Wheel and tire change, any FE improvement here?

[90crxHF]

Okay guys, I'm on a quest for 50MPG while trying to maintain 70MPH on the freeway. I know this is going to be very hard to do considering the 70MPH speed. I know that dropping speed by like 15MPH would probably get me my goal, but in Houston, everyone hauls ass. I'm getting better at not caring about this, but it's tough...

Okay for my next mod, do you guys think switching from a 195/50/15 to a 185/70/14 would be a wise choice? I plan on running them at a very high PSI to maximize rolling resistance. My current wheels are RPF1's and I'd be going to 14" steelies. I'm not getting rid of the RPF1's, I sort of want a spare set of Honda wheels anyhow....

Thanks in advance guys!! Oh BTW, I would plan on putting Mooneyes on these wheels for better aero...

[superchow]

Doing a little comparison I found the following information:

195/50 15 vs. 185/70 14: 185 tire will run 6.71% slower than the 195 tire at the same speed. That means that the 185 tire will show only 56.2 mph when the car is moving at 60 mph. So if you want to be travelling at 70 mph, your speedo should only read 65.6 mph.

Checked tire weight on Tire Rack and the selection is pretty meager for those sizes. Seems like tire weights are fairly comparable - so unsprung weight should stay close to the original size.

I did however notice that you can get a Bridgestone B381 in the 185 size. If I remember correctly this is one of the tires that did very well in a tire comparison in terms of rolling resistance.

So with all that in mind, you should be well on your way. I would experiment with some "drafting" to get you past the 50 mpg. By drafting I mean, following a larger vehicle at a distance you feel safe and comfortable with.

Sometimes I like to leave a larger gap, speed up just before a downgrade and then release the accelerator in gear and let the engine brake the car while catching the vehicle in front gradually. That way I can squeeze maybe a few miles of "fuel-injection-off coasting" while still having all ancillaries (A/C, power steering and brakes) operational in case of an emergency.

I'm curious to see other comments on this matter, as I may be going in a similar, but less drastic, direction. (205/55 16 current, 205/60 16 proposed.)

[MetroMPG]

Originally Posted by 90crxHF

I know this is going to be very hard to do considering the 70MPH speed.

You said it!

do you guys think switching from a 195/50/15 to a 185/70/14 would be a wise choice?

Did you check the circumference difference?

This tool shows you'd raise your effective gearing slightly, since the diameter of the 175 is larger.

And the narrower the tire, the better, in terms of aero drag. See here: http://ecomodder.com/forum/showthrea...g-cd-7475.html

You'd raise the ride height slightly, which is probably bad for drag.

But the smooth wheel cover would be a plus.

And even at the same pressure, two tires could have vastly different rolling resistance.

So... it's not easy to say for sure, but I'd do it.

(Actually, I'd do what Ben did on his CRX: put smooth alloys with the even narrower Insight tires on: http://ecomodder.com/forum/showthrea...ghts-9517.html )

[90crxHF]

Thanks for the replies guys!!

Yes, I've considered the ride height will be about 1/2 - 3/4" taller (tire differences are about 1.5" diameter), however to conteract this I will be installing a full under body tray very soon. I like that my effective gear ratio will go down, also it's a narrower tire which is better for aero and the wheel covers will help too. I'm only worried about them being a lot heavier. Currently my wheels with tire weigh less than 25lbs each...

Oh and is there any way to correct the speedo? Or will I have to make my own corrections when doing MPG calculations?

[superchow]

One more thought: Sealing panel gaps could smooth out the air flow. I would also check the underside of the rear of your car for any parachute-like objects. Mounting mud-flaps in front of the front tires may help offset the increased ride height? Good luck - I'm all for real world mpg improvements!

[90crxHF]

Originally Posted by superchow

One more thought: Sealing panel gaps could smooth out the air flow. I would also check the underside of the rear of your car for any parachute-like objects. Mounting mud-flaps in front of the front tires may help offset the increased ride height? Good luck - I'm all for real world mpg improvements!

I'm a long time lurker on this site, and I have since removed, tightened up, etc. most everything I could on the outside of the car. I am going to make a belly pan, and I want to do some sort of small bump across the rear hatch of the car to detatch the air properly and quickly from the car's behind. Anyone with a CRX make such a wing/spoiler yet??

[90crxHF]

Guys, I'm worried about the steel Honda wheels being way heavier than what I'm currently running and potentially making my MPG worse.

Current wheels are 15x7 RPF1 Enkei mounted on 195/50/15 Toyo Proxes4. My tires are getting to the point to where the need to be replaced.

How heavy is the steel wheel tire combo going to weigh? One of my wheel/tires right now wheel less than 25 lbs.?? Wouldn't the extra rotational mass hurt things more than help MPG?

[Ryland]

Stock CRX HF wheels are 13" steel wheels that are 4.5" wide and weigh around 12-13 pounds, stock civic vx wheels are also 13" by 4.5" wide alloy wheels but weigh around 11 pounds, same with the civic HX rims only they are 14" around, not sure how wide but in the 11 pound range, I think Honda Insight rims are a bit lighter, are also 14" by I think 4.5" with a smooth face to them so you could skip the wheel cover cost and wheel cover weight.
You might also check in to 1985 and maybe 1986 crx hf rear brake drums, along with the first gen honda insight rear brake drums as they were both aluminum drums instead of cast iron.

[cfg83]

90crxHF -

I am 90% sure that my Saturn has the 100 mm 4-lug bolt pattern that you have (let others chime in to correct me). If I'm right, and (BIG) *IF* you have the wheel-well space, you could use Saturn steelies and go up to 195/65/15 :

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

But, maybe that's just too insane.

CarloSW2

[ATaylorRacing]

I'd suggest going to some 155x80x13 tires on American Racing Estrella rims. I went from that size 13 to the 195x50x15 tire/rim package on my Geo Metro 3 banger and the tire/rim pacage is 8 lbs heavier per corner. The old tires had a max tire pressure of 35 while the new ones would take 51. In the end the gas mileage on flat ground/highway speeds remained the same due to much less rolling resistance, BUT the 1/4 mile times went from 19.9s normally to 20.6s. When the weather gets warmer I will return to the 13 set up. The Power Star 13" tires are 14 lbs while my Estrellas in 15" are 18 lbs. My 15 inch rims are 16 lbs while the Am Racing 13s are 12.8...BUT, the newer 13s can take more tire pressure than my old ones. Going back to my 1/4 mile ets, I did take an extra 32 lbs out of the car to make the wt the same as it was earlier with the 13s. When I go back to the 13s I will loose anoth 32 lbs so that my new wt will be 1598 compared to the present 1630. I have taken 132 lbs out of it, but added 32.

Another idea I was thinking about was to keep my 15s, but go to the 175x55x15 size that are 4 lbs lighter and less costly than a new tire AND rim set up!