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For those of you/us that have put bigger tires on your/our trucks, it has been said that you need to hand calculate your mileage because the computer is probably off now. I know that the speedometer is off....but isnt your odometer off too? If so, then how do you really hand calculate your mileage?
 

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'75 Power Wagon
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my odometer had to have been off while my dad was driving my truck.. the speedometer didnt even work for years.. we replaced like 3 different parts for the speedometer.. my truck was 3 tenths of a mile off.. but i finally got that fixed, and i think everything is working fine now.
 

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I'm Not A Post Whore! :(
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Deke goto the dealership and recalibrate your tire size, 315's are 604 revs per mile

then your speedometer and odometer will be correct and your computer will be accurate (as accurate as they are, anyway)
 

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Jeep Driver
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Even if your speedometer is 100% accurate, that trip computer estimate of your fuel economy is going to be off somewhat. I don't always hand calculate my mileage, but when I do it's a little off from what the computer says. Not too far off, but not exactly right either.
 

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Discussion Starter #5
Chauncy said:
Deke goto the dealership and recalibrate your tire size, 315's are 604 revs per mile

then your speedometer and odometer will be correct and your computer will be accurate (as accurate as they are, anyway)
Chauncy...I have considered that, but I cant see paying $43 for it. I guess you guys missed the point. I hear alot of talk about people hand calculating their mileage because they say that the computer is off..... but if you have put bigger tires on and your speedo is off, then your odometer is as well.....so you really cant hand calculate.....or maybe everyone that has read this has had their speedo recalibrated.
 

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RamFever
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My computer has been 1 mpg off since day one, haven't changed the tires or wheels. I speedo is not off so I need to hand calculate my mpg. I wish my computer was correct because I would love to get 17 - 18 mpg all the time.
 

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I'm Not A Post Whore! :(
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Deke we've all reclibrated our speedo's, it's worth the 43$

gotta think, on our trucks, the computers are extensive, not only does having the speedo off mess with our odometer and speedometer, it also messes with our ABS because the truck thinks it's going slower than it is, and all kinds of little stuff like that that wouldn't of mattered in a pre-comuter rig
 

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I have found that on my 1995 Dodge Ram the odometer and the speedometer are off by slightly different amounts.

The best way to check the odometer is to drive about 50 to 100 miles and check it against the mileposts on the highway. Equally good is to check using a portable GPS.

The best way to check speedometer is to drive at a steady 60 mph past AT LEAST 10 MILES OF HIGHWAY MILEPOSTS. (1 mile is not enough for accuracy)

Over 10 miles at a steady 60 mph it should take EXACTLY 10 minutes, which is 600 seconds.

Each 10 seconds more or less than the correct 10 minutes over the 10 miles of distance indicates you are fast or slow by 1 mph on the speedo. Each 1 second more or less indicates your speedo is off by 0.1 mph.

If you check and find that your speedometer and odometer are 'about' right, but your overhead trip computer still reads different than what you compute MPG to be by hand, that is usually a clue that at least some of your fuel injectors are clogged or your fuel pump pressure is low or high.

The overhead trip computer 'times' how long the fuel injectors stay open to keep track of how many gallons you have used. The trip computer counts on the injectors spraying the same amount of fuel for each millisecond of time as they did when new from the factory. If the amount sprayed is less due to partially clogged spray holes or a weak fuel pump not putting out the pressure, then the overhead trip computer won't calculate the right number of gallons of gasoline.
 

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I guess you guys missed the point. I hear alot of talk about people hand calculating their mileage because they say that the computer is off.....
Those people are full of crap...

but if you have put bigger tires on and your speedo is off, then your odometer is as well.....so you really cant hand calculate.....
Exactly...
 

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why can't you hand calculate if the tires are different size??? - the difference is linear and is easy to do. just get the ratio of the two circumferances (new/stock) then multiply the mileage on the trip odometer by that and you will have the actual distance traveled... right?
 

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I just tried it like that, and the calculated mileage based on what was reported by the trip odomoeter was still off by approximately 12.5%.
 

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it should work:
stock tire size lets say 33"
new tire size lets say 36"
33" circumferance = Pi*dia yeilds 103.67"
36" circumferance 113.09"
ratio is 113.09/103.67 which is about 1.09
now when you travel 100 miles according to the trip, then the actual mileage should be 109 miles...
maybe measure your tires with a tape measure to get exact diameters, cuz you know that tires arent exaclty what they denote.
 

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bigdrake said:
I just tried it like that, and the calculated mileage based on what was reported by the trip odomoeter was still off by approximately 12.5%.
I think you missed what he was saying. Yes, the trip odometer will be off with a tire size change. He's telling you how to factor tire size into the calculation for the correct distance.
 

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I think you missed what he was saying. Yes, the trip odometer will be off with a tire size change. He's telling you how to factor tire size into the calculation for the correct distance.
I didn't miss the point, I calculated just like that, and the multiplied mileage from the trip was still off.

Here's my scenario:

Dodge Ram 2500
Had BFG 315/70R17s put on
They set the computer for 285s since that was the biggest pre-defined setting and at the time neither I nor they knew how to manually set rotations per mile.

285 circumference (at least mathematically calculated): 102.76
315 circumference (calculated): 107.95

Ratio: approx. 1.05

Mileage reported over first drive: 115

What mileage should be: 135

Ratio: 1.174

Calculated trip odometer mileage: 120.75

Off: approximately 12%
 

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merc43amg said:
I wish my computer was correct because I would love to get 17 - 18 mpg all the time.
I wish my computer would lie to me like yours does. ;)
 

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I've driven that route a few hundred times with a vehicle that has an odometer that is correct...
 

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Discussion Starter #19
2-Stroke said:
it should work:
stock tire size lets say 33"
new tire size lets say 36"
33" circumferance = Pi*dia yeilds 103.67"
36" circumferance 113.09"
ratio is 113.09/103.67 which is about 1.09
now when you travel 100 miles according to the trip, then the actual mileage should be 109 miles...
maybe measure your tires with a tape measure to get exact diameters, cuz you know that tires arent exaclty what they denote.
Im going to try that and see what my actual mpg's are. Thanks for the info....everyone. But really, I was just making the point that you needed a calculation like the one above to TRULY hand calculate your mileage. It just occured to me one day that if the speedo is off, then so is the odometer.
 

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Dodgezilla
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Just do like I did when I was feeling too cheap to get the recalibration done. Use a GPS to figure your miles then divide the gallons into that. My GPS was only 2 grand so I needed to get some use out of it and it is more accurate then the odometer. Did I mention that my company paid for the GPS unit? :D Did I also mention that they pay for my fuel so why should I really care? :D :D
Anyways, yes the odometer will be off and you will actually travel farther than the odometer says so that may be a good thing. When I finally decided to get the recalibration done it cost me $85.00 so you should be happy with $43.00!! ;)
 
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