Bigger tires?

[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.

[ticopowell]

Great posts all! I just have a little to add, I put some bigger tires on my truck, 315/70R17's and I did notice a difference in mileage, I expected mileage to go down overall, but I drive a lot between 45 and 75mph and while there wasnt a large bump, it was noticable . I also enjoy the look, I put a 3" block in the front so my truck is a little "over leveled"(2" would make it level), but I love the look and the tires seem to fill the void quite nicely. I have some pics posted on my profile here but if you want to see any specific areas I can go take a pic for ya .
I did notice a large difference in off the line performance, and I use more gas to go, but once I am up to speed I didnt notice any difference. I have 3.73 gears btw, and if anyone wants to donate to me to put 4.10's or 4.56's in feel free!