Calculating actual mileage/MPG with oversized tires and stock gears - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > General Technical Discussions > Tires & Wheels > Calculating actual mileage/MPG with oversized tires and stock gears

Savvy Billet LED Tail LightsLight up the holidays with LED lights from JeepHutIntroducing MONSTALINER™ UV Permanent DIY Roll On Bed Line

Reply
Unread 10-24-2013, 07:33 AM   #1
Holder350
Registered User
1995 XJ Cherokee 
 
Join Date: Oct 2007
Location: Whitesburg, Tennessee
Posts: 5,291
Calculating actual mileage/MPG with oversized tires and stock gears

Make sure my math is correct.

Say your odometer reads 287mi and you just put 16 gal of fuel in your tank, your mileage would be 17.93mpg BUT you have 31" tires (or larger)

Stock tires on an XJ are 235/75R15

So doing the math to get that in a standard size would be

Code:
235mm X .75 = 176.25mm/25.4 (mm in an inch)= 6.93" 

6.93" X 2 sidewalls= 13.86" + 15" of wheel = 28.86"
Now it gets interesting.....

We have to find by what percentage the new tires are larger.

31"-28.86"= 2.14"/.2886 (this is one percent of the original tire size) =7.4%

So actual mileage would be 287mi + 7.4% = 308.2mi

Code:
308.2/16= 19.3mpg
Now, some will scream that I used diameter instead of circumference....and apparently they didn't pay attention in geometry.

28.86 X 3.14 (pie..... )= 90.62

31 X 3.14 (p...i...e..... )= 97.34

97.34 - 90.62 = 6.72/.9062 = 7.4% <- amazing isn't it.....ratios are the same because the circumference of a circle depends entirely on its diameter times a tasty pastry containing a filling of sweet ingredients.

So if you had say 33" tires on stock gears....

Code:
33"-28.86" = 4.14"/.2886 = 14.34%

287mi + 14.34% = 328.15mi


__________________
95' XJ Country - Stock Skids - D30/D35 Limited Slip - 3" Lift - 31x10.50R15 - 4.0 HO - AX15 - NP231

98' XJ Classic - D30/C8.25 - 4.5" RC Lift - 33x12.50R15 - 4.0 HO - AW4 - NP231
Quote:
Originally Posted by Azzy View Post
You can tell the character of a person in how they treat someone who can do nothing for them.
Holder350 is offline   Reply With Quote Quick reply to this message
Unread 10-24-2013, 07:48 AM   #2
WXman
Web Wheeler
2001 WJ 
 
Join Date: Jun 2003
Location: , Kentucky
Posts: 3,376
That's awesome, except you're using listed diameter and not real world measurements, so all your stuff is off.

Best thing to do by far is use GPS to verify the true difference in your speed vs. what the speedometer shows, then use that percentage to add to what your odometer is reading when you fill the tank.
WXman is offline   Reply With Quote Quick reply to this message
Unread 10-24-2013, 08:53 AM   #3
hustler905
Registered User
2002 TJ Wrangler 
 
Join Date: Mar 2010
Location: Canada, Eh! Aboot here!, Ontario
Posts: 3,677
The math does seem unnecessarily complicated and likely to show you an inaccurate number. I used mile markers on the freeway on a long drive.

e.g. Went from 108 to 208, but my odometer showed a difference of 110.
hustler905 is offline   Reply With Quote Quick reply to this message
Unread 10-24-2013, 02:45 PM   #4
Holder350
Registered User
1995 XJ Cherokee 
 
Join Date: Oct 2007
Location: Whitesburg, Tennessee
Posts: 5,291
I agree that real world measurements would be different, but those measurements could be found fairly easily with a real world measuring device.(in that case I agree that circumference would be a much more accurate)

I still think its close, and defiantly over complicated haha!

Guess I was bored and my mind got to wondering.......happens way to often

Sent from my phone where correct grammar is difficult.
__________________
95' XJ Country - Stock Skids - D30/D35 Limited Slip - 3" Lift - 31x10.50R15 - 4.0 HO - AX15 - NP231

98' XJ Classic - D30/C8.25 - 4.5" RC Lift - 33x12.50R15 - 4.0 HO - AW4 - NP231
Quote:
Originally Posted by Azzy View Post
You can tell the character of a person in how they treat someone who can do nothing for them.
Holder350 is offline   Reply With Quote Quick reply to this message
Unread 10-24-2013, 09:45 PM   #5
Dukcaln
Registered User
 
Join Date: Sep 2013
Location: Vacaville, California
Posts: 469
31 inch tires on stock gears for a 96 zj, what will the difference in miles be on the odometer? my stock were 225/70/16 @ 710.5 revs per mile. The new tires are 31/10.50/15 @ 685 revs per mile. So if I travel 200 miles what will the odometer read for stock vs larger tires? If stock was 200 miles, then the larger tires will read less miles on the odometer, right?
__________________
1996 zj-v8-79k miles-corbeau seats-exhaust-hood vents-3 1/2 lift-safari rack-arb bumper-amps-subs
Dukcaln is offline   Reply With Quote Quick reply to this message
Unread 10-24-2013, 10:52 PM   #6
Dukcaln
Registered User
 
