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How do I figure my MPG? Real Question; Figure MPG correctly
Topic Started: Jun 29 2009, 05:33 AM (1,510 Views)
Car Nut
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Have a 94 XFI with 175 65 14 tires installed. How do I correctly figure the MPG since it no longer has the factory 12 inch wheels? Last tank showed 53.76 mpg. Any math heads out there?
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ki4syr
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Metro Newbie
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Your getting roughly 56.45 mpg.

I'll let you smoke it over a few minutes then I'll tell you how I done it...........

























or just click here
Edited by ki4syr, Jun 30 2009, 07:18 AM.
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Car Nut
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I'm all smoked out. And the answer is? Enquiring minds would like to know. Thanks
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Johnny Mullet
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Fear the Mullet

I'm curious also :hmm
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dimetrodon


Mile per gallon means miles divided by gallons. But you need real miles and since your wheels are oversized, you are traveling further with each tire revolution than your odometer indicates. Just guessing at the ratio of the circumferences of 14's versus your 13's, you are probably traveling 5% farther than your odometer reading. So here is what you do: Pick a gas station where the ground around the pump is very level. Fill the tank until gasoline appears in the filler opening and stays steady for 10 seconds or so. Record the odometer reading in a pocket notebook. Drive a few hundred miles. Go to a gas station, refill to the same visible level and write down the new odometer reading. Subtract the first odometer reading from second. This is the miles you've traveled. Divide this number by the number of gallons you've pumped into the tank. If your odometer readings and your fuel inputs were accurate, this number is your gas mileage.

Now, how do you figure out your odometer error? Find a stretch of road with mile markers. I use the Illinois tollway. Pull up to a mile marker and write down the number. Write down your exact odometer reading to the nearest tenth of a mile. Drive a long distance (20-30 miles) on the same road and then stop next to another mile marker and record both the mile marker value and the odometer reading. Your oversize tires will cause the odometer distance to be less than the mile marker distance. Divide the latter by the former and you will get a number like 1.05 or 1.06. Mine at present is 1.061.

With the correction factor known, next time you do a mileage calculation, you can multiply either the miles traveled by this correction factor, or you simply calculate the gas mileage first and then multiply it by the correction factor.

Be very careful when writing down the mile marker readings. Don't do this unless you can do it safely. Keep a cheap calculator in your glove box along with a mini-notebook. I record every gas fill up with the odometer reading, the amount of fuel bought, and the resulting fuel mileage. This will make you feel good because your Metro is one of the most fuel-efficient cars around and when you factor in the price of the car and the modest expenses involved in keeping it running, you will realize that you are spending less money per mile traveled than just about anything else besides bicycles or gym shoes.



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mwebb
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FOG

assuming the correction is 5%
multiply 53.76 x 1.05 and the answer is 56.448 using http://www.moffsoft.com/freecalc.htm

my GPS has a speed function , at 50 mph per the speedo the gps thinks 52 mph , so i am roughly traveling 4% more than shown on the odometer ,
with 155 70r13 Firestone FR380 at 40 psi about 60 to 80f ambient.
in the interest of accuracy i could repeat the test at 100 mph, except there is no way the car will ever get to 100 mph and if it could , i do not think i would want to be the test pilot .

the mile marker method is pretty usable too , a good idea .
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ffpmrich
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Not sure if this strays off the topic but I have to ask you Car Nut. Have you always had the 14's on the car? I was wondering the effect on the mpg's and performance overall. I'm under the impression that the larger tires may hurt are mpg in the city,maybe not on the highway?
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Woodie
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Wow, those are some tall tires for a car that already has tall gearing. According to the Miata Tire Size calculator, your number is 1.086, or 8.6% error. That means your true mileage is 58.4 if the tire size is accurate. Best way to tell is the mile marker method, as tire sizes are not really comparable from manufacturer to manufacturer, plus air pressure and treadwear change the numbers.
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ki4syr
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Guess you guys didn't see my "or just click here" at the bottom of my post :D . Just using a Tire Calculator, click here .

I think I did the exact opposite calculation, subtracted when I should have added. The larger tires show "less" miles on the speed odometer. mwebb's answer is correct. That tire calculation website is pretty neat, gives you a pretty good guess to the difference in tire sizes.
Edited by ki4syr, Jun 30 2009, 07:16 AM.
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Car Nut
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Rims & tires came with the car when I bought it so I don't have anything to compare it to. Handles fine, still has plenty of space between fender, etc, with no rub issues. I would guess it has to handle better than the squirley 12's since there's more rubber on the road, and it has a wider footprint. Wonder if it's more stable on the hwy when it comes to dealing with cross breezes? 80% of my driving is around town under 50 mph. Try to use it for as many trips to Lowe's, HD, Wally World as possible. All I can say is I'm happy as a pig in s... when it comes to the gas mileage. Even more so based on Woodie's comment. Picked up a 6 degree cam gear that I intend to throw on soon. Looking forward to seeing how it does. ki4syr & everybody else, thanks much for your replies.
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ffpmrich
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Do I have the wrong idea about the 6 degree cam gear? I thought that that was the equivalent of or close to an xfi cam.
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