Largest Tire Diameter on a 6th Gen Civic (for lower RPM)

[UnCivic]

CarloSW2,

That's an interesting 5th gear mod and would be perfect for what I want to accomplish. Unfortunately, the only Civic transmission mod that I could realistically make would be to replace the long geared D16Y7 transmission with a shorter geared D16Y8 transmission, but this is contrary to the philosophy du jour.

Since I'm leaning towards the 195 tire size anyway, the options are really narrowed down to:
195/65 R14 OD=24 inches = ~+2%
195/70 R14 OD=24.7 inches = ~+5%

What I may do is get single 195/70 mounted to see if it fits. If it does, then I can buy the other 3. If it doesn't fit, then the 195/65's will probably fit, so the worst case is that I eat the cost of one tire mounting.

[MetroMPG]

Originally Posted by winkosmosis

Raising the car alone shouldn't increase aero drag except by the frontal area from the tires.

That's a blanket statement that you likely can't support by fact.

[cfg83]

UnCivic -

Originally Posted by UnCivic

CarloSW2,

That's an interesting 5th gear mod and would be perfect for what I want to accomplish. Unfortunately, the only Civic transmission mod that I could realistically make would be to replace the long geared D16Y7 transmission with a shorter geared D16Y8 transmission, but this is contrary to the philosophy du jour.

Since I'm leaning towards the 195 tire size anyway, the options are really narrowed down to:
195/65 R14 OD=24 inches = ~+2%
195/70 R14 OD=24.7 inches = ~+5%

What I may do is get single 195/70 mounted to see if it fits. If it does, then I can buy the other 3. If it doesn't fit, then the 195/65's will probably fit, so the worst case is that I eat the cost of one tire mounting.

Sounds like a plan. I'd be willing to do that test for my car also. Here is a comparison of the 195/70 R14 to OEM :



Your thread got me thinking that I will explore bumping up the tire size again when my current set needs replacing.

I mentioned the gear mod because my 2% isn't as big as the possible ~10% you were talking about. I don't know what your typical driving topography is, but I definitely think you are still fine at ~5%.

CarloSW2

.

[Vekke]

Hi I will find out soon if taller tires work for me. My original size is 185/65/14, now i Bought new summer tires and they come in size of 175/80/14. After long tire comparison I decided to bought Toyo 350 which should be LRR tires. I will put them on after 2-3 weeks. You should also notice that taller tires also fill better the wheel wells so maybe little bit less drag also

[Vekke]

Time to say some results of my tire swap. Now my rpm:s are 1940 @60 mph and my speedo is off by 10%. I can definedly see some improvement in my consumpion, specially in motorway speeds. I am now getting same or little consumption figures as in winter with higher summer speed limits...

I bought also smaller 13" tires to the back because those big 14" were little too big. And I dont get the same gain from the rear tires as I get from the front.

14" 175/80/R14 Toyo 350 with steel wheels weighted with hubcap 16,8 kg per piece
13 165/70/R13 Toyo 350 with aluminium wheel weigths only 11,6 kg

That is huge difference I i will chance my front 14" steel wheels for lighter steelies soon

[daqcivic]

Originally Posted by UnCivic

Since most of my driving is in 5th gear on the highway, in the interest of lowering RPM's at crusing speed to improve fuel economy, has anyone ever tried to increase the final drive ratio by using larger OD tires?

I haven't experimented with different diameters, but I can tell you a tire with significantly greater diameter will probably also have a higher tread width, which will eat up some of the efficiency you save with lower RPMs (as will the greater rotational losses already mentioned). I've noticed with my 195 (vs. 175 stock) tires I get noticeably reduced coasting distance, indicating that just 20mm adds significant rolling resistance.

To me the cost/benefit ratio doesn't make it worthwhile, as you could probably make up the same improvement with other (cheaper) strategies such as slowing down and doing simple aero mods.

[UnCivic]

16.8 Kg = 37 pounds. That's about as much as my 16x7 OZ Superturismos with the tires!!! Those are some HEAVY wheels. Stock Civic 14x5 wheel + tires weigh ~28 pounds. It has to be interesting driving with the mixed wheel types too!

[UnCivic]

Originally Posted by daqcivic

I haven't experimented with different diameters, but I can tell you a tire with significantly greater diameter will probably also have a higher tread width, which will eat up some of the efficiency you save with lower RPMs (as will the greater rotational losses already mentioned). I've noticed with my 195 (vs. 175 stock) tires I get noticeably reduced coasting distance, indicating that just 20mm adds significant rolling resistance.

To me the cost/benefit ratio doesn't make it worthwhile, as you could probably make up the same improvement with other (cheaper) strategies such as slowing down and doing simple aero mods.

I totally agree with this. With 185/65-14's I'd get 34 to 40 mpg. With 205/45-16's I'd be lucky to get 34 mpg.

I still haven't gotten around to trying 195/65-14's yet though... Actually, I've been thinking about a setup with 195/55-16 on a set of 16x6 rims. That's ~4% over stock diameter. The problem has been finding a set of cheap 16x6's to experiment with!

[daqcivic]

Perhaps going just 10mm wider will not add enough drag to be significant. I would wonder, however, about going to a 2in larger diameter wheel, as the wheels/tire package in such a setup would weight significantly more than what you have.

If had money to throw at a new set of tires and wheels, I might try going back to stock 175/75-14s, just to be able to compare MPG--but I would miss the extra grip in the corners.

[UnCivic]

I find it strange that more people haven't posted anything more about this. Surely there are others out there who have tried the oversize tire thing.

Anyway, after many months I managed to pick up a set of 195/60/15's and an old school set of 15" Integra at 5 wheels that I found on Craigslist.

So, here's the comparison...

Before
185/65/14 OD=23.5" % OD Diff = 0 MPG = ~34 to ~36

After
195/60/15 OD=24.2" % OD Diff = +2.9% MPG = ~32 to ~33

So even with the MPG correction for the diameter difference, I'm actually seeing a slight decrease in mpg.

Now, there are a few hundred disclaimers here.
1. A lot of my driving is in Pittsburgh PA on relatively steep hills.
2. Driving style is 'approximately' the same. However, I am not using any of the more exotic or serious hypermileing techniques. Actually, with my car, I can drive it really hard by shifting near the redline or really soft by shifting at 2,300 rpm, and MPG doesn't vary much.
3. The car has 10,000 more miles on it now than it did a few months ago.
4. I'm making no account for temperature / humidity difference.
5. I'm using 87 octane gas from different sources.
6. Tire air pressure is "about" the same, and neither set was overly inflated.
7. The weight of the 195's on Fat 5's is about 3 or 4 pounds heaver than the 185's on stock steelies.

And if you were wondering....
1. The effect of the larger OD tires is interesting. It seems to make the car roll better. Though this could be the result of a better wheel alignment than before.
2. The 195/60/15's don't rub during normal driving. However, if a heavy person is sitting in the passenger seat, the front tires do rub the inside of the wheel well when the wheel is fully locked left or right.
3. The difference in 5th gear cruising RPMs is minimal (ie, 2.9%)
4. The difference in reduced acceleration feel is less than minimal. (ie, more than 2.9%)
5. From a handling perspective, the 195/60/15's feel much more solid than the 185/65/14's.


I still want to try 185/65/15's... However, apparently this is not a common tire size (excepting snow tires) and I'm having problems finding a cheap or used set to experiment with.