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

Stock tire size on a 6th Gen Civic DX is 185/65 R14.

The engine is a stock 1.6L SOHC. In 5th gear at 60mph, the RPM's are something like 2,500.

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?

Since stock size is:
185/65 R14 OD=23.5 inches

Then, I think that the following might fit...
185/75 R14 OD=24.9 inches
195/65 R14 OD=24 inches
195/75 R14 OD=25.5 inches

I asked the guy at the local tire shop and told me that I was nuts. "That car is already designed for efficiency. Putting on bigger tires won't change anything."

However, how would cruising at 2,200 RPM at 60mph not use less fuel than 2,500 RPM?

Does anyone here have any experience with this type of lunacy?

The 8.5% RPM drop - from 2500 RPM to 2200 RPM from the smallest to largest circumferenct tires you list - will likely save fuel.

I switched final drives in my car, and dropped my RPM by 25% for a given road speed, and saw roughly a 6% improvement in fuel economy on the highway.

The potential trouble with doing a tire swap is you're also raising the car's ride height, which will probably increase aero drag. Maybe not a lot, but look at "eco" versions of European car models, and many have a lower ride height than the regular versions, on the order of about 15-20 mm.

As well, going from 185 to 195 width also increases drag by increasing Cd & frontal area (yes, slightly).

And lastly: would the rolling resistance of the tire you switch to be better or worse than the tire that's on there now? That's something to consider too.

So you're potentially changing a lot of variables, some of which oppose each other in terms of efficiency. If I were going to do this, I'd want to be able to empirically test it before spending the money. (Borrow someone else's rims with taller tires? And even then, are the rims aerodynamically the same as yours?)

And don't forget you'll also need to either recalibrate your speedo/odo, or manually do the calc to your distance to know if it's had an effect on MPG.

I have the same 2500 rpm at 60.

The 5th and 6th gen transmissions are interchangeable. If you can find a Cx/VX trans, you'll get that rpm drop while keeping your same wheels/rims. It shouldn't cost a whole lot more, either. I haven't done it because I don't do that much highway driving. If I did, it'd be done by now.

Remember, too, that larger diameter tires will have much worse rotational inertia. Nearly all the mass of a tire is on the outer diameter, at the tread / steel belts. Any time you change speeds, it'll be working against you - massively.

UnCivic -

Welcome to EM! I did a modest one-size-up tire mod :

http://ecomodder.com/forum/showthrea...ires-2642.html

The tire calculators on the net showed a 2% change, i.e. the speedometer will read 2% slow.

Since my odometer is calibrated for an OEM tire size, I did a GPS correction that agrees with the calculations, a 2% difference. Now, when I do my fuel logs, I add 2% more miles to the number of miles on the odometer. For example, if I drive 100 miles, I write down 102 miles in my fuel log.

CarloSW2

Ooh, right. I forgot that one.

Good tip on the transmission internals.

1. Swapping transmissions or components is unfortunately not an option.
2. Ride height is not a concern.
3. Drag ratio of car isn't a concern.
4. Drag ratio of tires is a concern. However, so is handling. I've used 175/70 R14's snow tires and find them to be too floppy. The 185 or 195 size seems to be the most realistic compromise here.

Right now with 185/65 R14's I get anywhere between 30 and 40 mpg. Once I got 44mpg but that was an exceptional case for a temporary job where I did a lot of travel along a long flat stretch of policed road with a 35mph speed limit. I was also running stock tires at 44psi.

Once upon a time in a former life I ran a with a set of 195/45 R15's. The reduced 22" OD provided much zippiness that was great for in-town driving, and autocross. However, 3,500 RPMs at highway speeds wasn't great for things such as fuel economy. The speedo was way off too. Radar signs would say that I was doing 75mph, but the speedo would be indicating a lot more than that. :)

I'm basically looking to do the opposite of this... bigger OD tires on 14" stock rims for taller final drive, less acceleration and potentially better fuel economy.

Has anyone actually tried this? If so, with what sized wheels, and what were the effects? For example, that little Honda engine only makes about 80 ft-lb of torque. Will the oversized OD tires render 5th gear useless for anything but downhill driving?

CarloSW2,

That's getting close to what I was thinking!... but you have a Saturn. Not a Civic.

Do you notice any difference in acceleration? What about RPMs?

Thanks

UnCivic -

At 2% I think I am fine. I drive like a granny, but I have enough acceleration in the lower gears when I need it.

I do have a 5th gear mod here :

http://ecomodder.com/forum/showthrea...swap-2779.html

This is a 21% change in RPM, not the ~10+% you are talking about. My 5th gear is a lot weaker than it used to be. I do have to use 4th strategically, but usually not on my main commute. But this *only* effects my 5th gear. I don't have a lot of hills where I live, so it isn't something I have to deal with often. Your tire mod will effect *all* your gears.

If I were you, I would target the 5.1% or lower change in tire size.

CarloSW2

Raising the car alone shouldn't increase aero drag except by the frontal area from the tires. He's not putting lift springs on the car and pushing the suspension into the airstream. IIRC, someone posted an article a while ago about vehicles raised off the ground being more aerodynamic

Someone on Miata.net switched to 10% larger diameter tires and saw a big increase in economy.

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.

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

UnCivic -

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 :

http://ecomodder.com/forum/attachmen...1&d=1236791447

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

.

