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.