MPG help

[notlimah]

Hey guys, so finally had a chance to get a full tank, and drive a good amount of 99% highway driving and trying to get as accurate number as I can. Only thing I'm having an issue with is compensating for my bigger tires and my speedo not being calibrated for it. Anyone know how to do that?

 

Here's my numbers:

 

Trip meter = 425.8 (has 285/70/17s when speedo is set for 265/75/16s so this is biggest concern)

Used 22.3 gallons of diesel

 

Hand crunching those numbers still comes out to 19mpgs which isn't terrible but pretty sure it's a little different due to tires/speedo being off. Thanks in advance!

[Dieselfuture]

https://tiresize.com/calculator/

285/70/17. 102.7 inches 

265/75/16.  99.4 inches 

63360 inches in a mile

63360÷102.7=616.94 rev/mile

63360÷99.4=637.42 rev/mile

That's 3.21% different 

425.8÷22.3=20.3mpg 

20.3 at 3.21% should be around 19.65 mpg

But I suck at math,  so... 

Edited July 21, 2017 by Dieselfuture

[test user]

Don't even start this method... It does not work. 

 

Grab a GPS and track your mileage/odometer. Do not attempt to calculate mileage from a percent offset it will not come close. But you'll find that if you track with a GPS total distance you numbers are correct.

[Mopar1973Man]

Opps... Wrong user I've been testing site functions...

[notlimah]

Well I know this method won't be the gnats a** accurate, I have no plans on getting a GPS so close enough will work for me as I'm really more just trying to get an idea anyways.

 

Here's what I came up with

 

old tire diameter 31.6

new tire diameter 32.7

 

31.6 / 32.7 = .96 - 1 = .0336 or .035

 

So trip meter showed 425.8 x .035 = 14.9 + 425.8 = 440.7 of 'corrected' trip mileage

 

440.7 / 22.3 would be 19.7mpgs so really not that far off from uncorrected but still nice to know it's a little more accurate. 

[Mopar1973Man]

Heck you can get on the OBDLink LX or MX dongles and log it on your tablet or phone. The OBDLink App was just updated to include tire offset so you can get correct mileage records. 

[notlimah]

Ohhhh that I did not know. I'll have to check that out. Going to be making another trip tomorrow. What do I need to change for an xzt being hooked up?

[Mopar1973Man]

Like with my Quadzilla I run slightly higher (71%) volumetric efficiency percentage from stock (63%). The volumetric setting calibrates your fuel consumption rate. Then I've got my ABS reprogrammed with @Chris O. CCD tool. But there is an offset setting in the vehicle editor setting for speedometer offset. This will calibrate your speed and distance.

 

http://www.obdlink.com/