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 ;)

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 ;)

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.

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!

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!

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.

Update
 

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.

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.

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% = ????????

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

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…

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.

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

Hero, Zero depends on the day.

Dave

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.

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

Dave

The Verdict...
 

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.

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).

I've changed my tire size, but kinda net'd my resulting mpg.

I bought larger wheels 17x7 compared to my 15x6. The 17's are actually a little lighter than the 15s, but the tires are heavier.

My Civic Si is geared close, so the larger diameter wheel made it longer, but since the tires are wider, I'm not sure if it made a difference as mpg's go.

I bet if I just replaced it with 15" rims that were lighter, I could have gained 1mpg.

There are so many factors, I think stuff like this falls into the "Trial and Error" sort of testing, rather than having a definitive answer before anything has changed. I'm glad to see you gave it a shot, and now we know. :)

Exactly... There are too many variables that effect efficiency. In my case, I just wanted to keep to 'real-world' driving to see what would happen. I could have overinflated the tires, but I kept them at 35psi. However, I think that over inflation would have resulted in more rubbing, and worse handling. The type of tire tread is a big factor too, but I had no control over this because of economy and availability. The condition of the engine. I know that at 162K miles that the my little Civic isn't as efficient as it was at 30K.

However, from this, we now know that.
1. Changing the final drive gear drastically affects driving characteristics and doesn't always result in more efficiency.

2. 185/65/15's and 195/60/15's will fit a 6th gen Civic hatch without modifications to the wheel wells. However, there will be some rubbing in some extreme cases.

3. It can be assumed that any size increase above this will probably result in more rubbing unless modifications are made to the wheel wells.

4. Even with the required wheel well modifications using something larger, say 185/75/15 would result in unacceptable vehicle handling (in my opinion).

Thanks for the testing and results, Uncivic. I'm surprised. Wonder what would have happened if you drove like me? I try to accelerate at best BSFC rate, which means shifting at 1700-2000 rpm in the lower gears.

I could've told you from day 1 that increasing the tire size would decrease your MPG's. Same deal on my truck. Stock size = 31" tires. Went to 32's and saw a 2MPG drop (on three different trucks).

Few factors here. (sorry if it's already been discussed---I just skimmed through). The biggest is the rotational mass. Even at the same weight, weight further from the center will require more power. Given the tires will weigh MORE with increased diameter (in most cases), it will rob you of efficiency.

As already mentioned, gearing might not be favorable. There was a guy who swapped in a 4.9 FD into his Integra and saw a 2MPG INCREASE (29MPG to 31). He is not the first one. Reason stated was that even though the RPM's increased, at higher speed, it made the engine work more efficiently. Aerodynamics plays a big role when you get up to speed, and higher RPM meant he was cutting through the wind easier vs. at lower RPM, he had to gas it more to maintain speed.

So the key is, find out where the best balance is. IMO, they got it right from the factory, as far as that goes.

With the increased 185/65/15 size I found short-shifting like that to not work so well because my engine doesn't produce enough low end torque. Especially on the hills around here. With stock 185/65/14's it is possible to shift at lower rpm's.

From experience, I can agree with that. Once upon a time I had a 1994 Del Sol. They were geared ridiculously low with RPM's at 70 mph being about 4,000. However, that car easily got 35 mpg. So, yeah, there is much more to fuel efficiency than just a final drive gear.

Anyway, I look at it this way... Honda (or any auto maker) has many engineers who are much smarter than myself, and they spend thousands of hours getting this stuff right. What are the chances that I could improve on that? It's not likely. However, it sure is fun to try different setups. :)

Its all about compromises do you want acceleration or good fe. Basic rule anyway can be applied if make your car more aerodynamic you can use taller gearing.

Yes, I'm sure the auto manuf. have tested various sizes and gearing for the best MPG's, especially now days. But I agree, it is fun to try and trump the engineers. :D

Going to skinnier tires, but the same overall height will help. I have tried to find another tire size for my Civic, but have had a hard time. My current size is the same at 185/65/14 and was looking for a 165 or 175 equivalent (keeping the overall diameter the same). But that would mean a 73 or 69-series tires (respectively)....which don't exist. But, the closest is 175/70/14's. I am wondering if it's worth it. Tirerack has several of these available. On my next tire change, I might spring for them.

Also, I have the HX rims, which are very light. If you're on stock steelies, I'd suggest trying to find a set of HX rims. Or even aftermarket 14" alloy rims would help.

cackalak,

I had a set of 175/70/14 snow tires on the Civic a few years ago. They are very narrow, and the ride felt very wobbly and disconnected. I got rid of them after one season. Unfortunately, I didn't keep track of fuel economy back then. However, they were full tread snow tires, so they're not the best tires for FE anyway.

Anyway, based on the above 'research'. Unless you're driving an aerodynamically modified civic on flat ground at speeds under 65mph, you'll probably find that you get the best FE with the 185/65/14's. Especially if you have lighter 14" rims.

Also, check Craigslist. That's where I found a cheap experimental set of 185/65/15's. I'm glad that I didn't pay full price for that!

If you do get a set of 175's please post an update.

Thanks.

Update...
 

In case anyone was interested...

After putting the 195/60/15's back on, FE has gone back up to a consistent ~34mpg...

and it handles so much better!