I really hate to sound noobish about this but I went from 205/60/15 to 225/45/17 and my car doesn't seem to get the gas mileage it used to get with the 15's. I cleaned the TB and everything but thinking it could be KS or IACV or even plugs (running BOSCH Plat +4). I have heard that sometimes the speedometer can be thrown off or your ECU needs to be programmed to recognize the new tire size. Car runs great otherwise, although sometimes the car does idle around 1500 or so RPMs before settling to 750. Any info or pointing me in the right direction is greatly appreciated.

 

lol. Whoever told you that was either really dumb or..really dumb. There is NO such thing as reprogramming the ECU for tire size. The reason you are getting worse gas mileage is because the wheels are just bigger, resulting in greater rotational mass/more power needed to turn the wheels. What kind of wheels/tires did you get? They are obviously much heavier than your stock 15s. As for the idle going from 1500-->750, that's normal. When you start the car, it will idle higher until it reaches the warm operating temperature. Good luck..

 

Using the stock 5th Gen 17"s off of a 2002. I was getting around 320-330 on a full tank and now I'm around 275-280. That seems like a big decrease but I could be wrong. that may be standard for bigger size tires and rims like that. The stock 17"s are heavier than the 15" Primax's that were on there.

 

funny 245/45/17 tires are actaully one half inch smaller in diameter. though the rims are most likely heavier. so rotation of a tire has nothing to do with gas mileqage. but the extra weight does.

 

25.86" 225-50-17

25.68" = 245-45-17

Where did you get 245 size from? The 205's he was using were too small to begin with


Stock size is either 205/65 or 215/60-15's, that's between 25.15-25.5"

speed accuracy now is better than it was with the 205's.

Doesn;t matter b/c the only reason you're getting worse mileage is like everyone stated, larger wheel.

 

get those darn Bosch spark plugs out of VQ motor , and replace with NGK's
You will have nothing but problems with German plugs in Japanese motor

and i bet your gas milage will improve !!!

Nick.

 

Okay, i'll change the plugs out but doesn't your speedometer needs to be adjusted for the extra tire size or something like that?

 

what are you going to do? spend hundreds of dollars getting a new ecu? i never see an aftermarket reprogram just for the tire size.

you might get a little off at higher speeds, but i wouldn't worry much about that.

 

No .

 

If you use a proportional tire size then NO, you will not need to reprogram your ECU. Reprogramming an ECU for tire sizes is necessary only if you have a truck or something and are going to put huge off road tires that are much bigger than the OEM size. If your outside tire diameter is close then you are fine. Tire pressure can have about the same effect as relatively close sizes.

This is a calculator that shows you the difference in sizes. Remember that once weight is put on the tires and there is air pressure in them that this will not be completely accurate but keep it close.
http://www.miata.net/garage/tirecalc.html

I would recommend getting your ECU reprogrammed for performance reasons (through JWT or TS), but that is a different story. It might help your gas mileage as well if you can help keeping your foot off the pedal.

The stock 17" 5th gen wheels are pretty heavy. A great mod for 5th gens is a set of lighter weight wheels.