Tire pressure/mpg?

[akdiesel]

I guess this is the place since it is on a HD truck.

Ever since I had my 285's installed I went from 17 mpg to about 11 mpg.  I checked the lugs after about 100 miles per the installers suggestions.  They were tight.  I had a different tire company put some tires on Grand Cherroke before and found 1-2 lugs loose on each tire.  I normally swap my tires around my self but these were both new tires.

After about 500 miles I checked them again. Still tight but then I also checked the air pressure.  All four were at 45 psi, when they should be at 65. :angry:  My ingnorance I guess.  I checked my speedo with a gps and found that I am of about 3.5 mph higher when doing 65 mph.  I am sure that my shift points are off of there sweet spot.

Could the 45 psi tire pressure make that big of difference in mpg?

I filled the tires to the specs, poured a bottle of Stanadyne fuel treatment and a full tank of fuel in.  I will post results when the tank is done.

[Shaners]

A couple of things here...

 

Did you account for the difference in mph's when you made your calculations?  For every 65 miles you logged, you traveled 68.5 miles which means if you got fuel at 325 miles you actually traveled 342.5 miles.  That could amount to at least 1/2 mpg (it's not much but I'm trying to help you find your lost mpg's).

 

Also, your in town driving will be affected too.  It is like changing rear gears in your truck when you up your tire size, so it takes alot more effort to get the truck rolling, meaning you will have to hold your foot on the gas peddle longer to achieve the same acceleration you are used too.

 

Thirdly, you have essentially raised your truck further into the air, and the extra disrupted air-flow will pull down your milage some.  Guys that have not changed tire size but have lifted their trucks, will normally report a loss in mpg's.

 

Lastly, low tire pressure will cause more rolling resistance and will reduce fuel economy, but your 45 psi isn't really too awful low.  Adding 10 psi to them should help some...

[RyanbabZ71]

you probably dont wanna run the 285s at max psi (65).  I run mine at 52 front and 50 rear.  No noticable change in mpg going from 265s to 285s

[gandolphxx]

Shaners, that was a great analysis

 

You really need to get the speedo corrected - dealer can do it or get a HPP3 or SCU2000 - I have also experienced some kickback with ABS when it isn't set correctly.

[hogger999]

Will correcting the speedo effect shift points?

I was really suprised with the loss of low end tourqe after I went from 245s to 265s

[gandolphxx]

I am way out on a limb here, but have experienced the same thing in reverse - reset, and it is fine. Any transmission guru's handy?

[akdiesel]

shaners,

I never even thought about the extra mpg hidden due to the oversized tires and the speedo being off.  I have about 50 miles on this tank right now and I am going to keep the tires at the 65 mark for some real numbers.  After the test I will drop them down to 50-55 psi.  

I have noticed a slightly better mpg in town than on the highway.  Avg. 65-70 mph corrected on the highway.

I have heard that the dealers can only adjust for 265's.  But even that will be better.  I hope.

[akdiesel]

I am still only getting around 12 mpg!  Not including the extra 1/2 mile or so for the bigger tires and the speedo differences.

I am going in to the dealer in two weeks for a so called "Computer Adjustment" to try and get my tranny back in sinc.  The guy making the appointment was not that familiar with the Duramax so he did not really have any idea what I was talking about.

Has anyone really ever had this done to there Dmax?

[Moonlight]

"If your speedometer has been calibrated for 245/75 R 16 tires, replacing them with 285/75 R 16's would result in the speedometer reading low by 7.20%" from the web site posted below ...

 

I have used the web site below for a couple years to compare tire sizes, speedo differences, gearing ratio's, etc.  Based on the fact the factory tire is a 245/75R16, I come up with a 7.20% difference for 285/75R16's.  If the speedo has not been calibrated and reads 60.32 mph your actually doing 65 MPH a difference of 4.68 mph.  So guys with 285's would compute my mileage by taking tripometer reading X 7.20% (.072) resulting in the difference to add to the tripometer reading to get total miles.

 

So if the tripometer reads 325 ...

 

325 X .072 = 23.4 miles difference

 

325 + 23.4 = 348.4

MPG = 348.4 / gallons

 

I actually run 265's so my difference is 5.11% I just remember the percentage and do the math when I fill up.  I actually keep a spreadsheet on the computer with the formula built in ... that way I just enter the date, tripometer and gallons and the mpg is computed automatically.  The web site is very useful ... check it out.

 

http://www.c5-corvette.com/tirecalc.htm

[Moonlight]

"As far as gearing goes, if you have a 4.11 rearend and 245/75 R 16 tires, changing to 285/75 R 16's would be the same as if you kept your current wheels and tires but changed your rearend gears to 3.81's"

 

From the web site in my post above ...