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.

[jmwilso2]

Hi Guys, thanks for all the input. I think what all this boils down to is that increased air drag is a factor but not significant. The torque issue can be solved by determining what additional power my mods are giving me at the rear wheels and only increasing my tire size enough accomodate the additional power I have over a stock setup. So other than getting on a dyno is there a way to estimate power gained from mods? Sure they all publish x additional hp but I doubt the published values are accurate. I've finally figured out how to list my vehicle and mods at the bottom of each post automatically so they should show up now. Hopefully I'll get some photos up soon but the truck is covered in road slop. Its so hard to keep it clean in the winter. If anyone has an idea what the mods might be gaining me I'd be glad to know. I am getting an extra 3 mpg according to the avg economy on the gryphon but that all I know. Which actually brings me to another question. Longshot mentioned getting a vacuum guage, do those read differently than the electronic fuel economy computer in the gryphon.

Thanks again

Joe

[Longshot270]

On the torque, do the math for the tires you plan to get. With the help of Pegasus's gear ratio calculater and some torque formulas Jack has posted on older threads on the same topic, you can use that to help figure out how much you are actually losing. If you are only going up a size or two like I did, that quantity of lost torque is not something you really worry about.

If you only go up a few sizes like I did then the power loss is not as much as you'd think. Yeah there is a SLIGHT decrease and your stop and go traffic mpgs MIGHT take a hit, but you are also upgrading for looks and you pay to play no matter the game. My stop and go city mileage didn't really go down (from 13-14ish), my 45-65 mph mileage went up some (17-21 to 18-23) and my 75-95 mph highway mileage went down a bit but I dont have numbers there because I'm not usually on the highway for more than 20 miles. These all have to do with my driving style though. I plan ahead so I'm never in a hurry.

Depending on the engine I can vouch for the gotts mod. I (independently) calculated up to 9 hp based on quarter mile time comparisons and MAF sensor gains on my 4.6L. I'm also using a drop in K&N filter. Funny thing is that K&N says I should get about 9 with their intake. If you have your doubts test it yourself.

If you want a good idea of your real horsepower without a dyno, go to a local drag strip. Make a few runs that are really close to each other and then weigh your truck. There are tons of formulas that will give you a general number including the one in the pegasus program.

The avg economy on any programmer has a tendency to read high or low. It is one of those things that makes perfect sense on paper but the real world makes it lose accuracy. I'd expect the same effect of the mileage on the truck readout. Similar formula but with different computations.

The vacuum guage is an old school mechanical device that tells you how LITTLE throttle you are using. It hooks up to a vacuum line after the throttle plate and reads the amount of intake manifold vacuum pressure. I think most of the ones you buy in the U.S. and such are measured in "Inches of Hg (mercury)", the original standard in vacuum operated devices. It does not tell you your mpgs but the philosophy is make as much vacuum as you can. The more vacuum means less air is getting past the throttle plate and into the engine...Less throttle = more vacuum = less air to the engine = less fuel usage = better mpgs. You could possibly put estimated mpg readings on it but it will be a multi tier field like on cheap volt meters. Then you'd be spending more time figuring out which number is applicable rather than looking at the road so I don't recommend it.

[Jackpine]

Nice reply, Longshot!

I would not be surprised to see a fuel economy increase in the 50 mph range with slightly bigger tires. I hardly ever drive at that speed in Arizona with our 75 mph interstates and 65 mph country roads (everyone runs about 80 mph and 70 mph on each of those).

About the only time I'm at 50 is in the city, where the speed limit is 45. (I'm keeping up with traffic).

- Jack

[Longshot270]

My programmer makes an electrical average of 15-17ish when I'm either going to or coming back from Cabela's. That includes jumps from 70 to 95 and maintaining that speed. I'm usually anxious when I go there and pissed off when I leave so I dont have any "normal" driving numbers.