In my never ending search for better fuel mileage, I have a curiosity around tire size. If I go to 265's or 285's I'll effectively increase my gear ratio but I'll also increase the width of the tire which increases rolling resistance. So I'm looking for some emperical evidence from those with larger tires. Did fuel economy get better, worse, or stay about the same?

Thanks

I tried it on an 01 Dmax and saw no MPG improvement empty, and actually lost a small amount when I towed heavy. I do drive my trucks hard though.

I tried it on an 01 Dmax and saw no MPG improvement empty, and actually lost a small amount when I towed heavy. I do drive my trucks hard though.

Rick,

Do you use 265's or 285's ?

In my never ending search for better fuel mileage...
The type of tire will make a measurable difference in fuel economy (due to rolling resistance), though you may have difficulty getting accurate information before buying them. In general, pure highway tread tires with long wear ratings have the lowest rolling resistance. Higher tire pressure helps, too.

But aerodynamic drag is a larger factor at "normal highway speeds" - a very general rule of thumb is that as a percentage of the total (aero + rolling), aero drag is about equal to the speed in mph, and it's probably even higher than that for an empty pickup truck.

Not intended to hi-jack the thread, but I am looking at the exact same situation. I would also like to hear from anyone who runs 235 85 16's

I went to the 265's, and they were both the same style Firestone. I ran the same pressure in both, but I was only running 40 psi then.

If you are driving easy and mostly highway, 285's 'should' help. With 285's instead of 265's you will lower your RPMs by about 200-250 on the highway, but if you drive hard...nothing will really help.

I was holding a solid 20MPG at 70MPH stock tires with only an Airaid filter and intake. I went to 305 50r20 tires and now do 17.8MPG. 2" taller and double the width killed my MPG. Looks good!

I was holding a solid 20MPG at 70MPH stock tires with only an Airaid filter and intake. I went to 305 50r20 tires and now do 17.8MPG. 2" taller and double the width killed my MPG.
Assuming the actual tire dimensions exactly match the size stated by the manufacturer:

For 305/50R20 tires, the diameter is 32.01" and the rolling radius is 16"

In comparison, the OEM 245/75R16 tires have a diameter of 30.47" and a rolling radius of 15.23"

Your aftermarket tire/wheel combination has only about 0.77" larger rolling radius (5% more) than the OEM tire/wheel.

But your mileage dropped by 11% because you increased the wheel width enough that it increased aero drag a little, and the low profile tires also have more rolling resistance.

Been runnin BF Goodrich 285's for a while now and I recently dropped my cruiseing speed from 75-68. MPG has dramatically improved. I drive 150 miles a day to and from work, 4 times a week. I used to burn 1/4 tank each way with cruise set at 74-75...Now, I use cruise set at 68 and I burn 1/8. Keeping the rpm's below 2k is the secret. I went from 15mpg, to a best so far on #2 diesel, to 19.8 mpg. I think the tires shouldnt make much difference. Its how heavy your foot is.

elvis, I have a thread called "who are we taking to". Can you post there and fill every one in. Also anyone else that would like to is invited.

http://dieselplace.com/forum/showthread.php?t=50936

Been runnin BF Goodrich 285's for a while now and I recently dropped my cruiseing speed from 75-68. MPG has dramatically improved. I drive 150 miles a day to and from work, 4 times a week. I used to burn 1/4 tank each way with cruise set at 74-75...Now, I use cruise set at 68 and I burn 1/8. Keeping the rpm's below 2k is the secret. I went from 15mpg, to a best so far on #2 diesel, to 19.8 mpg. I think the tires shouldnt make much difference. Its how heavy your foot is.

Various tires do have different rolling resistance. One thing I noticed is that highway mileage empty for me goes down noticeably with studded tires on, same size as factory same load range, same inflation, same speed, cruise set on 72. Going between portland or and spokane wa I normally get high 18's sometimes low 19's with factory tires. Switching to studs drops it to low 17's high 16's. don't think it is winter fuel because I have made the run with and w/o studs in the winter, same results. The studs are probably the biggest factor, but a true winter tire may be a factor as well.

I wouldn't be suprized if different tread patterns, tire compounds etc would have a small but measureable affect as well for highway cruising below 70mph or so. Above that speed aerodynamics and rpm swamp out most other resistance.

Elvis, thanks for the post. Don't assume the 245s I removed were new. 50k miles and the diameter was under 30". The double width thing can't help either. There is a picture posted some where. http://dieselplace.com/forum/attachment.php?attachmentid=14172&d=1137942846

With the tail gate down I lost .1 MPG. Out side air temperature and fuel quality mess with my economy a bunch.

