[quote author=shawngreen link=topic=123299.msg2610184#msg2610184 date=1221270604]
From a previous post bbf2530 mentioned that MPG would go down with a bigger wheel/tire combo. My question is why?
I upgraded from my stock 16" wheels with 205/60-16 to 19" wheels with 235/35-19 and according to
http://www.miata.net/garage/tirecalc.html the diameter is only .2 less with the 19s.
When I was in the process of changing to my new wheels I even weighed the mounted 16s vs the mounted 19s. 40 lbs 8 oz for the stock combo, 50 lbs 6 oz for the 19" combo. Since I've put on the 19s I am getting about 2 MPG less than I was on the stock 16s. The combined total of ~40 lbs shouldn't really mess up my mileage that much should it? I've driven with passengers and not had it affect my mileage that much, and my passengers have definetely weighed more than 40 lbs.
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Hi shawngreen. :wavey: It is not only the amount of weight (although that does also matter), but more importantly where the weight is located. This is an extra 40 pounds of weight located on the wheel/tire combo, and located further from the wheel axis than on a smaller wheel. Makes the effect on engine performance that much more pronounced.
It all has to do with the laws of physics, motion, leverage, torque and other factors. I am just making a guesstimate here, but that 40 pounds of wheels probably has the same affect as up to 400 pounds of passenger.
You have essentially increased the weight of your wheels by 25%. And that 40 pounds of weight is now located further from the center hub (or axle line). And in actuality, the true weight differential is more than 40 pounds, because more of that wheel weight is located further from the wheels hub (the axis of the wheel). In other words, you have more metal (aluminum) towards the outside of the wheel diameter than you did before, and less rubber.
And which weighs more, air filled rubber or aluminum?. Aluminum of course. So you have greatly increased the leverage (torque) needed to spin those wheels/tires. This is an oversimplified explanation, but it covers the basics.
Long story short, your engine has to work that much harder to get the wheels spinning
and work harder to keep them spinning. And by the way, something we do not normally mention (and gets lost in the discussion) is that your brakes need to work harder to stop them from spinning too.
So in the end, yes, that weight really will "mess up my mileage that much". Big wheels are a fashion statement. Think of them like high heels on a woman. Yes they look really good, but they are not practical. Big wheels look good in the showroom, just like high heels look good in the bedroom (and other places). But neither big wheels or high heels were meant for running. And if you wear them running, performance is going to suffer. A loose analogy I know, but relatively accurate.
And let me make it clear that I think big wheels look good. But no matter what anyone tries to claim otherwise, you cannot defeat the laws of physics. Those of us who have warned of the MPG penalties did not just decide to make this information up for the fun of it. It is simple science and common sense, and is what any automotive performance expert will tell you.
And no, I do not claim to be an automotive expert, but I do know where and how to research what the automotive experts say (although I did
not stay at a Holiday Inn Express last night). :wiggle:
Unfortunately, if you want better gas mileage, you will need to ditch the bigger wheels. It is just a fact of life (and physics, and science and...etc. etc.)
Good luck. :cheers:
