I got 315's on and now everything is off. How do I correct my MPG?

I calculated 19 but that can't be right my GPS says I am exactly 5mph off from the speedo. Am I just having a brain fart today???

 

The amount your speedometer is off will increase as you go faster...I think

 

Tire Calculator comparision for stock to 315s. Maybe it will help in calculations.

Stock Tire - 265/70R17....................Tire Compare - 315/70R17
Width: 10.43 in 265 mm....................Width: 12.40 in 315 mm
Rim Diameter: 17 in 431.8 mm............Rim Diameter: 17 in 431.8 mm
Rim Width Range: 7 - 9 in..................Rim Width Range: 8.5 - 10 in
Overall Diameter: 31.60 in 802.64 mm..Overall Diameter: 34.36 in 872.74 mm
Sidewall Height: 7.30 in 185.42 mm.....Sidewall Height: 8.68 in 220.47 mm
Radius: 15.80 in 401.32 mm.............. Radius: 17.18 in 436.37 mm
Circumference: 99.27 in 2521.4 mm....Circumference: 107.9 in 2740.6 mm
Revs per Mile: 658.2........................Revs per Mile: 605.3

Actual Speed: 60 mph 100 km/h.......Speedometer1: 55.1 mph 91.9 km/h

Speedometer Difference: - Speedometer Difference: 8.739% too slow
Diameter Difference: - Diameter Difference: 8.04%

 

Wow I think I am even more confused! So I need to add 8.7% to my calculation?
Or do I add 8.04% to my MPG calculation?

 

So if I got 19mpg I really got about 20.5%?

 

I think its 9% off of factory reading...

So, basically, @ 60, youre going 65, 70=76, etc

So multiply it by 1.09

 

There are a couple ways to do the math, but I like to convert my indicated miles to actual miles and then do the normal math for MPG.

I use 10% to keep it simple, but 9% is more accurate.

For 9%: Take your odometer reading and multiply by 1.09 and that is your corrected traveled distance +-. Divide the gallons used by the corrected distance and you get your corrected MPG. So if your odometer indicates 500 miles since your last fill and you filled up with 30 gal. 500 x 1.09 or 545 is your corrected mileage. 545/30 or 18.2 +- is your corrected MPG.


Now if you want to correct your speedo reading, multiple the indicated speed by 1.09. So 60 PMH indicated is closer to 1.09 x 60 or 65.4MPH actual. We all know speedos indicate high and you are actually moving 1 to 3 MPH less.

 

I have the 315's also.

Last week I had to make a run down to SC(725 mi.), so I used the odometer and compared with a Garmin CSX GPS I use for hunting. I had calculated 9% and had been using that, but with the extended no stop run, I figured this would confirm it....and it did.

Also, GPS also confirmed 5 MPH speedo off at 65, and pretty much even from 1 mph to 90.

My mileage was around 19.5, but got better when I got closer to SC...must be less gravitational pull toward the equator, thus less drag on the truck....that or the 10 degree weather in Ohio gets worse mileage than the 70's in the south...hmmmm...interesting

Brian

 

Why bother? Just take it to the dealer like I did and have them stick their computer on it and modify the program to suit your tire size.

 

ColdCase has Got it! This is the simplest way.

Now, if you want to get your speedo recalibrated, the input that the dealer will need is how many revs per mile. Just doing the math suggests that a 315 goes around 587 times per mile. However, you can't go just go by the math, unless you get things really copmplicated (and know things such as loaded tire radius, tire pressure, road surface, and more!). BFG sais that their 315 All Terrains go around 603 times per revolution, so that's the number to get your dealer (off by 2.7% compared to 'strait' math). The relative comparison of 9% is still good because the stock wheels will also be off to 'strait' math by about the same amount. Stock wheels & 'strait' math gets you 638 revs per mile, in actuallity this number is somewhat closer to 655.

All of the above is enough to make my head spin! Manual calc - multiply by 1.09 (if you don't get your speedo recalibrated, multiply your odometer & speed by 1.09). If you are getting the dealer to re-program the number is 603 revs per mile.

 

They cant adjust for 315s/35s on '06 models, best they can do is 285s...

 

So if I go from my stock 265's up to the 285's my odometer will show 9 miles (for arguments sake) for every 10 miles traveled? This would mean we could actually get more miles out of a factory warranty right?

I plan on going to the 285's and was going to have the dealer recalibrate everything but I don't mind doing a little extra math to get a few extra miles of warranty. Ooorrrrrrr..... Will the dealer say something if they notice I have not calibrated anything???

 

Yep, basically.

Just dont let them catch that at the dealership, or maybe say you didnt notice it was off...

285s are going to be off less though, 33s vs 35s.

 

I'll just play dumb and say I didn't realize it was off!!

 

They set mine for 285's, a little closer, only off a couple mph at 65. They cannot do the rotation thing on the ABS setup on the 2006 and up models.

CD