315's how much do the throw my gas mileage comp off?

[XcumminsX]

hey guys as you all can see from my sig i put on the 315's. I had the 265's from the factory. Since there is no re-calibrator yet for my truck i know my speedo is off, my gas mileage from the computer is off, my miles are off. . the overhead computer says i am getting 14 mpg... i know i am not getting this really, its because of these big 315's and its throwing it off. well untill i find a re-calabration kit Can anyone tell me the formula to figure out the mileage i really am getting. i do about 60% city driving so i expect it to really be like 16-17. thanks

 

[Northerner]

Your speed is 66. 5 mph at 60 mph on the speedo.

Your mileage is 66. 5 miles traveled if the trip meter shows 60

Your mileage should be about 9% better than your calculations show. Thats if I have your tire sizes right. So if your computer is accurate then 14 mpg. is 15. 5 mpg. (662 turns/mile vs. 601/mile)

 

[XcumminsX]

HEY good man. thanks alot.

 

[Moterhead]

I see that Northener has given you the numbers, so you have your answer. Here is where you can find out rotations per mile, height,etc and do a comparison of two different tire sizes.



http://www.csgnetwork.com/tireinfo4calc.html

 

[randyn]

Based on speedo vs gps, the correction factor I use in calculating my speed and mileage is . 083 The theoritical size of tires is rarely their actual size.



RandN

 

[Duck]

Just to beat this horse a little further, your mileage will be off by the same amount your speedo is off.

8. 5% (or 9) is the general consensus, so your true mph is 8. 5% higher than indicated, your true mileage is 8. 5% higher than indicated, and your true MTE is 8. 5% higher than indicated.

 

[alleyoop]

Hi ... . I'm new to the TDR , and its Great !

I took off the stock aluminum 17's with 265/70's and installed some Pro comp 33/12. 50-17's { 305's} on black allied wheels... ... and my speedo was off. I check the internet and found 3 sites for tire calculations..... 2 of which came out the same, and the third was a diff. number. The 265-17's revs per mile according to Dodge is 658 rpm for BF goodrich, the two internet sites said 638 rpm, and one site said 704 rpm ?..... So I wanted Rpm's for the 305-17's I now have on . The internet sites said ..... 596 rpm, and 662 rpm. I went to my dealer and they had them change the setting with there had held computer to 596 rpm to match the two calculations from the internet and drove the truck the weekend. When I was going 70 , it seemed to slow, so I broke out the GPS, and I was going 6. 5 MPH slower than what was shown. I called Pro Comp and the Rpm for the 305-17's is 642..... Took it back to dealer , they re-programmed again with 642 for rev's per mile and now speedo is just about perfect.

 

[600horse]

Rev's per mile

I don't mean to butt in but, when I need to know the true revs per mile all I do is: put the truck on level ground 1. mark a spot on the tire and the groundwhere they meet 2. roll the truck one rev. 3. mark the ground where the tire mark meets the ground. 4. measure the distance from mark one to mark two. 5. divide the distance from your measurement into 5280 feet. 6. this will give you the truest revs per mile. regardless of the published info. and it works right the first time every time. 600 horse