Jump to content

Welcome to Ultimate Subaru Message Board, my lurker friend!

Welcome to Ultimate Subaru Message Board, an unparalleled Subaru community full of the greatest Subaru gurus and modders on the planet! We offer technical information and discussion about all things Subaru, the best and most popular all wheel drive vehicles ever created.

We offer all this information for free to everyone, even lurkers like you! All we ask in return is that you sign up and give back some of what you get out - without our awesome registered users none of this would be possible! Plus, you get way more great stuff as a member! Lurk to lose, participate to WIN*!
  • Say hello and join the conversation
  • Subscribe to topics and forums to get automatic updates
  • Get your own profile and make new friends
  • Classifieds with all sorts of Subaru goodies
  • Photo hosting in our gallery
  • Meet other cool people with cool cars
Seriously, what are you waiting for? Make your life more fulfilling and join today! You and your Subaru won't regret it, we guarantee** it.

* The joy of participation and being generally awesome constitutes winning
** Not an actual guarantee, but seriously, you probably won't regret it!

Serving the Subaru Community since May 18th, 1998!

Guest Message by DevFuse

- - - - -

Pugs & MPG

  • Please log in to reply
5 replies to this topic

#1 Guest_Mac_*

  • Guests

Posted 14 August 2003 - 08:38 AM

(A really good question for my 100th post on the USMB)

Does anyone know if adding larger wheels on a vehicle results in increased fuel mileage? The same, or if it takes more fuel to turn a larger wheel, decreased mpg?

I know that putting on the larger wheels will skew the speedometer and odometer readings, which would also skew the mpg readings.

Wondering if anyone adjusted the odo/speedo and checked, or if any of our engineers used some of that college Calc and Analytical Geometry when they were bored silly to find an answer for this already.


#2 Guest_Chairotsu_*

  • Guests

Posted 14 August 2003 - 08:58 AM

If you do the measurement on the new bigger tire and can find the % of difference between it and the stock 185/70/13. You can apply that same % to mileage and rpms.

For example. A 28 inch tire would make the odometer about 12% off as far as I can figure. After that it's easy to figure gas and rpms.

Add a Weber, and you should get about 25 or 26 mpg with those 28s. Really. I also get about 25-26 mpg with my 26 inch tires and a Weber.

So, no, maybe the tire <em>size</em> might not make much of a difference in you mpg.


#3 Guest_calebz_*

  • Guests

Posted 14 August 2003 - 09:43 AM

i upgraded to 14 inchers with 185/75/14 tires.

this put my speedometer off by 10.1% according to the tire size chart that someone(skip?) used to have .. I have lost it again.

So when I check mileage, I fill up the tank, drive, fill up again.. check how many miles I have gone add ten percent, then divide that number by the number of gallons it took to fill up.

The only really big ding in the mileage that these larger tires have made, is due to the fact that they weigh a Whole lot more than the factory alloys I had before.

PS. could someone post the tire size calculator again?

#4 Guest_x silvershad0w x_*

Guest_x silvershad0w x_*
  • Guests

Posted 14 August 2003 - 10:53 AM

If you look at tire size as a function of the gear ratio alone, that taller ratio of larger tires decreases your RPM's through all the ranges. Of course we all know this. Now, at a lower RPM, the engine would burn less fuel all the time if it is operated within its load range. The ammount of fuel usage is determined w/ physics via the Ideal Gas Law. The law states:

PV = nRT

P = pressure,
V = volume,
n = number of moles (which is related to the mass of the gas, i.e. 1 mol = 6.023x1023 molecules of the gas, and n = mass (in grams)/molar mass(MM)),
R = the ideal gas constant,
and T = the absolute temperature.

Now, the Ideal gas law allows us to find the "perfect" mixture, which is called the stochiometric micture. Using this data, we can determine how much fuel is needed per volume of air. We know the volume of air in each cylinder, and so the fuel required is proportional to how much air we cram in the cylinder. This is also where Volumetric Efficiency (VE)comes into play. The volumetric efficiency (VE) is a percentage that tells us the pressure inside the cylinder versus the pressure in the manifold. We know the volume (V) from the displacement of the engine. Thus we can calculate the mass of air (M) in the cylinder (proportional to n) from:

n = PV/RT
=> M = n x MM = PV/RT x MM
= (VE * MAP * CYL_DISP) / (R * (IAT-32) * 5/9 + 273)) x MMair

All this garbage is how fuel injection is measured at the microprocessor level. Now, all of this science goes to pot in the real world, because of enrichment. The more you "lug" your engine, the further you press the pedal, which in turns tells either the carb or the injection computer (via the TPS) to dump a higher percentage of fuel than stoich+normal adjustments.

So, if you have injection, and you can access the computer data in real time, you could see just how bad you are "lugging" that poor engine and how much fuel is being "wasted" by the excess enrichment. The larger tires can cause the engine to be out of its powerband, which in turn causes more lugging. So you dump extra fuel down teh pipes, which will counteract the benefit of lower RPMs to some extent. This is largely affected by you engine tune up, and driving style of course.

Well, all the other guys explained how to get your MPG calculated, by using a ratio of tire circumference, and applying that to your actual odo readings, easy as pie. But now, with all this information, you can truly see what benefit or negative effect the tires would have on MPG, by collecting real world data! Simple design and build a precision datalogger for your vehicle (provided you have injection), or install and calibrate a precison flow meter, along with the dataloggin equipment, and you can get down to how many ounces of fuel per minute the tires are changing your actual MPG. Or, you could just put the old tires on, make a few good odo calulations, and then using the aformentioned conversion factor for larger tires, make the same odo reading and compare the two. I kinda like my idea better thou, it'd be pretty accurate.

#5 Guest_calebz_*

  • Guests

Posted 14 August 2003 - 11:18 AM


Way too much time on your hands John!

#6 Guest_carfreak85_*

  • Guests

Posted 14 August 2003 - 11:52 AM

Bigger + Heavier = More power used to turn them = Worse mileage. But the lover revs on the freeway may offset this slightly.

0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users