csutke Posted August 24, 2007 Share Posted August 24, 2007 (edited) 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 August 24, 2007 by csutke Link to comment Share on other sites More sharing options...
nissandoms47 Posted August 24, 2007 Share Posted August 24, 2007 You have 30x9.5 bfg ats? It shouldnt really be off since the stock tire size on a XE is the equlivant to a 29" tire.. Link to comment Share on other sites More sharing options...
csutke Posted August 24, 2007 Author Share Posted August 24, 2007 Im planning on goin up to either 31" or 32" soon Link to comment Share on other sites More sharing options...
Simon Posted August 24, 2007 Share Posted August 24, 2007 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. Link to comment Share on other sites More sharing options...
SnakeS Posted August 24, 2007 Share Posted August 24, 2007 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 Link to comment Share on other sites More sharing options...
Trainman Posted August 24, 2007 Share Posted August 24, 2007 Here is a tire calculator you can use to determine the difference between the stock tire and whatever you put on. It would be a start anyway. As long as you do not change your gear ratios, this should be all you need. Tire Calculator Link to comment Share on other sites More sharing options...
XPLORx4 Posted August 24, 2007 Share Posted August 24, 2007 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. Link to comment Share on other sites More sharing options...
FUELER Posted August 24, 2007 Share Posted August 24, 2007 You guys ever heard of a powerful magnet on the fuel line increasing MPG? http://www.tinet.org/~sje/mag_fuel.htm I have a company sending me one for free, granted that i'd test it and write a review. Link to comment Share on other sites More sharing options...
GrnMchn Posted August 24, 2007 Share Posted August 24, 2007 You guys ever heard of a powerful magnet on the fuel line increasing MPG? http://www.tinet.org/~sje/mag_fuel.htm I have a company sending me one for free, granted that i'd test it and write a review. Looks like snake oil, might as well get a vornado plus the grammer on their site crap like mine Link to comment Share on other sites More sharing options...
csutke Posted August 24, 2007 Author Share Posted August 24, 2007 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. Link to comment Share on other sites More sharing options...
XPLORx4 Posted August 25, 2007 Share Posted August 25, 2007 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. Link to comment Share on other sites More sharing options...
csutke Posted August 25, 2007 Author Share Posted August 25, 2007 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? Link to comment Share on other sites More sharing options...
XPLORx4 Posted August 25, 2007 Share Posted August 25, 2007 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. Link to comment Share on other sites More sharing options...
csutke Posted August 25, 2007 Author Share Posted August 25, 2007 good enough for me, thanks Link to comment Share on other sites More sharing options...
94extreme Posted April 21, 2008 Share Posted April 21, 2008 about the same as speed; ~10% more then calculated from the odo. Link to comment Share on other sites More sharing options...
csutke Posted April 21, 2008 Author Share Posted April 21, 2008 wow thats an old thread Link to comment Share on other sites More sharing options...
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