# Tire Size, Odometer & MPG.



## Davans (Jul 23, 2009)

I have a 2006 Dodge Ram. It came stock with 17" 265 70R tires. on it.

I just put 17" 285 70 R tires on it.

It has slowed the odometer about 3 mph @ 70 mph.( based on my Garmin GPS )

Is there any way to recalibrate the speedometer to the new tire size?

It is only off a few mph which is no big deal but my read-out for mpg has dropped from an average of 23.5 to 19.5 mpg. I would have thought that the slightly taller tire would slightly improve mileage overall especially on the highway.

To add a twist to the deal. The dealer just replaced 2 injectors the same weekend we put new tires on it.
(cummins 5.7 Diesel ) so I'm wondering if there may be some issue with that. That has caused the loss in mileage.

Any help would be appreciated.

Thanks.


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## contender* (Jul 23, 2009)

If I'm not mistaken (and I think I'm not because I've done the exact same thing to my Chevy 2500) The bigger diameter tire make your odometer actually read less mileage than your driving. In other words if it's 50 miles from point A to point B your odometer might register that you have only driven 45 miles. This is where your MPG diff is coming in. Your not really burning more gas your just not registering all your miles driven. Someone correct me if I'm wrong.


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## bat (Jul 23, 2009)

Sounds right to me..  do this using google search, search with the key words "calculate miles per hour with larger tires".  A few good links in there will tell you some tests to try.


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## BRANCHWYNN (Jul 23, 2009)

you just made it by a year. If its a 2006, a DODGE dealer should be able to recalibrate a 2006...you are out of luck on a 2007 model I believe. I can confirm for you tomorrow.

by the way...thats a 5.9l not a 5.7.

your whole digitals and read outs will be off just like your speedometer too....just get them to recalibrate.


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## jonkayak (Jul 24, 2009)

Not sure about a dodge but on Jeeps there is a small gear you change out on the end of the speedo cable. It's a 10 min fix on a Jeep but on Dodge I have on idea but it might be worth checking into. Also just call a 4wd shop they should be abel to tell you what it needs.


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## Mission (Jul 24, 2009)

dealer can reprogram it for the new tire size. i have same truck


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## Davans (Jul 24, 2009)

Thanks guys I'll call the dealer today.

And you are right Branchwynn....it is a 5.9 it was getting late when I posted this.

I'll update after talking to the dealer.

Thanks again


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## GoldDot40 (Jul 25, 2009)

contender* said:


> If I'm not mistaken (and I think I'm not because I've done the exact same thing to my Chevy 2500) The bigger diameter tire make your odometer actually read less mileage than your driving. In other words if it's 50 miles from point A to point B your odometer might register that you have only driven 45 miles. This is where your MPG diff is coming in. Your not really burning more gas your just not registering all your miles driven. Someone correct me if I'm wrong.



You sir, are correct.


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## 00Beau (Jul 25, 2009)

I did the same but the little difference from 265`s -285`s did not effect mine. 5.9 Cummins,05 3500 SRW. Checked on GPS also.


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## GoldDot40 (Jul 25, 2009)

300 short mag said:


> I did the same but the little difference from 265`s -285`s did not effect mine. 5.9 Cummins,05 3500 SRW. Checked on GPS also.



That's impossible for it to have no change.

Look at it this way...

265/70R 17: 31.30" tall and has 658.2 revolutions per mile

285/70R 17: 32.70" tall and has 636.0 revolutions per mile

The 285 tire is 3.37% taller which would yield 3.490% slower speedometer/odometer reading


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## Milkman (Jul 25, 2009)

I had a truck that the rear end was changed to one with a different gear ratio.... I got a speedometer shop to make a device to offset the difference.....   that was on an 86 with a dial  type speedometer.


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## 00Beau (Jul 26, 2009)

Bassquatch said:


> That's impossible for it to have no change.
> 
> Look at it this way...
> 
> ...


 Then it was off from being new because it is dead on, anytime you want to ride send me a pm and come on down.Had the truck since it was new! We will hit I20 and set the cruise on any speed you want! It even matches the digital signs put up in construction zones and by the sheriffs offices with radar on them. Matches hand held GPS and Street Atlas GPS in my laptop, so which one is off? Never checked it with 265`s but did when upsized so I would not get a ticket, but it was not off.


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## hoochfisher (Jul 27, 2009)

> The 285 tire is 3.37% taller which would yield 3.490% slower speedometer/odometer reading



sorry but incorect.

in this case, 265 vs. 285. the 285 is the width of the tire. 
the tire size breakdown goes like this,

width(285)/hieght(70r)/diameter(17)

tire size explainations can be found here:
http://www.buicks.net/shop/reference/tires.html#sizes


they are wider, so it will throw the speed sensor off some as the truck has to rotate more rubber. you can have it re-calibrated though. thats my suggestion.


