98 civic mods are hurting mpg

[oil pan 4]

Is it supposed to have an EGR?

[California98Civic]

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...

[Ecky]

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

[nickritter1992]

Originally Posted by Ecky

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

no elaborate

[nickritter1992]

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

[twj347]

Originally Posted by nickritter1992

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.

[Ecky]

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.

[California98Civic]

Originally Posted by twj347

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. ...

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.

[twj347]

Originally Posted by California98Civic

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.

[nickritter1992]

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