Lowering your car by getting shorter tires?
 

Something tells me I'm asking a very stupid question but here:

http://www.tirerack.com/tires/sizes/...sp?diameter=15

Also check this out to find out what i'm talking about:

http://ecomodder.com/forum/showthrea...ling-2813.html

you can buy tires that are taller or shorter (distance between wheel and tire tread)....

which would be most fuel efficient, a very tall tire or a very short one? assuming same tire width, rolling resistence, etc.

That's fine for a non-drive tire, but a smaller drive tire will have you doing more RPMs on the highway which will surely cancel out any aero benefit.

so what if i get really really tall tires then?

Then your highway revs will go down.

However it does slightly depend on your cars engine some are very in efficient at lower revs.. some are more efficient in lower revs.. but yes generally the slower you can get the RPMs for any given speed the better MPG you'll get as each RPM your pulling in a set amount of air and fuel into the cars engine..

The nice thing about tirew sizes are you can experiment pretty easily and cheaply.. no re gearing the transmission etc.. just swap out the tires, and you could even just do the drive tires if you didn't want to invest in a whole set.. at least initially.

However taller tires will raise the car that may be a consideration.. and your speedo will be off.. however a gps can tell you how far its off and you can then remember that 55mph on the speedo is actually 60 or whatever the gps gives you..

blackjackel -

If I had a rear-wheel drive, I would get "one size smaller than stock" front tires and "one size larger than stock" back tires. I *think* I would also lower the front end shocks and leave the rear stock. The lower fronts would (hopefully) stop some airflow from going under the car, and the larger rears would make all my transmission gearing "taller" (lowering my RPM at speed X).

Changing the angle might create a larger frontal area, but I think the positives would outweigh the negatives.

I'd have a Cooooooool-Rod that looks like a Hot Wheel!!!!!

CarloSW2

One thing to keep in mind when changing tire sizes: Your vehicle was engineered to handle, brake, etc with a specific wheel set in mind. Changing your wheels changes the engineering.

Another thing to consider, you won't be able to trust your speedo anymore for an accurate judgement on how fast you're traveling because smaller/larger tires means that your axil will spin faster/slower for a given velocity and will throw off the speedo by sometimes as much as 10mph or more. Happened to my friend, he had a Land Rover Defender 90 (sweet offroad vehicle I must say) and he put some larger mud tires on it and when he drove it, he drove past one of those real time speed signs and noticed it was about 12mph off what his speedo was reading, so from then on he had to adjust accordingly. Just know by how much you need to compensate on the speedo reading so you don't go getting yourself in trouble with the law is all I'm saying.

Or if you have a scangauge, you can set your speed multiplier to whatever you want and just look at that.

After much much much study, research, and deliberation, the results of which can be found here ( http://ecomodder.com/forum/showthrea...ling-2813.html ) I have decided to get new wheels and tires. My tires have another 5,000 miles on em, 10,000 if I'm really really lucky.... so.. I will be doing the following:

from 15" wheels to 14" wheels.

Tire size from 195-65-15 to 185-70-14

I used this tire calculation site to calculate the changes in measurements, its not much at all: http://www.miata.net/garage/tirecalc.html

my odometer would be off by .9% (if it says i'm going 60 MPH i'll be really going 60.5 MPH)

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/65-15 5.0in 12.5in 25.0in 78.5in 807 0.0%
185/70-14 5.1in 12.1in 24.2in 76.0in 834 -3.1%

The new tire would be .4 inches or 10 millimeteres narrower, or approx 1.5 inches if you count all tires combined.

I'm going from a GoodYear integrity to BridgeStone B381
This is going from a rolling resistence of 0.00955 to a RR of 0.00615 or a difference of 0.00340 in rolling resistence!

I expect at least 5 more MPG, if not more.

Any thoughts?
__________________

blackjackel -

Subtracting 3.1% implies losing MPG to me. I wouldn't go with a tire size below spec on the drive wheels. But who knows? It could make a good experiment. Do you EOC? It won't matter when the engine is off.

If you can, try to do a before/after GPS correction. I did the after, but I wish I had done the before. I know I am getting a 2% gain right now, but I don't know if I was already +X% to begin with.

CarloSW2

why wouldn't you go below spec on the drive tires? just so you wouldn't screw with the odometer?

Also, don't you think the reduction in wheel contact with the road coupled with the decreased rolling resistence of the B381's would well outdo the loss in mileage due to increased rpms? I would only have to do 27 more revolutions per mile....

And lastly, since you're in my area (I'm in west L.A.) would you like to get together to work on modifying our cars for better aerodynamics... I really don't know anyone who's willing to work with me on this. I realize it's an odd suggestion and I understand if you're weary about meeting a stranger from online :)

blackjackel -

You know, I thought about it and I don't have a good answer. I think I am biased in favor of "taller gearing" wherever possible. You are coming from the LRR + better Aerodynamics POV. I do know that some Lexus models have a suspension "auto-lowering" feature at highway speed, so you are emulating that.

For LRR, I think we should ask CapriRacer a question. I want to see a before(current tires)/after(proposed tires) of what the tread looks like. We can get "pristine pictures" of the tire tread on TireRack. I will post on CapriRacer's tire thread later tonight for details of what I mean.

Sending you a PM ...

CarloSW2

I've extensively studied this on pickups. choosing the optimum tire size is something the factory boys generally get right.

Remarks refer to driving wheels.

Go up in diameter and you lose MPG (even after recalibrating your speedo). Larger tires have a larger rotational moment of inertia and you have to expend more energy to accelerat them up to speed. Not a big deal if you exclusively drive the slab, but murder in town.

In a perfect world you could use the increased rotational energy in your bigger tires to coast further, but there is way too much traffic in my world for that.

Go down in diameter and your engine sppeds up and has to expend more fuel to overcome engine frictional HP.

With my 4x2, I split the difference. Kept the rear (drive) tires the stock size (235-85x16E) and reduced the front tires to 225-75x16E to lower the front and additional inch.

I too am biased towards taller tires creating a longer (lower number) final drive ratio.

However, Big Dave, you are assuming that if you go up in tire size, you go up in mass. That isn't always true ... or at least not significant. Often, if you go "negative 1" (the opposite of "plus 1" sizing), like what blackjackal is proposing, you are going with a smaller diameter rim ... which is usually lighter.

Me 3.

Not possible, well it is "possible" but it's not worth it.
You cannot go "negative 1" on a 9th generation Toyota Corolla, unless you plan to only use 14s on the rears because the front discs won't allow for anything smaller than 15s. Even then it's really questionable because you're going to spend X amount of dollars on new 14x5" rims, then being unable to effectively rotate the tires you'll be going through tires a lot quicker. I don't think it will pay itself off fast enough to warrant the headaches.

thebrad -

So the brakes require 15" rims, huh? That never occurred to me. That sounds like a "Toyota Corolla" forum question. I know about www.saturnfans.com for my car, but I don't know about a Toyota Corolla forum where I could ask a question like this.

CarloSW2

Yeah the front rotor is 10.8" + 4" for the caliper, you need a 15" rim. I used to own a 2004 Corolla but totaled it in fall of 2007. 9thgenCorolla.com is the forum/resource for 2003-2009 Corollas. I also wouldn't recommend downsizing the brakes on that car since they are already somewhat underpowered.