Join Date: Sep 2013
Location: Vacaville, California
Posts: 469
So I think I figured it out. My odom reads .8 for every actual mile I travel. So 200 miles on the odom is actually 250 miles traveled. So my last tank I figured 11.11 mpg. But since it is .2 off, it was actually 13.8 mpg. All city! Makes me feel better especially since I am lifted and have 31 inch tires.
__________________
1996 zj-v8-79k miles-corbeau seats-exhaust-hood vents-3 1/2 lift-safari rack-arb bumper-amps-subs
Dukcaln is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 01:07 AM   #7
Holder350
Registered User
1995 XJ Cherokee 
 
Join Date: Oct 2007
Location: Whitesburg, Tennessee
Posts: 5,291
225/70/16 calculates to 28.4"

31-28.4 = 2.6/.284 = 9.15%

So take your odometer reading plus 9.15% and that's your actual miles traveled.

200 odometer miles = 218.3 actual miles

Sent from my phone where correct grammar is difficult.
__________________
95' XJ Country - Stock Skids - D30/D35 Limited Slip - 3" Lift - 31x10.50R15 - 4.0 HO - AX15 - NP231

98' XJ Classic - D30/C8.25 - 4.5" RC Lift - 33x12.50R15 - 4.0 HO - AW4 - NP231
Quote:
Originally Posted by Azzy View Post
You can tell the character of a person in how they treat someone who can do nothing for them.
Holder350 is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 08:26 AM   #8
WXman
Web Wheeler
2001 WJ 
 
Join Date: Jun 2003
Location: , Kentucky
Posts: 3,376
See, every tire runs different. One 31x10.50 will be slightly different than the next 31x10.50. Also, the markers on the highway aren't perfect. They are installed by workers who measure off a mile at a time and there's lots of room for error.

I'm telling you guys... the easy way to do this and eliminate error is to use GPS and see what the % difference is between true and speedometer. Then add or subtract that % as needed to the odometer reading when you fill up the tank. If you do it this way, it takes all the measurement and guesswork and variances out of the equation.
WXman is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 08:52 AM   #9
misfit77
Registered User
1996 ZJ 
 
Join Date: Sep 2012
Location: tucson, az
Posts: 652
http://www.miata.net/garage/tirecalc.html

Helps with all my tire size questions
misfit77 is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 09:01 AM   #10
S8NNG8
Registered User
1995 XJ Cherokee 
 
Join Date: Feb 2012
Location: Helena, Montana
Posts: 4,528
I just round it off to 10%. 200 miles on 31's would be about 220+-. The XJ was also calibrated to a 225/75R15, not a 235.
__________________
D3<!L!>3D
S8NNG8 is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 09:33 AM   #11
Holder350
Registered User
1995 XJ Cherokee 
 
Join Date: Oct 2007
Location: Whitesburg, Tennessee
Posts: 5,291
Quote:
Originally Posted by WXman View Post
See, every tire runs different. One 31x10.50 will be slightly different than the next 31x10.50. Also, the markers on the highway aren't perfect. They are installed by workers who measure off a mile at a time and there's lots of room for error.

I'm telling you guys... the easy way to do this and eliminate error is to use GPS and see what the % difference is between true and speedometer. Then add or subtract that % as needed to the odometer reading when you fill up the tank. If you do it this way, it takes all the measurement and guesswork and variances out of the equation.
I use a GPS also on the 98 with 33s and have found my calculations within a couple miles over the course of a tank. (I use 14%)

Also didn't realize 225/75/15s were actual stock size.

Sent from my phone where correct grammar is difficult.
__________________
95' XJ Country - Stock Skids - D30/D35 Limited Slip - 3" Lift - 31x10.50R15 - 4.0 HO - AX15 - NP231

98' XJ Classic - D30/C8.25 - 4.5" RC Lift - 33x12.50R15 - 4.0 HO - AW4 - NP231
Quote:
Originally Posted by Azzy View Post
You can tell the character of a person in how they treat someone who can do nothing for them.
Holder350 is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 10:14 AM   #12
mschi772
Registered User
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 2,360
Quote:
Originally Posted by Holder350
Also didn't realize 225/75/15s were actual stock size.
There were other stock sizes, but the stock speedo gear is most accurate for 225/75r15. 215 and 235 both appeared as stock depending on year and options. Some had 16" wheels, and I'm not sure what the stock tire size was there.

The speedo gear is SO easy to remove and change. Why bother doing all the math when you could just pop a proper gear in there to make you odometer and speedometer accurate again?

Sent from my phone
__________________
Might trade rebuilt 231 for new/rebuilt 242. Contact me.

New sig rules suck.
mschi772 is offline   Reply With Quote Quick reply to this message
Unread 10-25-2013, 03:26 PM   #13
Dukcaln
Registered User
 
Join Date: Sep 2013
Location: Vacaville, California
Posts: 469
my stock tire size per my sticker on the door was 225/75/16
__________________
1996 zj-v8-79k miles-corbeau seats-exhaust-hood vents-3 1/2 lift-safari rack-arb bumper-amps-subs
Dukcaln is offline   Reply With Quote Quick reply to this message
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.
Note: All free e-mails have been banned due to mis-use. (Yahoo, Gmail, Hotmail, etc.)
Don't have a non-free e-mail address? Click here for a solution: Manual Account Creation
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.


Thread Tools






Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.