calculating mpg with different tire
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Senior User
Joined: Mar 2006
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calculating mpg with different tire
I installed 295/75/16 on my psd I was told to add anywhere from 4%-12% when calculating mpg is there any truth to this and is there a actuall % to add or does it just make people feell better to add 12% to your calculated mpg thanks
Of the next Riffraff East
Joined: Oct 2004
Posts: 8,679
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From: Concord NC
From what I've been told, the vehicles come from the factory with as much a 7-10% error in the speed odometer. I don't know that for sure. I do know that when I went from stock to 285s my speedometer was a close as it has ever been. Now with my 315s I am off around 10%. IMO the only way to tell for sure is with a GPS or radar. Match the GPS reading against the reading on your speedometer, calculate the differance. You can spend some money and recalabrate your truck to match the actual speed or do like me and remember to allow for the differances. I know that when my speedo is showing 70mphs I am going 77.3mphs according to the GPS. This is handy to know because they are folks out there with radars that will show you how much your speedo is off and give you a ticket with the numbers written down for your future referance.
Senior User
Joined: Feb 2006
Posts: 420
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From: Alamo city
Hed, there are a few tire size calculators out there that will provide some guidance. However I've got the most help from the one at 1010tires.com
Then again, you can always swipe the wife's measuring tape and measure the circumference of your tires, to compare to the stock tire measurements to determine the percentage of change.
Then again, you can always swipe the wife's measuring tape and measure the circumference of your tires, to compare to the stock tire measurements to determine the percentage of change.
Senior User
Joined: Mar 2006
Posts: 171
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From: Houston, TX
I just did some calculations tonight regarding my tires. I have 305's.
The key lies within the diameters. Here is the method that I used to figure the difference in MPG's, MPH, and total miles. I have verified all of this with my GPS. It is accurate.
1: Find the true diameter of the tires. (ie: 305=>32", 285=>31")
2: Calculate the revs/mile. REV/Mile=20176/(tire diameter). (ie: 20176/32=630.5)
3: Take the ratio of the revs/mile of the new tire over the old tire. (ie: 305's/285's ----> 630.5/650.83=.96875)
4: Multiply your mileage or speed by this number to give you the true value. ie: MPG: 15*.96875=>14.53; Speed: 70*.96875=>67.81 mph
*my speeds were verified using a Garmin GPSMap76
WB
The key lies within the diameters. Here is the method that I used to figure the difference in MPG's, MPH, and total miles. I have verified all of this with my GPS. It is accurate.
1: Find the true diameter of the tires. (ie: 305=>32", 285=>31")
2: Calculate the revs/mile. REV/Mile=20176/(tire diameter). (ie: 20176/32=630.5)
3: Take the ratio of the revs/mile of the new tire over the old tire. (ie: 305's/285's ----> 630.5/650.83=.96875)
4: Multiply your mileage or speed by this number to give you the true value. ie: MPG: 15*.96875=>14.53; Speed: 70*.96875=>67.81 mph
*my speeds were verified using a Garmin GPSMap76
WB
Senior User
Joined: Mar 2006
Posts: 171
Likes: 0
From: Houston, TX
crosbo got there before i did... ah well
I would like to add a note on where I got the diameter of the 285's. I back calculated it from the Ford's Rev/mile.
I just toured the above website, looks like a very nice one.
My method provides very similar results to their calculator
I would like to add a note on where I got the diameter of the 285's. I back calculated it from the Ford's Rev/mile.
I just toured the above website, looks like a very nice one.
My method provides very similar results to their calculator
