2nd Gen Non-Engine/Transmission Calculating gas mileage with oversize tires

[FTwardoch]

Guys,



Anyone know the formula for calculating the speedo/odometer correction for running 315 tires vs. stock (by the way, what IS the stock tire size???



TIA

 

[RBallard]

I watched mile markers on the highway, maybe not the most accurate but pretty close. I also compared to radar setups you see along construction zones telling you how fast your going. When I was running 315's, mine was 1/10th of a mile off. When I fill up I would multiply whatever miles by 1. 10 and divide by gallons consumed.



Find somebody with a GPS gizmo to get the most accurate numbers.

 

[TBeckman]

It depends on what your truck is programmed for, when I went from the stock 245/75 R16 to the 285/75 R16 I just looked at revs/miles and made the correction, 7. 5% in my case. I go 10% on the speedo to be conservative.



If you went 245's to 315's it would be a 13. 5% correction

If you went 265's to 315's it would be a 9. 2% correction



245-680 revs per mile

265-654 revs per mile

285-632 revs per mile

315-599 revs per mile



Tom



Just noticed your second question, your stock tire size will be on the sticker in the door jam.

 

[fourwhls]

To be just a little more accurate. Calculate the actual tire diameter or go to the manufactures website and find the rev/mile. With this difference you can figure out the percentage your speedo is off. 265/285/315 etc is the width of the tire not the diameter



I just went through the same scenario and this is how I determined how much I was off.



Old Tires 265/75-16

265/25. 4=10. 433 (divide the width by 25. 4 to convert to inches)

10. 433*. 75=7. 825 (The middle number is a percentage. Multiply the width by the percentage and it gives you the amount of sidewall. )

7. 825*2=15. 65 (Multiply the sidewall by two. )

16. 65+16=31. 65 inches (Sidewall plus rim size=diameter of tire)



New tires 315/70/17

Using the same equation the diameter of the tire is 34. 36 in



Now to determine the percentage the tires are off by use the following equation:



((New Diameter - Old Diameter) / Old Diameter) * 100



((34. 36-31. 65) / 31. 65) * 100 = 8. 56%



You can use the new and old rev/mile numbers also.



@30 ~ 32. 57

@50`~ 54. 28

@70 ~ 75. 99



Hopefully you get the picture.

 

[NWhitney]

Dealer help?

Maybe this one time were the dealer can help you. My dad just put new tires on his '99 and they were 265/55-16 so the spedo was off by 10%+. He was able to take the truck to the dealer and tell them what the revs per mile is on the new tires and they reset the spedo for FREE. Shocked me... . I thought the dealer charged to let you come in the door let alone do anything... oh well.

 

[FTwardoch]

To be just a little more accurate. Calculate the actual tire diameter or go to the manufactures website and find the rev/mile. With this difference you can figure out the percentage your speedo is off. 265/285/315 etc is the width of the tire not the diameter



I just went through the same scenario and this is how I determined how much I was off.



Old Tires 265/75-16

265/25. 4=10. 433 (divide the width by 25. 4 to convert to inches)

10. 433*. 75=7. 825 (The middle number is a percentage. Multiply the width by the percentage and it gives you the amount of sidewall. )

7. 825*2=15. 65 (Multiply the sidewall by two. )

16. 65+16=31. 65 inches (Sidewall plus rim size=diameter of tire)



New tires 315/70/17

Using the same equation the diameter of the tire is 34. 36 in



Now to determine the percentage the tires are off by use the following equation:



((New Diameter - Old Diameter) / Old Diameter) * 100



((34. 36-31. 65) / 31. 65) * 100 = 8. 56%



You can use the new and old rev/mile numbers also.



@30 ~ 32. 57

@50`~ 54. 28

@70 ~ 75. 99



Hopefully you get the picture.

Awesome info and help - thanks. First tank worked out to 19 mpg... ... ... ... ... ... ... ... . love my Cummins.

 

[NVR FNSH]

According to my GPS my speedo is off ~8% with new 315s and ~5% with used (on the verge of being replaced) 315s. I would speculate that 13% is too high and you're not getting that good of mileage.