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## Davans (Jul 27, 2009)

hoochfisher said:


> sorry but incorect.
> 
> in this case, 265 vs. 285. the 285 is the width of the tire.
> the tire size breakdown goes like this,
> ...





Look again my friend.

Both tires sizes states are 70 R's
the change was from 265 to 285. which does indeed change the height of the tire.

Short Mag may have changed from a 265/70 to a 285/65 or 60 which may have kept his height about the same.

But the example BassQuatch used is correct.

I went from a 265/70/17R  ( 31.4" according to mfg. )
To a 285/70/17R ( 32.7" according to mfg. )

It has slowed the speedo/ odometer down abit.

Approx. 3-4 mph @ 70mph. which I expected. I just didn't think it would through off the mpg read by 3-4 mpg.

I'll be going to the dealer to see about recalibrating it this week and repost the results.

I do appreciate everyones input.

Thanks


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## hoochfisher (Jul 27, 2009)

> the change was from 265 to 285. which does indeed change the height of the tire.



the first set of numbers in a tire size indicate the width of the tire, second set indcates the hieght of the tire, third set indicates the rim/wheel diameter.

this size breakdown is clearly stated in the link i posted.

a cut and paste of link:

First, lets look at metric sizes of today. Ex. 225/70R15. The first number(225) represents the cross section width of the tire at it's widest point(sidewall) in millimeters. The second # (70) represents the Aspect Ratio. This a big term meaning that this tire's sidewall hieght is 70% of the cross section width, or 70% of 225mm. The R means simply Radial tire construction. And finally the 15 represents the rim size. 




you said you went from 265/70/17 to 285/70/17.

 this means only the width of the tire has changed.

if your tire dealer said that the 285 is taller than the 265, i'm sorry, but they feed you a whole big bunch of horse hockey.

only in inch rated tires (example: 33/10.5/15) is the height listed first


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## marktaylor99 (Jul 27, 2009)

hoochfisher said:


> you said you went from 265/70/17 to 285/70/17.
> 
> this means only the width of the tire has changed.



According to your post, the second number is the aspect ratio [70% in this case]. 70% of 285 (199.5mm) is more than 70% of 265 (185.5), Therefore the tire is taller.

The height of a metric tire is found by taking the aspect ratio, multiplying by the width (as above), multiply x2, (because you gain height from the sidewall above and below the rim) convert the rim size to mm and add to the sidewall number. Then convert back to inches, if you don't like the metric system.


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## Davans (Jul 27, 2009)

Hooch i appreciate your attempt to help, but you don't seem to understand the info the YOU have  givin us in the link.

Go back and take a good look at YOUR link and read it carefully.

Better yet use the " Tire Size Calculater " at the bottom of your link.

Try it for yourself and it will show you that the 285 IS taller than the 265 in the same 70R. and 17" wheel size.

You can go to any tire mfg. website and compare tires sizes and see that  this is in fact true.

Thanks again.


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## merc123 (Jul 27, 2009)

I bought a SpeedCal for my mustang to compensate for both tire size and rear end gears.  Don't know if they have them for the truck tough.  It's basically a box that uses switches and spliced into my transmission harness.


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## 00Beau (Jul 27, 2009)

Nope mine are 285/70/17 BFG All Terrains 10 plys. On my second set, and are going to 285/70/17 BFG Mud Tires in about 10,000 more miles. I am serious though, my speedometer is correct, not trying to debate with you guys, just been driving the truck since 05, would not blow smoke to mess ya`ll up.


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## jimbo4116 (Jul 27, 2009)

hoochfisher said:


> the first set of numbers in a tire size indicate the width of the tire, second set indcates the hieght of the tire, third set indicates the rim/wheel diameter.
> 
> this size breakdown is clearly stated in the link i posted.
> 
> ...





http://www.dummies.com/how-to/content/reading-a-tire-sidewall.html


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## mossyoakhh (Aug 11, 2009)

Just run factory size tires intended for your truck,then you want have to worry about it! Just a thought!


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## Malum Prohibitum (Aug 11, 2009)

hoochfisher said:


> The second # (70) represents the Aspect Ratio. This a big term meaning that this tire's sidewall hieght is 70% of the cross section width, or 70% of 225mm. . . .
> 
> you said you went from 265/70/17 to 285/70/17.
> 
> this means only the width of the tire has changed.



Multiply 265 by .70 and then multiply 285 by .70 and tell us if they are the same height!


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## Davans (Aug 12, 2009)

mossyoakhh said:


> Just run factory size tires intended for your truck,then you want have to worry about it! Just a thought!



The truck came with 265 17 70r's  Michelin Highway Tires.

This is a 1 Ton 4wheel drive truck with a cummins. The original tires left alot of fender well and not much traction in an off road situation.

Seems to have made perfect since to go to the 285 A/T.

The truck looks better and has better off road / pulling capabilities. 

Whats not to like

But thanks for your insite.


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