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

[drew1d]

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.

[UnCivic]

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).

[SentraSE-R]

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.

[cackalak]

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.

[UnCivic]

Originally Posted by SentraSE-R

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.

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.

[UnCivic]

Originally Posted by cackalak

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.

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.

[Vekke]

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.

[cackalak]

Originally Posted by UnCivic

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.

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.

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.

[UnCivic]

Originally Posted by cackalak

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.

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.

[UnCivic]

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!