I need new tires. The truck has 265/75 on it now. I was thinking about going back to the stock 245/75 size since they are cheaper and lighter. Is there any advantage to keeping the 265 size? Note, I mostly drive highway.

Also, how off is the speedo with the bigger tires on it now?

 

the speedo is about 2.5 mph slow at 65 with the bigger tire. but i would stick with the 265 they look a lot better and wont wear as quickly being they have a little more surface area. but the 245 will get you better mileage being it is less rotating mass.

 

get some 285
speedo will be off by 3mph

 

Tire Diameter (in inches)
245/75/16=30.47

265/75/16=31.65

0r a idea is "265/70/16=30.61" will not thow the speedo off by much and still a wider tire,with a better ride, but wider tire less mpgs

my two cents, hope this helps

oh
and the web site
http://www.gordon-glasgow.org/tirecalc.html

 

im gonna compromise with my next tires. im doing 255/85/16's which means its a 33, but not much wider than my stock 245's. I get height though.

 

My faulty common sense told me that a taller gear ratio would get better gas mileage on the highway. However, I now believe the gear ratio for best gas mileage depends on the number and size of hills and other factors. I have the 3.54 rearend with 245 tires and it is too tall for the hills I drive up. I am turning about 1950rpm at 70 mph. So, going up hills I have the throttle open as far as I can without coming out of overdrive and I can't maintain the speed limit. However, my engine seems really strong around 2200rpm so I think I would get better gas mileage by going with smaller tires or 3.9x rearend. Even though I would be turning more rpm, I think the engine would be moving my truck easier. I used to think low rpm was the key to optimal gas mileage but now I think it is a combination of rpm and throttle position and engine power band. If I change the rearend ratio I'll let you know how it affects my gas mileage.
Peace.

 

Thanks. I found a local tire place that has used tires for $27 total, including mount and balance. I needed a tire immediately today. I'm getting another one by the end of the week. Within a month, I'll have four 265/75s all with over 60% tread for about $110.

BTW, I will probably have one kind of tire up front and another in the back, but the savings is definitely worth it.

 

if you are climbing a 4% grade hill or steeper
it does not matter much for MPG whether
you have 3.21, 3.55 or 3.91 diff gears
or whether you are in OD (ratio 0.69)
or in 3rd gear (ratio 1.00)

Why?

Because at high percent throttle openings
the MPG does not change much.

MPG is low because you are using high horsepower to climb the hill.

I found this out on the steady steep grade of I-75 at Jellico Mountain (Pine Mtn)
using the instantaneous MPG feature of the Oztrip aftermarket Trip Computer.

The Fuel Economy Calculator program from Performance Trends will even predict for you at what hill grade there is no longer any MPG difference between being in 3rd Gear or Overdrive.

If your throttle has opened up
because you are climbing a hill
or towing a heavy trailer
there does come a horsepower load
where the MPG is the same whether in 3rd gear or Overdrive.

This is similar to the same science
where a 3.9V6 Ram may get better MPG in city driving or low speed,
but there comes a speed threshold on the highway
or at a certain steepness of hill
or with a certain load in a trailer
that a 5.2V8 Ram will get equal MPG to the V6
and then above this
the V8 will actually get better MPG.

Want to get better MPG climbing hills?

Slow down a few mph while climbing a hill
then speed up a equal amount while going downhill
to get back to average speed
..... this is the annoying behavior you see 18 wheel trucks do

Overdrive gear is to be used when the throttle opening is too low
and creating 'Intake Pumping Losses'
because a high vacuum exists inside the intake manifold,
which forces the pistons to 'fight against this vacuum'
as they try to descend on the intake stroke.

Note to engineers: i know no such thing as a vacuum exists and what is really happening is the throttle is restricting the free work the pressure of the atmosphere would otherwise do to help the piston descend, as the v versus p graph shows with its wonderful clockwise and counterclockwise loops for the Otto cycle.

 

This is very informative. You seem well educated so I will ask. All things being equal, will going to a 265 width tire from a 245 decrease fuel economy significantly? Also, all things being equal, will increasing the weight of the tire by 5 pounds decrease fuel economy? I'm guessing yes to both. In reality I would be doing both, so I suppose that adds up to fuel economy loss.

Thanks for any help/insight you can provide.

 

i run 285's and love the taller and wider tire. my gas mileage was always 13 regardless of the tire size. my speedo was off alittle but nothing major. i fixed the plenum and did a SCT tune and i am getting 16-17.