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OperationSmile.org
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I have been reading the latest Diesel World while my wife has been in the hospital and it brought me to wonder some things. They are doing some testing with a 6.0 and mileage. Im sure some of you have read it. Anyways they upgraded the tire size from a 255/75r16 to a 305/70r16. That resulted in a 1.7 inch increase in tire size which caused a speedometer and odometer error of 5.6 percent which meant that the speedometer and odometer were reading low. So my question is if tire size is causing the odometer to read less than the actual mileage that means that my calculations of 13mpg is off by alot. This also means that when my speedometer reads 70 and Im sitting at 2k rpms I am actually running 65mph or so correct. The article doesn't say what formula they used to figure this out so does anybody know? They are also claiming 24mpg with a bed cover, exhaust, intake and xzillaraider chip. Sounds pretty good to me. What do yall think.
 

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im out 6 k/hr so for every 100km I drive its really 106km .. thats going from 265-295 tire
 

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I read that article also. When my speedo reads 60 I am realy going about 65. I have to multiply my mileage by 1.0857 then I can divide that by the # of gallons I buy to figure my mpg. I truly get about 15 mpg mixed driving as opposed to 13.5 uncorrected. Made me feel better about my truck.
 

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See thats what doesn't make sence to me. If they went to a 305 from a 255 and I went from a 265 to a 315 wouldn't my speedometer and odometer be reading even slower than theirs. Not faster? The smaller the tire the faster it is having to spin thus more rotations per mile which would mean higher speed right? a larger tire means less rotations per mile thus allowing for a slower speed to be read at the wheels.
 

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Another thing that people aren't aware of....it is cumulative.

If your going 30mph and it's 3mph off,....that doesn't mean that when your driving 60mph,..that your actually going 63.

I don't have the exact numbers calculated,...but at 60mph,..your closer to 64.6mph....but even then...it is depending on tire size.
 

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your speedo is driven off the rear diff.. tone ring on the ring gear.. so for every axle revolution you make a larger tire will travel a farther distance a smaller tire will go a shorter distance.. this is why gear change in these trucks dont throw out the speedo so you will go faster and farther with a larger tire then the speedo says
 

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I'm running 315/75 R16's and cruising at 70 (speedo) I'm actualy doing 75 MPH. I was told there was a 10% error with that tire size.
 

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I see what your saying. But I don't completely understand it.

Wouldn't it be 10% across the board?

You said if your 3mph off at 30 you won't be 3mph off at 60, thats true, but its still 10% error, you'll be 3mph off at 30, 5mph off at 50, 7mph off at 70, 10 mph off at 100.....and so on. The only thing changing is the mph you will be off at a given speed.

If this is the wrong way to look at it, please correct me so that I will know.
 

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Question: How do you guys who say "My speedo reads X, but i'm actually going Y" know what you are actually doing??

Speeding tickets?
Roadside "you are going this fast" signs?
Car next to you with a digital speedo?
Dyno?

I would only trust the last one on that list.
Is there a better way to figure this out?
 

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Question: How do you guys who say "My speedo reads X, but i'm actually going Y" know what you are actually doing??

Speeding tickets?
Roadside "you are going this fast" signs?
Car next to you with a digital speedo?
Dyno?

I would only trust the last one on that list.
Is there a better way to figure this out?
GPS

For my truck the Speedo reads 65, GPS shows 62.
And all of my mileage is figured via GPS. I was quite pleased this week on my 789 mile trip from San Diego to Moab netted me 19.06 mpg unloaded at 62mph (with my POS Edge set at stock).
 

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:doh:

forgot about GPS, probably cause i don't have one.

Are they accurate for MPH and MPG? IDK, i'm asking. :shrug:
 

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The GPS that I'm using has an average accuracy of 15 feet. The GPS speed is dead on when compared with radar or laser.
 

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If your mph is off,...then your distance traveled is off also....so it would be nearly impossible to figure mpg's.

A member here is an electrical engineer,....and he designed and made a speed control device...kinda like the TRUESPEED only better.
 

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don't you guys use your programmers to correct your speedometer? That's what I bought my hypertech for and it's bang on when checked with my GPS.

and a side note about the mileage - not only larger diameters make the difference, but the weight of the tire makes a big difference too. With my stock tires i easily got 19-20mpg, but with 295's I'm down to 17-18mpg.
 

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don't you guys use your programmers to correct your speedometer? That's what I bought my hypertech for and it's bang on when checked with my GPS.

and a side note about the mileage - not only larger diameters make the difference, but the weight of the tire makes a big difference too. With my stock tires i easily got 19-20mpg, but with 295's I'm down to 17-18mpg.

That is why my buddy made his own speed control device, and now he markets them....because alot of the programmers lie,...and say they can adjust for tire size, odometer, tranny, etc,...but for most of them...that is BS.

Those company's think we are all idiots and they assume that we will never check to see if it actually does what it says.....and more often than not....they don't do anything for the modifications you've made.
 
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