question for a tuner

I_luv_HP&trq · **question for a tuner**

ok so i used my tuner to tell my cars computer that i'm running with 18 inch rims. I've never made this adjustment before but thought i would try it. I used the equation that the manual said to use. However i found out on the way to work that my spedometer was at 60 MPH and my RPM's were at 2,500. it should have been around 2,000. Then all of a sudden i'm flying past cars thinking i'm only going 60 so i'm assuming something got screwed up when i made the adjustment. It felt like the car was at it's max limit and didn't want to go past 60. it was the same feeling when you reach 115 MPH and trying to hit 120 but it takes a while after 115 MPH.

Plus it took my car a while to shift out of third. I'm not running aftermarket gears so that's not the problem. I'm going to just change my tune back, but was wondering if anyone knew why this was happening. I thought telling my car the correct size of my wheel would help but apparently it didn't. anyone know the cause?

justin00stang · **Re: question for a tuner**

What kind of tuner do you have? what size are your tires? what size rear end gears? what rev's per mile or tire size value did you put in the tuner?

I_luv_HP&trq · **Re: question for a tuner**

i have a diablosport predator tuner. my tires are nitto 245/45 ZR18. I have stock gears on my 99. and i put the rev's per mile at 1120. the equation in the manual i have says to divide 20168 by the diameter of the wheel and thats how i got 1120.
I already set my car back to stock and it works fine but i was just curious why it didn't work like i thought it would. even though the problem is gone i just wanted to find out what got screwed up so i know for future reference.

jimmy_beaner · **Re: question for a tuner**

ummm, diameter of the WHEEL, not rim... I'm no tuner, but maybe this is why... saying an 18" wheel would make you run REAL fast like you said you were.

I_luv_HP&trq · **Re: question for a tuner**

i call the wheel and rim the same thing. so what did i measure wrong then? am i suppose to measure the end from one side of the tire to the other? my manual just states "divide 20168 by the diameter of your wheel". it's probably an easy mistake i made but i'm tired as hell from work and can't think right now....

jimmy_beaner · **Re: question for a tuner**

measure from bottom of the tire to the top of the tire... through the middle... you have to include the tire, not just the rim

lowflyn · **Re: question for a tuner**

I believe the real problem is that your tires are off by 10 in sidewall height. I don't exactly know how to fix this, but yeh, you're gonna need a measurement of the wheel plus the tire. That's the only way to calculate this correctly.

**SpectorV** · **Re: question for a tuner**

If you buy the CORRECT size tires for your rim size you will not have ANY problem with your speedometer. I have no idea what the correct aspect ratio would need to be for that size, you would have to do the math and im to sleepy to do that lol.

DO NOT raise your rev limiter unless you KNOW what you are doing, its just bad and gets you NO where unless you bump it a bit for an auto car to keep the rpms up as you shift into the next gear.

You dont need to go over 115mph.... if you do you are rather stupid (sorry but unless you are on a closed course going that fast is just ... stupid... if not for you everyone else on the roads you are endangering, of course it does have its time and place... very rare though.

Be careful tunning, talk to someone that knows the car and how to tune it to help you keep from messing stuff up and setting it right.

justin00stang · **Re: question for a tuner**

The problem is that the revs per mile # you came up with is way too high, stock is around 820, the GT sized tires are just lightly larger, around 810 revs per mile, not even really enough for you to notice on the speedo.

jimmy_beaner · **Re: question for a tuner**

that's why I said the number he put in would suggest a WHEEL with diameter of 18"... which would be a very very small rim+tire combo.

I_luv_HP&trq · **Re: question for a tuner**


Quote: Originally Posted by SpectorV If you buy the CORRECT size tires for your rim size you will not have ANY problem with your speedometer. I have no idea what the correct aspect ratio would need to be for that size, you would have to do the math and im to sleepy to do that lol. DO NOT raise your rev limiter unless you KNOW what you are doing, its just bad and gets you NO where unless you bump it a bit for an auto car to keep the rpms up as you shift into the next gear. You dont need to go over 115mph.... if you do you are rather stupid (sorry but unless you are on a closed course going that fast is just ... stupid... if not for you everyone else on the roads you are endangering, of course it does have its time and place... very rare though. Be careful tunning, talk to someone that knows the car and how to tune it to help you keep from messing stuff up and setting it right.

........ i don't speed like that anymore, but when i did i was on the freeway at 3:00 in the damn morning on my way to work. i was the only car on the road for miles, so no thanks for your concern and calling me stupid.
..... and i don't know anybody to talk to around me about tuning. I've moved every couple of years and went to several high-schools so the only people i can talk to about cars is online and that's not always the easiest with people trying to prove someone wrong and show off what knowledge they have while ridiculing them.

ok so i made a mistake but fixed the tune when i got home. i don't normally mess with the tune but thought maybe it would help me with mileage if my car knew the size of my aftermarket wheels. i learn from my mistakes and move on.