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Old 10-13-2017, 11:40 AM   #1 (permalink)
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98 civic mods are hurting mpg

hi everyone I have been lurking around the forums for some time now I have some unsolved questions first I will walk you through my setup and whats happening

1998 civic LX 215k miles 5 speed manual

when I first bought the car I was getting around 36mpg combined (70hw/30 town) recently my mpg has been going down having done basic maintenance not sure if its because of winter and gas blends but now I'm getting 33mpg with no driving habits changed any ways....

maintenance done

switched from 185/65/14 tires to 175/65/14 tires at 50psi 23.5 inches in diameter to 23 inches in diameter I thought the width would help me

New primary O2 with ebay header (old manifold was cracked and sensor went bad) plugged the cat I left the secondary 02 sensor out

changed oil and filter
timing belt done at a shop timing was set
new clutch
not sure about plugs and wires but it doesn't have any problems idling

cold air intake, I've only read people gaining mpg on these cars with this I have even see a VS on a geo metro forum and there was zero difference in mpg

I am wondering, anyone from the north notice a 3mpg loss from summer blend gas to winter gas ? also I am considering stepping up to a 175/80/14 (26inch tire) to help my gearing, 60mph is 2600rpm I'm having a feeling losing 0.5 inches on my tire diameter hurt me as well I'm considering putting the car on hondata s300 as well and tuning for mpg via Wideband 02 I haven't seemed to find anyone on here who has done so I did this in my 90 Ls1/t56 Rx7 with hp tuners and I get 29mpg hw on e85 makes 470rwhp/490ftlbs all motor 370 C.i. runs 17:1 afr down the highway so this 33mpg is frustrating I've also considered removing the PS pump on my car for a mpg bump

I drive with the cruise control on everywhere

any advise would be great I'm really considering the taller tire, from a 23inch to 26inch if anyone has fit such a tire on a car please share ! and also share mpg gains
also considering removing ps pump and trying to do the warm air intake
I did all these mods at once and then winter came. so theres so many factors I don't know about and would like to hear from the pros I would like to get this to 40mpg without any "silly" driving habits I.e. shutting the car off etc and leaving the alternator on

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Old 10-13-2017, 11:53 AM   #2 (permalink)
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The taller tires will probably help. It would feel less than fabulous to get new tires so soon after having just fitted these, though.

Warm air intake is your way forward, not CAI. Cold air = more mass flow into the engine, = more fuel capacity = more power. Warm air means less mass flow, less fuel, less power, requiring wider throttle openings to maintain the same power. Lower pumping losses.

Fire up the Hondata monitoring and see what AFR it's giving you. Will it permit lean burn? Can it be tweaked to do that? With the second O2 sensor it may not, and I think some of your fuel economy potential went away with that second O2 sensor.
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Old 10-13-2017, 12:08 PM   #3 (permalink)
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Quote:
Originally Posted by elhigh View Post
The taller tires will probably help. It would feel less than fabulous to get new tires so soon after having just fitted these, though.

Warm air intake is your way forward, not CAI. Cold air = more mass flow into the engine, = more fuel capacity = more power. Warm air means less mass flow, less fuel, less power, requiring wider throttle openings to maintain the same power. Lower pumping losses.

Fire up the Hondata monitoring and see what AFR it's giving you. Will it permit lean burn? Can it be tweaked to do that? With the second O2 sensor it may not, and I think some of your fuel economy potential went away with that second O2 sensor.
car needs to be converted to OBD1 then chipped with eprom then Wideband 02 would be 230$ for that mod if I tuned it at causing conditions just don't know if its worth it

tires are cheap If you factor in megs gained over the course of a year with 25k miles a year it will make up for its self
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Old 10-13-2017, 12:39 PM   #4 (permalink)
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Nope, you want taller tires unless you regear.

See if your EGR is plugging up. Seems they like to plug up around 200k miles.

I use both warm air and cold air depending on operating conditions.warm air for economy cold air for power, controlled by my switchable cold/warm air intake.
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Old 10-13-2017, 12:43 PM   #5 (permalink)
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New tires have greater rolling resistance than worn tires, all else being equal, wider tires have lower rolling resistance than narrow ones.

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Old 10-13-2017, 12:45 PM   #6 (permalink)
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Smaller tires with a lower load rating will have higher rolling resistance at a given pressure. New tires tend to have higher rolling resistance than worn tires due to the flexing of the tread. Did you change the model of tire? It may have higher rolling resistance. The model of tire is more important than the size or pressure when it comes to rolling resistance. Of course your gearing is a bit shorter now as well, which will mostly affect highway mileage.
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Old 10-13-2017, 01:29 PM   #7 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Nope, you want taller tires unless you regear.

See if your EGR is plugging up. Seems they like to plug up around 200k miles.

I use both warm air and cold air depending on operating conditions.warm air for economy cold air for power, controlled by my switchable cold/warm air intake.
doesn't have a egr valve ....
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Old 10-13-2017, 01:31 PM   #8 (permalink)
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Quote:
Originally Posted by twj347 View Post
Smaller tires with a lower load rating will have higher rolling resistance at a given pressure. New tires tend to have higher rolling resistance than worn tires due to the flexing of the tread. Did you change the model of tire? It may have higher rolling resistance. The model of tire is more important than the size or pressure when it comes to rolling resistance. Of course your gearing is a bit shorter now as well, which will mostly affect highway mileage.
changed to lionhert tires some cheap tire to get on there I would like to keep this car cheap
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Old 10-13-2017, 01:39 PM   #9 (permalink)
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+1 on the EGR. Mine had gotten snotty with less than 150,000 miles on it, I picked up a couple of points when i cleaned it out.
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Old 10-13-2017, 02:20 PM   #10 (permalink)
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I fitted two new tyres to the front of my Jazz, and the FE went right down.

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