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Actual Mpg?


csutke
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With all the recent talk of MPG I was wondering if somebody can either tell me the formula or point me in the right direction to get actual MPG. What I'm looking for is the formula that takes in to account the gear ratio, tire size, etc... I know that the mileage that I'm getting is wrong as I'm just going off of the ODO and gallons put in.

 

Thanks

Isaac

Edited by csutke
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I have no idea on the manual calculation, but personally, I use my GPS to see how fast I'm going, and then do the math based upon what my speedo says. My speedo is about 20% too slow.

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would a electronic device like this one: http://scanguage.com/products/ give you mileage? I guess it would still be off if you have bigger tires

 

Yes, it will give mileage, and it allows you to enter a speed correction factor to compensate for larger tires. (I used a GPS to find the compensation factor for my truck, which is 3%.) The easiet way to calculate your actual fuel economy is to:

 

First, make your speedometer/odometer accurate (or at least be aware of how inaccurate it is). The inaccuracy is expressed as a PERCENTAGE, not in MPH. If you speedometer reads 65 when you're really going 68, the correction factor is ACTUAL divided by REPORTED, (68/65), or 1.046, or 104.6%. That means when your trip odometer reads 200 miles, you've really traveled 200 x 1.046 = 209.2 miles

 

Here's how to calculate your MPG for the last tank of gas you used:

Refuel your truck up completely until the pump shuts off.

Reset the trip odometer to zero.

Drive until your tank is below 1/4-tank left.

Refuel your truck completely until the pump shuts off.

Note the current trip odometer mileage.

Note the quantity of fuel you just put in.

Divide trip odo by gallons to calculate MPG for that tank of gas.

 

Note: you MUST reset the odometer at each FILL-UP, and you MUST fill-up the tank each time to continually track your fuel economy.

 

It's helpful to use the exact same gas pump to be most accurate, due to differences in the way each pump clicks off. Naturally, this is not always practical, so you may find that there is usually some error in the exact amount (+/- .5 gallons) used.

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Yes, it will give mileage, and it allows you to enter a speed correction factor to compensate for larger tires. (I used a GPS to find the compensation factor for my truck, which is 3%.) The easiet way to calculate your actual fuel economy is to:

 

First, make your speedometer/odometer accurate (or at least be aware of how inaccurate it is). The inaccuracy is expressed as a PERCENTAGE, not in MPH. If you speedometer reads 65 when you're really going 68, the correction factor is ACTUAL divided by REPORTED, (68/65), or 1.046, or 104.6%. That means when your trip odometer reads 200 miles, you've really traveled 200 x 1.046 = 209.2 miles

 

Here's how to calculate your MPG for the last tank of gas you used:

Refuel your truck up completely until the pump shuts off.

Reset the trip odometer to zero.

Drive until your tank is below 1/4-tank left.

Refuel your truck completely until the pump shuts off.

Note the current trip odometer mileage.

Note the quantity of fuel you just put in.

Divide trip odo by gallons to calculate MPG for that tank of gas.

 

This is how I do it now, but for me to get the actual MPG i need to figure out how off my spedo is and then use the actual miles traveled divided by the gallons?

 

Also now i have 4.3 gears, if i were to regear to 4.6 with 31" tires would that make the spedo and odo correct?

 

Thanks for all the help everybody.

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Also now i have 4.3 gears, if i were to regear to 4.6 with 31" tires would that make the spedo and odo correct?

 

Most likely, yes. At least, it did for me. Your actual results may vary. Instead of swapping gears, it's probably a lot cheaper to just get a GPS reading of your speed or odometer to calculate the % error, or travel a known distance and see how far off your odometer is. Some highways have mile-markers that you can use for this purpose, too.

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i was thinking about regearing already so thats good to know. So let me see if i have this correct:

 

to figure out how far off my spedo is i take the difference of actual MPH and Speedo MPH

 

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

30"/28.9"*60=62.3

Actual divided by reported

62.3/60=1.038

1.038*100=103.8% correction factor

 

Then to find my actual MPG I do the following:

 

So lets say that that my Speedo says that Ive gone 200 mile but ive actually gone 207.6

1.038 (correction factor)*200 (reported miles)=207.6 actual miles

 

Lets say that I used 10 gallons to go that far so my MPG would be 20.76

207.6 miles/10 gallons=20.76 MPG

instead of 20 MPG

200 miles/10 gallons=20 MPG

 

Correct?

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Yes, your math is correct, assuming that your speedometer was accurate before the tire swap, and that the actual tire size is really what it says. The actual diameter and circumference of a tire varies widely based on its construction, tread depth, etc. In fact, different brands and models of same-sized tires can often be different actual diameters. That's why it's best to acquire your actual speed with a GPS or other known-accurate method. However, in the absence of such equipment, your approach is good enough.

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