"Tire effects on mileage for dummies"

[CChinman]

I have looked through older posts but am looking for an explanation that even I can undersatnd as to why a bigger tire, BFG 315, screws up odometer/mileage computations. As I go down the freeway, the mile markers match up with my odometer, not exactly scientific but that's all I got.



2003 3500 QC LB 4X4 BFG 315s LINEX

 

[JMoberly]

Take a full roll of paper towels and mark a line with a marker from the center straight down. Match the line up with the edge of your desk and roll it one full revolution across the desk. Measure this spot.



Now use up 70% of the towels or so. Then repeat the experiment with the thinner roll.



Same deal with tires. The bigger one will cover more distance with each turn so if your computer knows ( 1 turn = 3 feet ) it will still think this when 1 turn of your new wheels cover 3. 5 feet. Thats an extra 6 inches every turn. Of course these values are not accurate but you get the idea.

 

[grfraz]

The BFGoodrich website gives you the rpm's (revolutions per mile) of their tires, if you look at this spec chart you will see that the 315 is listed at 601 rpms. This means that when that tire has rolled 601 complete revolutions your vehicle will have traveled 1 mile. If you look at that same chart and find the factory size tire (265/70R17) it is shown to have an rpm of 654, this is a difference of 53 rpms. Our speedometers are calibrated to a factory tire spec, this calculation is based on rpms of the tire. The output shaft of the transmission will turn, at a constant speed, the same number of revs per mile every mile.

There is 63,360 inches in a mile, dividing 63360/rpm of said tire size will get you a circumference of 105. 42 inches on the 315 and 96. 88 inches of cicumference on the stock tire. Now, by multiplying the 53 additional rpms of the transmission by the circumference of the taller tire, 105. 42*53 = 5,587 additional inches the vehicle has traveled for every recorded mile of the transmission.

Isn't this clear as mud or what?



same principal as above post I was just a slower typer.

 

[betterthanstock]

I have looked through older posts but am looking for an explanation that even I can undersatnd as to why a bigger tire, BFG 315, screws up odometer/mileage computations. As I go down the freeway, the mile markers match up with my odometer, not exactly scientific but that's all I got.



2003 3500 QC LB 4X4 BFG 315s LINEX

Did you buy used? Did someone reflash computer for your tires already?

When I changed mine, I got a difference of about 10% between indicated and markers.

 

[CChinman]

Yes, bought used. No idea if the computer was reflashed for bigger tires. I could email seller to find out. Didn't know it could be reflashed to take that into account. Learning new things everyday. Thanks

 

[grfraz]

You can go into the computer and set the pinion factor for revs per mile of tire size. If you got it used I'd almost garrantee that yours has been adjusted for larger tires.

I do not know what the difference is, my taller tires were install at dealership Oo. and pinion factor was corrected when I picked the truck up.

 

[archer2500]

What is the story with putting on slightly larger tires (285 75 R17's) to get better mileage?

Has anyone done this, reflashed and actually seen a positive difference in their mileage?

 

[Bob4x4]

I lost 1mpg by going to 315's with 4. 10 gears



Bob

 

[Bucky205]

Tire Size and Mileage

In my opinion many people think they are loosing more mileage on a larger tire than they actually are. If not corrected the odometer in a stock truck is based on a 245/75/16 tire. If you are computing your mileage with a pen and paper and are spinning 285's, 245's were stock and you used 25. 2 gallons to travel 285 odometer miles, roughly your actual is 285\245 = 1. 16 285miles*1. 16 = 330. 6 miles 330. 6/25. 2 = 13. 11 MPG. I think a lot of people would feel better about their mileage if they corrected their odometer for their tire size. One of the effects on changing tires is rolling resistance. All terrain tires tread patterns offer greater resistance than a street tire and therefore take more energy to turn. This combined with the additional tire weight of an all terrain can reduce mileage. All in all, in the same tire it is pretty much a wash. The added circumference of a larger tire offsets the weight. I have tried different tire sizes in the same tire and normally it works out to within + or - 1 MPG.

 

[Bob4x4]

In my opinion many people think they are loosing more mileage on a larger tire than they actually are. If not corrected the odometer in a stock truck is based on a 245/75/16 tire. If you are computing your mileage with a pen and paper and are spinning 285's, 245's were stock and you used 25. 2 gallons to travel 285 odometer miles, roughly your actual is 285\245 = 1. 16 285miles*1. 16 = 330. 6 miles 330. 6/25. 2 = 13. 11 MPG. I think a lot of people would feel better about their mileage if they corrected their odometer for their tire size. One of the effects on changing tires is rolling resistance. All terrain tires tread patterns offer greater resistance than a street tire and therefore take more energy to turn. This combined with the additional tire weight of an all terrain can reduce mileage. All in all, in the same tire it is pretty much a wash. The added circumference of a larger tire offsets the weight. I have tried different tire sizes in the same tire and normally it works out to within + or - 1 MPG.

My truck speedos are accurate... ... ... I have access to all the parts and software needed :-laf



Bob

 

[Cattletrkr]

I lost 1mpg by going to 315's with 4. 10 gears



Bob

I lost about 0. 5mpg towing when I went to 245/70/19. 5. More than ate up the savings from better tire wear. I can't complain too much though, I've got 75K on the tires and they measure out to have 60K or so left.



I'll go to 225's next time and see what that does to mileage if anything. That is... if I still own the truck if/when the tires wear out.