Calculating MPGs w/ 265s

[Piney]

I asked this question several months ago and someone posted a formula to use to calculate the mileage with 265s. I've done a search and cannot find that post ANYWHERE! Anybody know???

[TimofCharlotte]

Piney,
Here's all you need to do. I'm gonna try and help it make sense, and maybe it will be easier to remember. If not...simply remember 1.04 as what you wanted to know.
Let's start with what's at play here. Assuming we're dealing with '75' profile tires only, I'll give some calcs for various sizes, the stock 245/75, then other popular sizes, 265/75, 285/75 and for those reaching for the sky, 315/75. I used info from the BFGoodrich tire spec file on the KO tires that seem most popular among alot of the group here. I wouldn't expect much deviation among other tire manufacturers for the same size tire.
In that spec file, they provide 'Revolutions per mile at 45 MPH'. As you look at the numbers, you will see that the 245/75 has 680 rev/mile, the 265/75 has 654 rev/mile, the 285/75 has 632 rev/mile and the 315/75 has 599 rev/mile. Ok, you probably see the pattern here. The bigger tires rotate less for the same distance traveled, makes sense, right?
So, in order to calculate MPG or MPH, you need to multiply by a ratio of the size you were calibrated with (the 245s), divided by the size you are running, the 265's.
If you now have 265's, and travel 100 miles as indicated on your speedo, you will actually have travelled, 100 x(680/654)= 104 miles. The ratio in this case is 680/654=1.04 , so if you are calculating mpg, just multiply that ratio by your mileage before dividing by the number of gallons used.
If I used 5 gallons in 100 odometer miles, with 245, I would calculate 100/5=20 mpg. If I run 265's, I would calculate (100x1.04)/5=21 mpg. Remember that for the most part, you're truck will only do so much work, so I would expect 'uncorrected' mpg to go down with new tires. I've occasionally heard people say their mileage sucks (decreases) with the bigger tires, and they may not be taking this affect into consideration.
Similarly, if I am running 265s, when my speedo indicates 55 mph, I'm actually going 55 x (680/654)=57 mph.
You can do this same ratio technique with tire diameter if you know the height of each. Just ratio the height/diameter of the new tire, divided by the height/diameter of the original tire, and the ratio should work out the same since revolutions are in direct proportion to diameter.
The revolution figure is probably more accurate because it is determined 'at speed' so it may be taken out some small errors using 'at rest' measurements.
Here's ratios for other tires, versus stock 245's
265/75 Ratio = 1.04 (55 mph on speedo actually 57 mph)
285/75 Ratio = 1.076 (55 mph on speedo actually 59.2 mph)
315/75 Ratio = 1.135 (55 mph on speedo actually 62.4 mph)
Note also that since this is a multiplier, at higher speeds, you will be off more mph than at lower speeds. With 285s at 80 mph on the speedo, you're actually going 86 mph. Sounds like a cheap way to beat the 95 mph top end, eh? ...or...an easy way to get a ticket when you think you're not speeding that much.
Hope that helps. I may have got a little carried away, but I got off work early, and at 2:30AM there isn't much going on... :D :D

[ 05-05-2002: Message edited by: TimofCharlotte ]

[ 05-05-2002: Message edited by: TimofCharlotte ]

[ 05-05-2002: Message edited by: TimofCharlotte ]</p>

[DmaxMaverick]

Piney

Figure your mileage as normal with the odometer miles and multiply the result by 1.035. The formula is simple. Figure mileage as normal with indicated miles and gallons. Multiply the mileage by the % difference between the diameter of the original (245's) and the 265's, and add it to the original figure. The multiplier for 265's, upgraded from 245's is 1.035 (3.5% difference). [corrected mileage = uncorrected + 3.5%]. The formula will work for any tire size change. The multiplier has to be calculated, though. OK?

[TimofCharlotte]

DmaxM, I think you may have had the right approach. I had to go back 3 times to make sure it wasn't too confusing.
Should have kept it simple. Oh well, live and learn. :D :D

[Wally]

Or you could do it this way. Chances are that the the odometer didn't match the original tire size precisely. So next time you get on an Interstate or any other road with mile markers compare your odometer reading to the mile markers. Hint; use the trip meter, it shows the tenths. Best if you can do this for a 100 miles. 10 miles will be really close. Any way, travel a known distance by the mile markers, record mileage on odometer at beginning and end. Divide these two numbers. Example: mile markers=10, odometer=9.6 10/9.6=1.042 from now on use this number to calculate fuel mileage. You don't have to apply this number until you have calculated mpg.
Another example; 350miles/17gallons=20.59mpg X 1.042=21.45mpg.
Sorry for the remedial math lesson :D

[SledZep]

Wally, you evidently trust the people that put those mile markers in a whole lot more than the ones who did your speedometer. Not sure if I would. GPS would probably be the best bet.

[Wally]

Those mile markers are surveyed in with a GPS that is many times more accurate than anything you or I can afford to buy.

[Piney]

Thanks for the help. I love all the examples using 20+ mpg. When I applied the formula I went from 7.8 mpg to a whopping 8.0 mpg!! I guess for you duramax guys to see numbers like that, you'd have to be dragging a house around behind you :D

[AndrewF]

Excuse my ignorance , but do the 265's fit on the same rim as the stock 245's ?? :confused:

[Piney]

"Excuse my ignorance , but do the 265's fit on the same rim as the stock 245's ??"

AndrewF,

Yes they do. Some are even running 285s.

[mackey_62]

Try this link guys.
Can type in our 245-75 stockers and compare them to any size you want.

http://www.venommotorsports.com/howto/wheeldiameters.htm

[Nels]

Or you can have your own home version here (Excel file):

http://members.lsol.net/gop4evr/unlinked/tiresize.xls

I made this up a while ago, before I knew about the calculation sites on the net.