Calculate milage w/265's

[CCPerformance.com]

Does anyone know what factor or formula to use in order to correct MPG with Firestone 265's. These are 1" larger than factory 255's.

It appears MPH is off about 2 mph at about 75mph

Thanks

[mackin]

CCPerformance.com,

Here you go .......

===>http://www.intercotire.com/html/calculators.htm

MAC :D

[CCPerformance.com]

Thanks Mac:

Perhaps I'm dense, but how do I calculate MPG? I now know actual MPH. How do you use this info to calculate true MPG? Per teh MPH formula, the MPH ranges from about +1.3 - +3.6 depending on speed. You should be able to calculate the MPG based on the effective gear=ratio change of the tire size increase.

Thanks

[mackin]

CCPerformance.com,

The only conceivable way would be if you drove only one speed ...... To many variables ....... For instance all you need is true MPH ...... You will not get it without recalibrating your speedo .... Cause, if you went from 30" OD to 32" OD ...... @65 actual would be 69.3 only 4.3 off ..... @ 75 , actual is 80, a full 5 off .... So no way to figure actual amount traveled with your onboard mileage record keeper unless you recalibrate "it" ....... Make any sense? It is not consistently off ...... One cannot judge by gear ratio, terrain, speed, coasting, again to many variables ......
Me, if I'm out of fuel, I don't ask why, I just fill it up .......
MAC smile.gif smile.gif

[CCPerformance.com]

Hey Mac:

I don't think you understand my question. I could care less what my mph is. My only concern is with MPG. ie, how many "actual" miles traveled per haw many galons of gas used in my tank. I've gone from 30.6 tires to 31.7, therefore increased OD by 1.1. Consequently, my trip meter/speedo is assuming I have a smaller tire than what I actually do, therefore I am actually traveling more miles than my trip meter is telling me by some factor. I need to know how many "actual miles I am traveling based on an additional 1.1" of tire. I currently have 3.73 rear end so this 1.1 is perhaps making this an effective 3.63, etc. Get my point? On paper this is no differant than changing my axle gear. The rollout on my tires will effect how many miles traveled & this is what your speedo/trip records.

For example if you jacked up your rear end and put a brick on the gas pedal your truck will record miles cause the tires are rotating regardless of the speed. MPG will be effected by efficiencies at various speeds etc. but I need teh raw formula or factor.

Does this make any sense or am I off base?

Thanks again

[mackin]

CCPerformance.com,

I understand what your trying to do, but this info is programed into the computer of yours, and my truck, and I feel it is a constant mathematical equation ...... This info is gathered from the transmission, from different shaft speeds ....... You're trying to back in and get results after the fact ..... One common denominator is not going to do it ...... Brick on the go peddle maybe, constant rpm and speed, gear ratio by being in final gear,multiply miles traveled by the difference in gear ratio , but not in real time, variable gathered info....... I got to quit, my head is smoking ........ Way to much thought ..... if I think of something when my head cools off I let ya know .....
MAC smile.gif

[pinehill]

CCPerformance.com,

No rocket science here. Calculate your mpg using the odometer reading, then multiply that mpg by 31.7/30,6= 1.036 to correct for the increased diameter. smile.gif

[CCPerformance.com]

Finally.....Thanks Pinehill. I knew it would be fairly simple.

[mackin]

CCPerformance.com,


Well, there you go .... Now pump them tires up to the max. .... And keep your tape measure handy for when you squat them tires with some weight.... Hope it squats evenly, in all four corners . I think you'll be back raving about your fuel economy .......... HE HEEE LOL ;)
MAC :D :D

[ 05-26-2002: Message edited by: mackin ]</p>

[Dawg]

I thought I had this figured out, but feel even more dense than usual. I went from 245's(30.5" dia) to 265's(31.6" dia) So I divided 30.5" by 31.6" to give me a .965"% difference in tire size. Is this correct? If so, I should be able to calculate my MPG as usual then multiply the result by .96 then add it to it to get actual MPG. Or adjust my odometer miles right from the start by multipling the odometer miles by .96 and adding that result to the odometer miles then calculate my MPG as usual. Am I thinking correctly ?

[ 05-27-2002: Message edited by: Dawg ]</p>

[mackin]

Dawg,

It's 31.6 (div By) 30.6 =1.033 ====&gt; I still believe it will be a guestament at best ....... To many variables ...... Take in to consideration your tires squat ...... So true OD is not what is addvertised ...... You'all see what I mean? Get "stanley" out and go take Peek..... .See ......

MAC

[ 05-27-2002: Message edited by: mackin ]</p>

[CCPerformance.com]

Dawg, divide the other way, 1.03 your increase your distance not reducing. Your tire is approx 3% larger than stock.

[Dawg]

Okay, that make more sense! But, How do you get the 3%?

[Dawg]

So to further complicate things... can't I just multiply my MPG by 3%? Man I'm slow!! Been a long time since my school days!

[CCPerformance.com]

Dawg:

I run till the light comes on, approx 375 miles. It takes about 22.5 - 23.0 gallons to fill. So 375 x 1.03=386 miles per tank. Divide 386 by gallons, 386/22.5=17.15 mpg. This is about as accurate as you will get unless you have your truck flashed for 265's. I'm not sure if this is possible, I've heard yes and no.

[CCPerformance.com]

PS, mac

regardless of tire squat, I don't think the roll-out changes very much if at all. A circle's OD remains "fairly" constant whether you squash it like an egg or it remains perfectly round.

To test this, the next time you rotate your tires, put a line on your tire and the ground below. roll the tire one revolution. Measure the distance traveled, i.e. tire roll-out. Now take your tire off and do the same thing. The numbers should be very close. If not, you can factor this error into your OD formula. At this point your splitting hair and are llikely more precise than even the stock speedo.

[CCPerformance.com]

True Mac:

But you current speedo is calibrated for the stock tire size. If you do the math you will see that the diff in MPH at any speed is fairly accurate (per GPS) with the correction. I know my speedo is off over 2 mph at about 70mph. 70 x 1.03= 72.1

1.03 x any speed will be damm near exact per GPS, which is more precise than the speedo anyway.

[Wally]

Gee whiz, this ain't that hard. Use the mile markers on an interstate. They are surveyed in with an extremely accurate GPS. Pick a mile marker, record trip meter mileage and the mile marker number. Run a set distance. Easiest if you can measure over a hundred miles.
Example at 50 miles. Trip meter records 47.2, mile markers read 50. Divide 47.2/50=.944, subtract .944 from 1=.056, add 1, 1.056. From now on figure your mileage as you normally would and multiply the result by 1.056. Voila!
The reason to use the mile markers is that chances are way better than average that your odometer was not right with the factory rubber. This method rules that error out.
The same number you derive for the MPG calcs can be used to correct your speedo. 70mph(indicated) X 1.056=73.92mph(actual)