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

[SentraSE-R]

At some point, you'll run into wheelwell clearance problems, and tire rubbing. That's why most tire shops recommend no more than a 3% increase in diameter when replacing tires.

[UnCivic]

3%... that sounds about right...

185/65/14 = +0% = No rubbing
195/65/15 = +2.9% = Rubbing only under extreme circumstances.
185/65/15 = +4.1% = ????????

[dwtaylorpdx]

Ok,, so its by NO means a eco rig, But I also drive a 1994 Jeep wrangler and I was running, 285/65x16 tires, Heavy mud tread. I switched to 235/85x16 tires same tread. Similar height, but 3" narrower. I gained almost 5 MPG. (Yes thats a 5 not a .5)

So there is example however gross

Dave

[UnCivic]

Irony,

After yesterday’s posting, I found a pair of 185/65/15’s on rims for cheap on CL!!!

185/65/14 OD=23.5 inches
185/65/15 OD=24.5 inches
4% difference.

I have a total of 8 miles on them now. The effects on the civic are fairly dramatic. It’s a different car. It feels like it’s bigger and sitting higher. The shift points are noticeably changed. The handling is much more squishy than the 195’s.

The fronts did rub a little when turning out of my steep driveway. The 195/60/15’s never did that. However, everything seemed good on the road

Stay tuned for MPG and other comments…

[UnCivic]

285/65 R16 OD=30.6 inches
235/85 R16 OD=31.7 inches
3.5% Diff


I'd like to see +5MPG in my civic, but, I'm not sure that anything is going to change in my case. Even after 8 miles, with only 80 ft-lb of torque max, I know that I'm going to be doing a lot more downshifting. This will probably make the average RPM's go up in the long run. It would be ironic if my FE went down.

[dwtaylorpdx]

Just like car racing, small changes sometimes send you into the weeds.....

Hero, Zero depends on the day.

Dave

[rkcarguy]

We did this on my wifes 1998 Civic DX automatic because it cruises on the freeway over 3000rpm. We went from 175-65-14's to 195-60-15's that we got from a newer Civic owner that got rims... for $100. We've seen an increase from ~30mpg to ~33mpg average, and that doesn't factor in that we are probably going more miles than the odometer says because of the increase in tire size.
They handle better, look better, and fill up the wheel wells more than the 14's....no rubbing problems.

[dwtaylorpdx]

You might get more gain with the speedo calibrated. ITs part of the ecu package these days...

Dave

[UnCivic]

Okay, I put about 600 miles on the Civic with the 185/65/15's

Contrary to all preconceived logic, the net efficiency result with the 4% correction, was only 30 mpg.

I was getting 35mpg before? Why did I loose almost 5mpg?

1. To make smooth shifts the PRM’s had to be taken up higher. This applied to all gears, but was especially the case from 1st to 2nd. Normally, when driving with a light foot, I could shift from 1st to 2nd around 2500rpm without a problem. With the larger OD setup, shifting at anything less than 3000rpm resulted in unsatisfactory bog-down when taking 2nd gear. This resulted in more lead-footed in 1st gear takeoffs and more cruising around parking lots and side streets in 1st gear.

2. 2nd, 3rd gears got used a lot more for regular driving. 4th gear became the new highway gear in traffic. 5th gear was great above 60 mph on flat roads or downhill. However, it was pretty useless for going up any long hills or for driving in traffic.


Other points of interest were:
1. The car felt like it was a bigger car. That was kind of neat.
2. Brake action changed a lot. There was a good bit of initial fade. However, the wheels didn’t lock up as easily so I could apply much more braking force. I can’t say if the net result was better or not.
3. The 185/65/15 were way too floaty for my liking. For driving on anything-but-smooth Pittsburgh roads, my preference is for the 50 to 60 aspect ratios. For a larger OD setup, something like 195/50/16’s might feel a lot better. However, I have never seen 16” wheels with 195’s mounted to them.
4. Rubbing was about the same as with the 195/60/15’s. It only happened a couple of times while maneuvering in a parking lot or doing fast turnouts onto main roads.
5. Having to keep the revs up higher to shift meant that I probably drove faster.
6. 2nd gear went all the way to 70mph!. That was kind of fun.
7. 3rd gear pulled "well" from 70 to 80, but that was it. Not enough torque.
8. Top speed was only around 90mph. It would only go faster on the downhills.
9. I think that this setup could work in the case where most driving was on flat ground, or in a car with a larger engine with more than 80 ft-lbs of toque.

It was an interesting experiment, but the Civic was more than glad to have the 195/60/15’s put back on.

[UnCivic]

For any Civic DX of that generation stock size should be 185/16/15

175/65 R14 OD=23 inches = -2% from stock
185/65 R14 OD=23.5 inches = 0%
195/60 R15 OD=24.2 inches = +4%

175's were under sized, so your odometer would have read more miles than you actually traveled. For example, 100 miles would really have been 98 miles. Also, your 4th gear (automatic) cruising rpm's would have been higher (probably around 3300 at 65mph). Conversely, with the 195's, 100 miles is really 104 miles, and 4th gear cruising rpms are lower (probably around 2900 at 65mph for the automatic).