Brian

 

[NIsaacs]

Actually the first number in the tire size is the diameter, the second numbers are the width. The bigger the first number, the bigger the diameter, the smaller the number of the second one is wider.

 

[SBrandon]

Actually the first number in the tire size is the diameter, the second numbers are the width. The bigger the first number, the bigger the diameter, the smaller the number of the second one is wider.

Actually the first number is tire width. Second number is sidewall aspect ratio to tire width.



315/75r16= 12. 4 wide 9. 3" side wall heigth and a 16" rim = 34. 6 tall!





fourwhls is correct.

 

[NIsaacs]

If the first number doesn't relate to tire diameter, what does? For example, if you are using an 85 series tire, isn't the width all the same in any given tire size? Like 215-235 or 255, are not the bigger numbers a bigger tire. Then in the 75 series, or wider, don't they use 225-245-265 and so on to mean larger didameter ? Then the 70-60- and 50 series are all wider yet. I understand that the larger the first number, the tire might get a little wider, but was under the impression that the series number was the main factor for the width. If I am all wet and giving out wrong info, I am :{





"NICK"

 

[Forrest Nearing]

first number is width is milimeters... 2nd number is the sidewall height as a percentage of width...



so a 300/50 would be 300mm wide, the sidewalls would be 150mm tall.



24. 5mm per inch.

 

[Bill Weber]

Put 315's on my new 2004. Went on a trip to San Antonio. Checked my trip Odometer against 100 miles of mile markers. Also, Checked my time, using a stop watch, with the cruise control on. Did some calcs and my speedo & odo were both off by about 9%. Any slight error on the mile markers is too small to worry about in 100 miles. Came up with the same numbes in 20 -25 mile trips.

 

[NIsaacs]

Forrest, thank you for the simple, yet perfectly understandable lesson on tire size. In all the years of being around these dumb metric tires, I have never fully understood what the size ment. I just thought I did, (my bad). I did some random tire size math with your formula and wallaa... ..... I come up with tire diameters that are very close to what the manufacturer advertises.



Fourwhls, I went back and reread your post, guess what, you said the same thing, only more technical, (it went over my simple head the first time). Now it makes sense.



SBrandon, I want to thank you for the correction to my post and bringing it to my attention with out so much as a simple repremand. You guys are Thanks again!!





"NICK"

 

[Luke Warmwater]

With my 315s my speedo says 35mph and local radar says 39. I pretty much know when my speedo is saying 75+ that I am asking for a hefty ticket

 

[MBean]

Okay, I'm REALLY not trying to kill a dead horse... But on this whole tire size thing, I REALLY need some help. Would someone PLEASE explain to me WHY if I had my stock Michelins (265x75R16) off the truck, off the rim, in the tire shop, sitting next to new Big O (265x75R16) they were the same size (exactly all the way around), but when he showed me the Big O (285x75R16) it was 1" taller (again, off the truck, off the rim)?. ?. ? Also, because I was skeptical of the tire man trying to play hokus-pokus games on me, I measured the height of the truck from the ground to the point in the front at the bottom of the front quarter panel where it meets the bumper and at the rear at the top of the black bumper cover. The truck was 1/2" taller with the 285x75R16 than it was with the 265x17R16. The Michelins were all but new and the Big O's were brand new, of course. Now I have "upgraded" from the Big Os to BFGs and again went with 285X75R16s and the measurements are the same as with the Big Os.

BTW, I don't mean to be argumentative, but I know how to measure and the measurements where taken in the same exact spot (@ the tire shop). Please keep it in mind that it may take you more than once so if you want to e-mail me that would be fine, too. Thanks SO much.

 

[klaybus]

I bought a Abbott ERA box & it recalibrates your speed sensor so your speedometer works correctly. You can check it with a GPS & using the mile markers on the highway.

This thing works great & it was pretty reasonable for $145.



http://www.abbott-tach.com/era.htm



Clay

 

[Bill Weber]

MBean: be aware that not all tires of the same size are the same size. I have found at least 1" difference in width and more in circumference with tires of the same "size". If size makes a difference, take a tape measure when buying. Some of the difference could be tread depth or design of the tread. Or, just plain differences between manufactures.