Quote:
Originally Posted by jmwilso2
Hi Everyone. Just wondering if anyone has an opinion or experience with slightly larger tires on their truck. I dont mean huge mudders but a set that are a little bigger would help fill in what in my opinion is a huge wheel well that looks pretty empty with the stock rubber in it. I know conventional wisdom is that bigger tires equal worse fuel economy. But if I didnt get too carried away and just get them a little bigger, might a larger diameter tire actually improve my milage instead of hurt it.
Thanks
Joe
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Hi Joe -
I've given this a lot of thought and don't think you're going to improve the gas mileage by going to larger tires. Yes, the engine will operate at a lower RPM for a given speed (and this sounds good) but:
1. The load on the engine will be higher during acceleration due mostly to the "effectively" lower gear ratio. This means more gas used getting up to speed.
2. The wind resistance will be higher at cruising speeds due to the increase in frontal area caused by the larger tires. In addition, it's possible that raising the vehicle due to the larger tire radius will increase interference drag (another wind resistance effect) between the bottom of the vehicle and the road. I'm guessing here, but I doubt the interference drag will be decraeased.
3. At cruising speed, the more torque will be needed at the drive axle since the force opposing forward motion (road friction, wind resistance) is the same or higher. This means that force actually applies a higher resisting torque on the axle due to the longer moment arm between the axle and the road. More engine torque requires more fuel.
In addition, since the brake surface is not increased, your stopping distance for a given brake application will be longer.
Now, take what I've just said with a grain of salt, because fuel usage depends heavily on vehicle speed. It's possible that installing bigger tires and driving at a different speed COULD save gas. The problem is, I suspect that new speed would have to be slower, and most of us are not willing to drive that way. The vehicle engineers have tried to match the vehicle aerodynamics, power output, gearing and tires to achieve the best possible fuel economy at the "Federal Standard" for highway and city driving. Obviously, there's a compromise here, and it's possible there are potential tradeoffs. Trouble is, we don't have the lab data to determine the effect of changes.
What we CAN see though is the effect of even a mild change in engine load while maintaining a constant RPM - what you would see if you drive with the cruise control set on level ground, then on a slight uphill climb and a slight descent. If you monitor the instantaneous fuel usage, with the same speed and RPM in all three conditions, you'll see a drop while climbing and an increase descending. But, it won't average to the level ground usage due to inefficiencies. The effect of bigger tires is the same as an uphill climb.
Anyway, this is my analysis.
- Jack