ok, i got 280 mile out of 27 gallons of gas which comes too 10.37 per mile. however; since i have 305 on my truck, i know my speedometer is off. For example; when the speedometer says i am going 60 mph, i am usually passing cars on a 65mph highway. My question is, how could i caulate this into my gas mileage.

 

I will take a stab at it. I went to 1010tires.com and did a comparison to what I have as stock equipment tires (245/75/16) vs. a 305/70/16 and it came out to be that the bigger tires would be "slower" by 7.71%.

Stock Tire - 245/75R16 >Search Tires
Tire 1 - 305/70R16 >Search Tires

Overall Diameter:
30.46 in
773.68 mm

Overall Diameter:
32.81 in
833.37 mm


Circumference:
95.69 in
2430.5 mm

Circumference:
103.0 in
2616.2 mm

Revs per Mile:
682.8

Revs per Mile:
633.9


Actual Speed:
60 mph
100 km/h

Speedometer1:
55.7 mph
92.8 km/h

Speedometer Difference: -
Speedometer Difference: 7.714% too slow

Diameter Difference: -
Diameter Difference: 7.17%

So, using this math you could basically take your miles traveled * 1.0771 = 301.58 adjusted miles to your 27 gallons. That brings you to a whopping 11.16 MPG using this method.

If you had different original size tires, or different sizes than what I used, you can go to that same site and compare the 2, get the % difference, and go from there.

Either way, you didnt gain much in your MPG's, but its something!

 

Sorry for the lame formatting.. the html from their site didnt copy/paste as well as I would have liked.

Here is the link you can compare tires: http://www.1010tires.com/TireSizeCalculator.asp

 

Ready to do some math?
First you have to figure out the circumferance of your tires.

1. Say you have 305/65R x 16 inch (Just a guess). Take 65% (that the 65R part) of 305 and you get 198.25 millimeters (305 x .65 = 198.25). So the height of the tire from bead to tread is 198 mm.
2. Convert the height of the tire to inches and you get 198.25 / 25.4 = 7.8 inches
3. Now add the radius of the wheel (16 inches/ 2) to the tire height and you get a total radius of 8+7.8= 15.8 inches
4. Using 2*pi*radius you get a curcumference of 2*3.14*15.8 = 99 inches.
5. Do the same thing for the stock tires. For example if your stock tires were 225/70R x 16 the circumference would be 89 inches.
6. Divide the circumference of your existing tires by the stock tires to get the difference. So 99/88 = 1.11 or an 11% difference.

That means when your speedometer says your going 60 mph, your really going 11% faster or about 67 mph. The faster you go, the greater the error. This also means that your odometer will register 11% less than actual. When your odometer reads 89,000 miles you've actually gone 100,000 miles.

So to finally answer your question, if your odometer tells you you went 280miles on a tank of gas, you really went 280 + 11% or about 308 miles. You're really getting about 11.4 mpg.

 

WOW, I think i need to go back to college.[8D]

sweet thanx for the info, so if i really baby my truck i could get 12mpg.

here is another question; when do i need to chang my oil? In other words, how do i know when 3000 miles is up.

 

You can use the same principle for that too..

Depending on your variance from stock, just multiply that by 3000. In my case, if I upgraded from the stock 245's to 305's (which I plan to) my 3000 miles indicated on my odometer would actually be roughly 3230 miles (about 230 off over the course of the oil change interval)...

I'd say a safe bet would be somewhere between 3200-3300 to cover all your possible variances in tire sizes, etc. Be safe and change it close to 3000 and you will be just fine.

 

Take your truck out on the highway and compare your odometer reading to the mile markers. Go 10 miles according to the mile markers and see what your trip meter records. For example, if you travel 10 miles by the markers and your trip meter just shows 9.5 miles, mpg = 10/9.5 x gallons / miles. That will correct your mileage calculation. You also could use other increments,i.e. 35, 50, 100miles according to the markers. Thelonger the distance, the more accurate you will be. Be sure to use the trip meter because the tenths value will be important, especially on shorter check distances. This is a simple and ultimately more accurate method assuming the state department put the markers out correctly. Also, keep a running total of gallons and miles over a month or so. That will give you a true picture of your average mileage instead of what you get on one trip.