As for the math, pie are round, cornbread are square!

Tread design can very much affect rolling resistance. If you have a "coarse" or mud type tread, that tire is going to drag much harder, than a smoother tread. The reason is that the lugs slapp the pavement in a way that creates drag and scrubs speed. They also squirm a lot when they hit the pavement, because the lugs have no support around them to hold them still. That squirming action will create resistance. Believe it or not, there is also some aero drag as well, but very little.

Rubber compounds play a part as well. There are certain compounds that roll easier than others. Sometimes, those compounds give up either run out miles or wet traction to achieve the lower rolling resistance.

I think the question was about the size, or more correctly the height and would a larger diameter tire yield better fuel economy. I am not sure how much it will, the reason is because it will take more torque to achieve the same amount of pulling force with the larger tire, because you are effectively gearing the truck higher. A larger tire goes a longer distance/revolution than a smaller tire. Now, what can happen is that the extra power to get the truck moving with the larger tires, might outway any cruising efficiencies that you gain. If a tire drops your RPM's by 100 at the same speed, then theoretically, you should use less fuel. However, if the engine has to do more work to get there, then you negate that affect.

I have to think that Aero plays a much bigger part in the MPG game on these trucks than anything. As the speed increases, the aero drag increases at an exponential rate. For example, some years ago, I was involved in some fuel tests to show the difference different tires made on semi trucks. We weighed fuel to a hundreth of a pound. Kenworth told us that on their T-600 model trucks that from 55 mph to 65 mph cost you 1 mile/gallon. When we did the test, we ran a series and only changed the speed of one of the trucks to 65 from 55. The result was 1mpg, just like they said. So aero plays a huge part in it, as well as speed.

Stolen from another post on another group:

"Rock-Solid Rules
§ Every 2% reduction in aerodynamic drag results in approximately 1%
improvement in fuel economy.
§ Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy
by 0.1 mpg.
§ Worn tires provide better fuel economy than new tires, up to 7% better
fuel economy.
§ Used lug drive tires can get up to 0.4 mpg better than new lug tires.
§ Ribbed tires on the drive axles provide 2-4% better fuel economy than
lugged tires.
§ Every 10 psi that a tire is underinflated reduces fuel economy by 1%.
§ The break-in period for tires is between 35,000 and 50,000 miles.
§ Tires make biggest difference in mpg below around 50 mph; aerodynamics is
the most important factor over around 50 mph.
§ The most efficient drivers get about 30% better fuel economy than the
least efficient drivers.
§ Idle time is costly. Every hour of idle time in a long-haul operation can
decrease fuel efficiency by 1%."

here is the link to the acual paper:

http://www.everytime.cummins.com/every/pdf/MPG_Secrets_Whitepaper.pdf

At the very leasr it's pretty interesting reading.

-Curt

If you are running a tire 10 psi underinflated for the load, you are in a whole lot more trouble than 1% fuel reduction. On a light truck like these that could be around 20% of the total pressure in the tire. That could be really bad on a hot day.

I wish there were a way to publish load inflation tables for Light truck tires on this sight. That would help everyone out. Just weigh the axles with the heaviest load that you would carry, match the axle load to the proper air pressure recomended for that load, and you are done. No guessing. That way, you wouldn't run to much air, which can reduce runout miles significantly and you won't run them underinflated as well. I suppose we would have to get the tire companies permission to do that.

Curt, LOVE that avatar.

I have lost about 1 mpg switching from the stock 245 to 265 with the same driving habits. Rides nicer and looks better than stock so it is a good trade-off for me.

I put 225/70/19.5 Michelin XRV on my 3500 DRW and got about 2mpg better on freeway then the stock tire and wheels. It rides alittle harder then stock but I like the mpg. It also seems to track a bit better.


Mike

I realize this is a little off subject but I make a lot of trips (empty) from SW, AZ to Nor Cal, freeway all the way, same speed, same road each time, 700 miles RT. My mileage has always been very steady. Last 2 trips I went from 55 to 60psi in all tires and saw a 1.6 mpg increase. Never would have thought 5 lbs would make a difference.

I put on 285,s and lost about 3 or 4 miles to the gal.

Would there be any advantage in going to a 17" rim with 285's instead of 285's on a 16" rim?

buy a 2000 dodge. put 315's on it, and get at least 20 mpg doing 70 mph all day long.

my new lbz is a pig.

Here's the deal. The stock setting is for a 31" tire. When you increase tire size you need to have it reset with the tech II. The computer uses wheel speed to gather info that directly affects driveability. My son, who is a GM tech changed mine with the tech II to see the 265-75 size.

how about with 315-75-16's ???