Larger tires = greater circumference = more distance covered per revolution. This means for the same given speed your engine, transmission, and entire drive-line will be turning slower. This could mean for better gas mileage at speed, as you are rolling at a lower RPM, but it will be offset by the need for additional torque. The larger the tires the more torque it will take to turn the wheels and drive the vehicle. This will make your truck feel slower off the line as you won't have enough power to turn the larger radius as quickly. This will probably cause you to stomp on the gas to get up to speed and therefore hurt your economy.
Economy will also be impacted by the added weight of the larger (and heavier) tires, especially if you go for a width increase as well.
Exactly how it will effect you will depend on what you are running now, how your truck is geared, and how you drive. If you never ever jack-rabbit off the line then the impact will be less than it would be for someone who frequently does launch rather agressively.
31" tires are probably a safe bet for most Ranger applications. For what it's worth, that's the maximum size tire Ford has the Ranger officially certified for. Meaning the computer is programmed to accept gear ratios and tires sizes up to 4.10 and 31" respectively. Anything beyond that and you'll either be limited to after market correction methods or left w/ an innacurate speedo/odo..
2003 Ford Ranger FX4 Level II - Silver - Stick - fuelly
2009 VW Jetta SportWagon SEL - Grey - Stick - fuelly
2001 Suzuki Bandit 1200 - Red
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