Mileage correction with larger tires

[2016outdoorsman]

Ok, my stock 2106 Ram outdoorsman had 265/70-17 tires. I upgraded to 285/70-17's a while back. I assume the gas miles my computer shows is incorrect because of that size difference? Seeing the new tires are 3.5% larger, I assume I can add that to the computers mileage to get my corrected gas mileage. Or am I missing something?

 

[indept]

With AlfaOBD you can enter the correct size so speed & mileage will be correct.

 

[Jeepwalker]

If you hand-calculate the mileage for a while, that'll give you the best idea, and most accurate measurement of what your mileage actually is. Fill up at the same pump (vehicle level), same station for a couple months. Write down the numbers, maybe even the temps, and perhaps what the truck computer was reading generally at the time of fill-up. Then average out your actual mpg's over the month or two after you've done it a while. Repeat in the warmer summer months for better year-round accuracy.

I'm guessing you probably lost .5 mpg with the larger tires, based on what others have reported in the past. Other factors come into play: Were your new tires heavier 10-ply, or a lighter weight load-range C, tire? Some tires come in at 55#'s ea while others can be up to almost 80#'s a tire. Were the old ones a street-style tread, for example, and almost bald (low friction), vs perhaps, a brand new heavy off-road tire with course tread blocks with more weight and road friction? I was pondering the move to the larger tires too. Fill out the wheel wells more. If you have a mechanic buddy with a Snap-on scanner, those can change the Ram tire size too. You select from a menu of sizes pre-programmed into the vehicle's computer. Love to know what you discover!


Cheers!

.

 

[RamDiver]

Ok, my stock 2106 Ram outdoorsman had 265/70-17 tires. I upgraded to 285/70-17's a while back. I assume the gas miles my computer shows is incorrect because of that size difference? Seeing the new tires are 3.5% larger, I assume I can add that to the computers mileage to get my corrected gas mileage. Or am I missing something?

That's close to the same error I have with my winter wheels and yes, you can allow for it with your mileage calculations if you don't care to fix it with AlfaOBD or JScan, or another app.

As far as I know, the Security Gateway Module (SGW) wasn't used in 1500s before 2018 which is a huge bonus. You could simply plugin directly to the OBDII connector with a wireless interface instead of ripping the dash apart to install a bypass cable like my Ram truck.
That is if you wanted to correct this error.




https://tiresize.com/tyre-size-calculator/
.

 

[danielmid]

There's a few ways to correct, stand alone correction computer like this one I have for sale (link), AlfaOBD (Android software and an OBD bluetooth adapter, you can read about it here), or full computer tuner with unlocked ECU and all.

 

[2016outdoorsman]

I know bigger tires will get worse mileage, along with the computer showing incorrect mileage too. proably 1mpg total difference. What are the advantages to getting it changed to the correct diameter? The mileage being off doesn't really bother me. I drive my vehicles into the ground no matter the mileage Lol

 

[danielmid]

Potential: speedo being off, mileage on odometer being wrong, mpg calcs being wrong, if it's a big enough change the transmission shift points could be off.

Nothing major, but enough things that it was worth it for me to correct with a programmer.

 

[RamDiver]

I know bigger tires will get worse mileage, along with the computer showing incorrect mileage too. proably 1mpg total difference. What are the advantages to getting it changed to the correct diameter? The mileage being off doesn't really bother me. I drive my vehicles into the ground no matter the mileage Lol

You can see in my previous post a 2.1 mph error at 60 mph. That's no biggy provided you always remember, your speedo is not accurate. And when your mind is somewhere in the clouds while you're rushing to an appointment, you always have to remember that error increases with speed. Look at the chart I posted for reference.

Now at 70 or 80 mph, a local cop or state trooper won't care about your speedo accuracy problem, he/she will just right the ticket up and you pay the fine plus the potential increase in insurance costs due to demerit points. If you're confident you will never forget, excellent and good for you.

When I looked to correct my tire size error, Jscan was the least expensive app but if you decide to make changes, you might look at the enormous myriad of changes you can make with AlphaOBD for a few dollars more.

I've had my truck for 1 year and have yet to correct my tire size error. It's on the list and I have some of the parts to make it happen. Mine is much more of a PITA than yours. I have to access the SGW behind the radio and install a bypass cable.

And the bypass cable has to be removed after changes or at least before I visit a dealership when they need OBDII port access.
.

 

[2016outdoorsman]

I've had oversize tires on every truck I've owned. So I realize the speedo is always showing low.

 

[pacofortacos]

The speedo error isn't a set mph (it varies by speed), mine was off 3-4 mph with those tires on at 70 mph.

A dealer can also change it - have them set it to a 275/60/20 as they are almost the same size.

 

[Daily Driver44]

Should you choose to get a "tuner in a can", most of them have a option that lets you change tire sizes.
That is if you haven't already! lol!!

 

[Jeepwalker]

Changing the tire size isn't any *special tune*. It's a built-in feature of the truck's computer that lets the size to be changed per whatever tire size your truck has, or you put on. It's actually was really nice Ram allowed that feature. A lot of other mfgrs allow it too. It doesn't require any special programming or tuning knowledge ....Any mechanic or shop (or you) with a decent scan tool (or AlfaOBD) can select from a drop-down tire-size menu. It's quick thing to do. And the tire size can always be changed back (or to another size) whenever you want.

 

[2016outdoorsman]

The speedo error isn't a set mph (it varies by speed), mine was off 3-4 mph with those tires on at 70 mph.

A dealer can also change it - have them set it to a 275/60/20 as they are almost the same size.

Correct, it's a percentage difference in the circumferences. That's a good idea! I wonder how much they would charge?

 

[pacofortacos]

Depends on how nice they are
Doesn't take long, they just go into the computer and change the code.
I would think 1/2 hr or hr charge at most.

 

[flyinverted]

The issue with the Gen5 Rams is that if you go over a certain tire size, the BCM will disable the front and rear parking sensors.