2nd Gen Non-Engine/Transmission Larger tires, How to check milage?

[cbaldwin]

I've got 285/75/R16's on my truck and didn't know how to figure my mileage with the larger tires. Is there a % on what to multiply your miles indicated on the odometer with to come up with the correct mileage?

 

[DennyDay]

Tire Change

If your OEM tires were 245's, the difference will be a little over 10%. I got an Abbott box from Jannetty to go with my 285's, speedo and odometer on the money with new tires, off a little as the wear. If you have a trip computer, the MPG will not be correct, but you can check the old way when you fill up.



Denny

 

[klenger]

Tire size calculator

 

[cbaldwin]

Thanks for the info, this will really help me keep track of milage and correct speedo.

 

[BroncoMarc]

Here's another way to calculate how far you are off. Drive down the highway 10 miles using the highway mile markers. Now look at how far your odometer has advanced. Say it's advanced 9 miles. That means your reading 11. 1% slow (100 / 90 = 1. 111). So, if your speedo says you're doing 45 miles an hour, 45 * 1. 111 = 50 mph actual.



I've used this method to correct my speedo in my 73 Bronco when I've change tires or gears. For example, I tried that and my odo advanced 8. 75 miles in 10 highway miles. To correct it I changed the gear on the end of the speedometer cable. I had an 18 tooth gear. 18 * . 875 = 15. 75. I switched to a 16 tooth gear and my speedo was much more accurate. Of coarse this won't work on a Ram since it doesn't have a speedo gear

 

[LSMITH]

Borrow a GPS unit from a buddy, run down a flat straight stretch of road and take some readings. 285 BFG's on my 99 are right at 10% error when about half worn.

 

[Berrigan]

All 285s are not created equal. Close be not the same. Assume that your speedo was accurate with stock tires. Measure the circumference of the new and old tire. If the new tire is 5% larger then multiply you mpg by 5%.

 

[MWilleford]

Calculating

On your next road trip on the interstate, write your truck OD and the mileage marker down once in route. Later, do again (as long a you dont exit). The farther you go the more accurate the factor.

Your correction factor will = the actual mileage per the mile markers, divided by your truck mileage per the OD. I came out to somewhere around 1. 08.

After you get this factor, mulitply it by your trip OD to get actual mileage. GPS works well also.

Factor= (Interstate miles traveled)/(Truck miles traveled)

 

[michaelsgreene]

Here's my experience on adjusting the speedometer/odometer for different tire sizes.



Do not rely on any charts or internet sites that claim to tell you the difference in the speed of rotation of the tires. There are enough differences between brands and models of tires that this information is pretty much useless.



I tried measuring the tire height and coming up with a ratio to correct the ratio, but it was still way off.



I have always thought that the dealer could reprogram the computer to adjust for tires but on my truck, it gave me the choice of 215/85 or 215/85 M and S. That didn't work for my new 255/85s. When he started mumbling about violating emmisions regulations by changing the tire size and the dealer not being permitted to recalibrate it, I just walked off.



I purchased the Abbot ERA and it worked like a champ. If you have 4 wheel ABS you will have to make a simple mod to it to get it to work. That info is on the site here somewhere.



Before installing the ERA, make a test trip on a long continuous stretch of highway, up to 100 miles is good, but ten will do. Use the Abbot formulas and charts to determine how to set the switches inside the box.



Go out and try it with GPS or mile markers and it will probably be a little off the first time. Go up or down one setting and try again. It took me four tries, but now it is dead on.



If you decide to do this find the thread on here about it.



Michael

 

[krs]

Hmmm,



Install 285/75 when truck is new and drive 110,000 miles under engine warranty.

 

[brandonwhite]

How are you guys getting 10% with 285's? I have 35" tires on my truck (315/75/16) and I have verified with GPS that I am 9. 5 percent off.

 

[Hummin Cummins]

I was wondering that too.

I get about 8%

8%, 10% close enough for me. My mileage has been consistant within 1/2 mpg since I bought the truck. All calculated by hand. If I see a MAJOR change then I will be concerned.

 

[MidwestMotorman]

years and tires

The 99s had 245/75 tires. The later ones had 265/75 tires. I found out that the stock tires new were too undersized for the speedo calibration. I was actually putting on more miles that I had actually driven, and that got worse as the tires wore. (Sounds like a D-C trick to get it out of warranty faster... )



I went to the 265/75 and found them be off by 3. 3% A little wear will put them right on. Tire pressure and load also makes a big difference in speedo accuracy too.

 

[illflem]

Re: years and tires

Originally posted by nathanbush

Tire pressure and load also makes a big difference in speedo accuracy too.

I don't understand how this could be. With the steel belts in tires how can the diameter change? The amount of rubber on the road should make no difference.

 

[MidwestMotorman]

Maybe I should have omitted the word big. It does make a difference, but not a huge difference. Put a tape measure around the outside of your tire at 40 psi. Now air it up to 80 psi. You will see the tape stretch apart. You are increasing the outer circumferance of the tire, and that will make a difference (sometimes only very slightly). Further proof of that, take a limited slip rear truck. Put one side at 40psi, and the other at 80psi. You will definitely get a pull due to the rotational difference from different circumferance. My truck shows a pull at 2psi difference.



I also meant to say that every tire is unique in its size, and unique in the stretch abilities of the rubber and belts. Also the tread length shortens as the tire wears down. When you talk about % difference in a tire, it really does not take too much to be 1% different. That is . 8" of an 80" around tire. A lot of little factors add up to create a difference between trucks.

 

[2-ND TIME]

tires

The two tire manufactures should have the RPM's you should be able to calculate the difference then compare it to the original tire. hope this helps.

 

[michaelsgreene]

IMHO you can do all the testing and measuring you want, but until those tires are spinning down the freeway, you will have significant errors.



Riddle me this Batman!: I went from 215/85s to 245/85s for better mpg. The old tires were 28 inches tall the new ones 33 in. 28/33 is 85%, so the tires should rotate about 15% slower. These are actual tire size measuremens with the tires on the truck under load. For some reason the actual rotation speed difference turned out to be about 8%. Enough to gain 1. 5 mpg improvement but not logical.



Any and all mathematicians are invited to work on this.



Michal

 

[MidwestMotorman]

For some reason the actual rotation speed difference turned out to be about 8%

Well... it could be that your speedo was 7% off to begin with, making a total difference 15%. Don't forget to multiply your miles travelled by 1. 178 to get the actual miles travelled (33/28 is where this came from. ) Have you travelled the interstate and verified the actual distance travelled with mile markers or used GPS?

 

[michaelsgreene]

In looking at this again, I guess I messed up the formula. I used 1-(28/33) instead of 33/28. Not sure why I did that. That is a common statisticians tool to change the point of reference to distort the outcome.



Hmmm...



The original approximation was by tire size measurement. When that was way off, I went to mileposts to get close and then a couple of touch ups using GPS. It is dead on now, somewhere around 8 %.



Michael