Gen-III SBC or I'd say you could even install 3.08:1 gears and cruise closer to 1600.