Bigger tires?

[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

[Jackpine]

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

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

[jmwilso2]

Hi Jack thanks for the reply. From an aerodynamic stand point I cant see the slight increase in frontal area being a major significance. My thinking being that if the tire is say 2-3 inches larger in diameter, half of that increase will be inside the wheel well and thus not part of the frontal area. This means that each tire will have an extra 8-12 square inches of frontal area. If this was the case on a vehicle that is already highly aerodynamic I could see it being a significant factor but not so much on a super draggy truck. As for the interference drag change that is a really interesting point I had not given any thought to. I agree it probably wont decrease it. I cant really see it increasing it either though. But as with the frontal area issue I think the change would be too small to notice. Its the increase in engine load that really has my attention. Your point that the engineers at Ford chose the diameter of the tires, the rear axel ratio and tranny gearing to match up with the power output of the engine to give maximum drivability and economy is exactly what has my curiosity. With the mods and gryphon in my truck I should be getting a bit of extra power to the rear end so would this throw the engineers balance out of whack, and would a slightly larger tire restore the equalibrium. I am at a loss but I have lots of time to figure it out before I need to replace them. I may just go out on a limb and try it to see what happens. But I'd like to get as many opinions as possible first.

Thanks for the input

Joe

[hampsterzone]

I have been where you are and respect your thought process. You must consider ALL factors! What type, what size, how wide, ect. I think the looks of bigger tires out way the minimal loss in fuel economy. Unless of course you go way bigger, then you are making a choice between fuel economy and the benefit of bigger tires. A great size for our trucks w/out lift kits is 265/75/R17. You might have slight differences depending on rim size. Basically, you have to be real and consider where you do your driving. If you go off road a lot then consider that. I use my truck for work that is on a mountain so I bought a reddi lift kit and run 35" tires. But I recently discovered I need to chanted gears to restore HP/torque. I decided to go w/ 4.56 gears. Just consider the pros and cons and then buy what makes YOU happy. You are the driver, not anyone else. What have you upgraded on your truck or are you just starting out? If you need any more advice, just kit me up. I am sure someone on here can and will help you. Much luck and respect.....

[Jackpine]

Joe - I agree that slightly bigger tires (without lifting) probably adds only a minor amount to the frontal area drag. Since interference drag is a super complicated thing, I can't say how much effect that would be either. But, I DO think there will be more resistance to forward motion at a given speed, and not less.

The part that really concerns me is the loss of forward force on the road caused by translating the available axle torque through the longer moment arm (the tire's radius). It's easy to calculate this loss. Going from 31" tires to 33" tires, increases the moment arm by 1 inch, which is a 6.45% increase. This means the "motive" force (push) at the tire circumference is decreased by 6.45% for a given axle torque. That's a significant reduction in the factor that keeps you moving forward. It will have to be made up for somehow, and that somehow translates (in my opinion) to more fuel and air per charge in each cylinder (to get that 6+% back). The applied axle torque will have to be increased by 6.45%, to keep you at equilibrium (a given, constant speed).

- Jack

[Longshot270]

I went from a 235/70-17 street tire to 265/70-17 cooper atr's and I had a slight increase in mileage. It all depends on how you use it. I recommend a vacuum guage so you can keep tabs on your driving. It will probably give you more mileage than any other mod you can do because instead of modifying some aspect of the truck it modifies some aspect of you.