OC_Scion

i was driving my friends xB the other day and he has the aem intake and i noticed that in 5th gear goin like 75 mph's his rpms were only at like 3000 - 3100.

now i dont have the intake and when i am at 75 mph, my rpms are at like 3900-4000.

are his rpms lower because he has the intake? or is my car just weird?

thanks

Munch

Does he have bigger wheels on his car, like 17,18,or 19's?

OC_Scion

no, he had the stock hub's on his

BigOrangeXb

Manual or auto? An intake can't change the rpm's at X mph. That's determined by 1st through 5th gears and final drive ratio in the trans. Different size wheels will change it, small wheels turn faster and big wheels turn slower at X mph.

Rayman

So if you have bigger wheels your RPMs will be higher because more power is needed? That would mean shorter engine life possibly? Hmmmm. Now I wonder if I should get 17s?

kdanie

RPM vs speed is a math function of engine speed, gear ratio and over all tire diameter (NOT WHEEL SIZE). I have 17" wheels and 215/45 tires on my Xa and they are .25" shorter in dia. than the stock tire/wheel combination. Not noticeable at all.

Power does not make a car cruise at a lower RPM, it does make it cruise with less throttle opening at the same RPM.

ken

hnefrdo

prolly going up a slope the time without the intake, so yer steping steeper and pulling the rpm up to maintain speed.

BigOrangeXb

Sorry, I should have said overall tire diameter.

A taller tire will turn less RPMs at a certain speed since it has to roll longer to complete 1 revolution. Think of it this way: If you cut a tire across at one place and laid it out a 225/50/16 tire would be longer than a 205/40/16 (just used those sizes as an example). If you look an www.tirerack.com at the specs page of a certain tire. It'll show you overall diameter and revolutions per mile for all the sizes offered.

jackmott

No, if the speedo says X mph the rpm will always say Y rpms in a given gear. uphill, downhill, or thrown off a cliff!

and bigger wheels will change hte ACTUAL speed, in a given gear at a given rpm, but NOT the indicated speed. Your speedo will just now be wrong.