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

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

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.

New tires have greater rolling resistance than worn tires, all else being equal, wider tires have lower rolling resistance than narrow ones.

Barry's Tire Tech

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.

doesn't have a egr valve ....

changed to lionhert tires some cheap tire to get on there I would like to keep this car cheap

+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.

I fitted two new tyres to the front of my Jazz, and the FE went right down.

Is it supposed to have an EGR?

Sorry for your difficulties. Well I drive a 1998 Civic DX, basically the same engine.

A 98 Civic LX does not have EGR.

I suspect some of your changes are the cause. You are missing a downstream O2 sensor? Isn't there a CEL lit on your dash? You have a non-standard exhaust header and mods to the electronic controls. All these and the new tires are sources for the possible losses in MPG. And despite confidence in your standard driving technique, our butt dynos are just too imprecise and too vulnerable to confirmation bias. Fuel changes, driving changes, colder weather... all could be producing or contributing to this decline.

I don't agree that narrower tires will have higher rolling resistance than wider. The contact patch on wider tires is larger. Did you buy LRR tires? Let the tires break in.

I hope some of that is helpful. Keep posting updates...

I have to ask - are you correctly calculating the difference in tire circumference into your MPG calculations?

no elaborate

I put the hot air intake on I will post results if the Hot air intake is better or worse than the CAI

if someone has experience with the change in diameter in tire vs FE please chime in
also was at the pump the other day and noticed 87 oct has 10% ethanol in it and the 91 said it was E free so gonna make that switch after the Intake results

The 185/65-14 tires are 2.2% larger than the 175/65-14 tires. Your odometer will read lower with the smaller tires, so you need to multiply your miles travelled by 2.2% when calculating mileage. So your 33mpg from your last tank is actually 33.7mpg.

As far as going to 91 octane for 0% ethanol, it won't be worth the extra cost. E10 fuel has 2.5% less energy than E0 fuel, so in theory you'll get 2.5% better mileage with 91 E0 fuel, but lower octane fuels burn faster, so this will offset some of this improvement. I also bet you're paying more than an extra 2.5% for the 91.

Right, as twj347 says, you need to adjust because your speedometer and odometer are wrong now, with different sized tires. They measure wheel rotations, but if each rotation takes you more or less distance, it won't be accurate.

Actually he switched to the smaller tires, which means that he is counting more distance on his odometer not less. The smaller tires spin faster and they turn more times per mile, so therefore in the same distance as the larger tires they will cause the odometer to count more miles.

Yes, you're right. That would make his mileage 32.3mpg instead of 33mpg.

put the hot air intake on with 185/65/14 tires and just got 35mpg ...... same commute every week filling up at the same pump

Cool! Your tires are also getting broken in better now after a 1000 miles, and that might be contributing to higher numbers.