is going from a 275-70-18 to a 315-70-17 tire going to kill my gas mileage?

 

I do not have a mileage answer, I would also like to know. I have 275-70-17 tires on my F-250, they look like little donuts in the huge wheel wells. I don't mind the rims, just want a larger outer diameter tire. Post some pics when you get the 315's, I sure would like to see how they look.

 

Yes, anytime you go up in tire size you will do the following. Increase rotating mass, increase ground friction, and increase wind resistance. All 3 of these working against you will hurt mileage. Plus you're lowering your overall gear ratio, which will kill what power you have plus put higher stresses on the transmission.

 

Conversely, you will lower your revs at set speeds.

And if your geared real low, as in 4:10s, this definately isnt a bad thing. And could net you BETTER fuel mileage on the highway.

 

i have a v-10 with 4.10s.

 

going from a 275-70-18 to a 315-70-17 will change the gearing from 4.10 to 3.95.

 

I just checked on both tire specs...

I didnt realize that the difference in diameter was so minimal.

And the 315-70-17 is 12" wide. This will only hurt your gas mileage. If you were simply increasing the diameter, you might see some benefit. But going with a wider tire, I think it will do more harm than good.


275-70-18 = 10.9" treadwidth 33.2" in diameter 628 revs per mile.
315-70-17 = 12" treadwidth 34.5" in diameter 601 revs per mile.

The next step, if you want to get real technical about it, is to find the actual weight of the specific tires we are talking about.

 

I have the 315's with the diesel and I get 12-13 in the city and around 17 on the highway.

 

What gears does your diesel have? And what speed do you maintan on the highway?

 

I replaced my tires with 35 12.50x18 and I have no change in MPG or bottom end. My 2005 F350 6.0 Diesel rides better and stops better! The ride has improved. My 1998 F250 5.4 that I lost due to the recall fire was not the truck to change ti tire size. It lost bottom end power and the MPG went down by 5 - 8 MPG with 2" higher and 2" wider!

 

Sorry, but the braking comment is physically impossible. There is no possible way that something with a higher rotating mass can be stopped by an identical braking system than something with a smaller rotating mass. Its just not physically possible. It may SEEM that its a little better, but if you do a brake distance stop of the 2 tire sizes you'll see that the brake distance is longer for the larger tires.

While the wider tire has a higher friction to the road, it is still not enough to overcome the rotational force to provide better braking.

 

In wet weather it mad a big difference in stopping and when I accelerate from a stop in wet or dry weather it has much better traction. The stock tires were junk I got rid of them at 15,000 miles. The Rancho steering stabilizer made no difference over the stock one. I added a Leveling kit at the same time I put the tires on. The leveling kit, shocks and tires made the truck much better than stock. Stopping is better in the dry surface than the junk stock tires.The stock tires had the ABS working more than I ever wanted it to work. The tires I have are Cooper SST 35x12.50x18 take a look at the tread and the tire is wider. I have purchased things that I thought would improve the performance and I did very little but people at F150 online will not tell you that a product is better than it is. I purchased a K&n Air Filter System for my 5.4 and the magazines told me it would improve the mileage but when I installed it it made no difference! A waste of money but it looked cool and had a better sound.

 

While you are correct that the overall grip on the road is increased over the stock tires, due to the increased footprint, the braking ability of the truck still does not improve. Any added braking ability, as you say, will be in adverse conditions. However on dry surface road larger tires will hinder performance. Why do you think that people have to upgrade their brakes when they go to very large tires? Also one of the reasons the F450/550 use a larger brake system. If your ABS system was going off with the stock tires on anything other than wet or icy roads you're either panic stopping all the time, or you have something wrong. My truck never had any issues with the ABS activating in stock form, and you have a much improved system compared to the 99-04. I could easily feel the difference with my 97 F150 when I went to 35s. I had some of the original MTRs which were great for gripping in slick conditions, but normal use was greatly hindered.

 

The brakes are larger on the 2005 F350. I have two 2005 F550 at work we use every day. You cant compare a 1999 F250 to a 2005 F350 the front end is not the same. We just got rid of a 2000 F550 at work And the 2005 are much better. The ABS is OK and I travel from Pa. to Virginia every weekend. On I-95 the avg speed is about 75 MPH and sometimes someone dose something stupid and I have to avoid them! Oh yes at times pull a 8000 LB. trailer and the stopping is much better and my wife feels much better also(thank god). I think the 33" tires that were stock and the 35" tires are not that much of a change for the size of the suspension. It may be hard for you with the theory system you have worked out but sometime you need to experience the difference when you make a change like I did. I put 2" larger tires on my 1998 F250 5.4 and it took away the bottom end. It was not much bigger but it killed the MPG also! I have to say the tires are not steel belted they are Kevlar a much better riding tire. I have very little road noise and the people at work are impressed with the tires. When these Cooper tires Ware out I am going to put 37" tires on and lift it 4". I know this will make a difference so the rears will have to be changes. I want to keep it stock until the warranty runs out.Don't expect to make changes and feel no change! Every action has a reaction!Go with bigger tires and pay a little more it's worth the time adding it and it feels good.When I drive the 10 wheeler at work we have a belayed time with our brakes because they are air. So we have to allow more room to stop. When I dive my F350 it feels small to me and it's a nice looking truck.:santa:

 

I never disagreed with you about the 05 brake increases, infact just the opposite. My point was that my truck with a smaller, though similar, braking system and 31" tires never had a problem. An 05 with all the upgrades and only 33" tires should have absolutely no problem. and either panic stops or a weak system is the only reason your ABS should be going off. However, there are no brake differences between the F250 and F350. The front axle is 100% the same for a given year brakewise.

I do know about the F450/550 brakes. I regularly drive them at work, and I will gross 26-27k with them. Last year going through TX with a 36' flex draper on the trailer I grossed out at 27,250lbs. Our 06/07 F550 will be delivered end of next month so I can't comment on that particular model until then.

I have no doubts the the truck looks better or rides better with new tires with the extra sidewall cushioning.

As for the theory comment, I don't know if I'd go along with that. Basic physics there. Braking distance is directly related to speed and how much mass must be stopped. Force = Mass x Acceleration. The more force the longer it takes to stop given an unchanged system (brakes). Bigger wheels or tires adds mass, thus making a vehicle harder to stop. It may FEEL better, but real world numbers might disagree, though how much is unknown. The seat of the pants meter isn't always right. I deal in vehicle field testing, and this is almost always the case.

If it feels like it stops better and thats great, but I would say the opposite based on physics of the system and my first hand experiences going up in tire size. I don't doubt that going from a 33 to a 35" tire is going to dramatically decrease braking ability, but I wouldn't say that its going to increase.


On a side note, and to take this thread even further off track, I hope you have better luck with the SST tires than a guy at work has had. He got a set of 285/70/17s on his 04 F150 and he only managed to get 23k out of them. They didn't wear funny, they just wore that quick. He liked them, and thought the same as you did for noise and handling. They pretty much did what my MTRs are doing.