hey, does anyone have any thoughts on how to correct this? i'm on here just about every day posting messages and trying to learn more about cars, but lately i've been experiencing some very ****ty fuel economy. it started getting BAD after i installed my header+dp and my rims. hard to tell which did it since they were installed within hours of one another, but i'm guessing it's the rims. why would exiting the gas more freely hamper economics?

here's some specs for ya: 18x8 +42mm offset. i'm sure that with the 225/40/zr18 tires, there's a lot more for it to pull. so i guess here's where the question really comes in... what are my options for getting better fuel econ again? i'm down from 32 to about 24.5 and i commute around 70mi/day. not cool.

btw: the only recomendation i've received thus far was to add more teeth to my gears... umm..

 

How bad is bad. You might want to give us a fairly accurate ballpark number if anyone is going to be able to offer any help. That and of course the obligatory: are you sure you're not suffering from a case of the lead foot.

 

headers give you better exhaust flow allowing more fuel to enter the cylinder. any kind of performance upgrade that increases the amount of fuel mixture, speed of travel of fuel, etc is going to lessen mileage.

next question: is your speedo off? if that's off, then yes, new wheels and tires that don't equal the stock wheel/tire diameter will cause your speedo to read slower. if you press the gas to get your speedo to match the current limit (which of course you're overshooting) then you will use more fuel because you involuntarily engage in the leadfooted happiness of driving

 

adding you bigger wheels and tires is causing you speedometer to run slower than you expect. (example it might be reading 60 mph but you are actually going 70 {just using round numbers})

so keeping that in mind lets say you drove for an hour and put in 3 gallons to fill it up. you would think you were only getting 20mpg but you would actually be getting 23.33 mpg. Now that is just an example.

since your engine is breathing better both in and out you are burning more fuel. so lets just say you lost 10% for each of the above items (wheels, dp, & exhaust), that would be about 9% of which only 6% is acutal loss. my guess is yo are still actually getting around 26+ mpg.

You should be able to get the correct reduction gear for you speed cable that would correct you speed accuracy. Then re do the math.

 

Your stock wheels and tires rotate 833 times per mile, the 225/40/18 turn 804 times per mile, this means that when your driving at 60mph you are really traveling at 62.2 which is a 3.6%
difference. So if you take your new gas mileage at 24.5 and times it by 3.6% you will see that it is 33 mpg.

your gas mileage is correct watch your speeding.

 

you multiplied 24.5 by 36 not 3.6, nice try though.

 

smart #%$ static. When i multiplied 24.5 by 36 i get 882

out of school to long?

 

Please gentlemen. Easy.

 

Hi hobie just kidding, guess it wasn't funny. free hugs!!

 

No harm, no foul. Sarcasm isn't easy to detect in text.

 

sorry i didnt put the % sign.

 

andy, where did you get those rotation numbers for tires? i would like to get that printed out.

 

Here is the link

www.miata.net/garage/tirecalc.html

 

wow how in the hell did you get the ES to get 32 mpg?!?! i am running stock and am getting 24/25 mpg (which the little sticker from the dealer said i would get in the city) unless you are constantly on the highway. if you are then i am just a dumbass and you can ignore me.

 

thanks andy, that is a great info site!