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2 Quetions one PSI and MPG

C

cj11

NAXJA Forum User
Location
Easton Pa.
Just upgraded to 33x10.5x15 from 32x11.5x15 has any one done this swap and what did the MPG go down to?

I have 97 4" lift what type of air pressure are or would you run in them the shop filled to 50psi feels ok but dont know where would be good starting point.

Thanks for any help.
 
I can't run my tires any higher than 35 psi. I drive mine daily on the highway and anything higher than that I start wearing out the center of my tire. Can't help you on the MPG, my speed-do isn't working properly.
 
I run like 25psi in my 33s on the street, but they're 285s (11.5s.) At 50psi you're gonna have a pretty rough ride and the center of the tread will probably wear out far before the edges - use the chalk test.

mpg is around 10 to 17 depending on how hard I'm stomping on it, that's with stock gears, your transmission type and gear ratio and a million other things are going to affect your gas mileage so no one can really predict it for you.
 
All stock gears auto tranny t/c

I checked my speedo with the 32's @ 70mph I was doing 75
with the 33's @ 70mph i am doing 79

At one point found a calculator that said with 32's I was about 18% off per MPG.
 
Optimum tire pressure will vary by tire manuf. and vehicle weight I like to use the "chalked tread" method to determine the proper tire pressure for my rig. http://www.jpmagazine.com/techarticles/31802_tire_pressure_psi/index.html

As for MPG, this is relative to gearing but generally speaking, if you increase the diameter of tire then you reduce the effective final drive ratio, which means it will require more fuel to push you Jeep down the road.....so your MPG will decrease.

Correcting this with proper gearing is the key to finding the proper balance between power and MPG. FWIW, I run a 285/75/16 tire inflated to 32lbs and average 16mpg at a 70mph average freeway speed in a 2000 XJ/4.0L/AW4 trans with 4:88 gearing. At 70mph, my engine spins at approx 2700 RPM’s.
 
XJEEPER said:
, if you increase the diameter of tire..... your MPG will decrease.

.
Click to expand...
yes and no, on city streets stop and go you are trying to turn more weight and blah blah blah, kills you mpg

BUT

on the freeway, or constant speeds, your tires are covering more ground per rotation than stock tires (when engine speed is the same)
around town i was getting 15mpg average, the freeway, i was averaging 25....
50psi is stupid, what tires are you running? and what load range? id go back to the shop and ask why they did that and raise hell, its a safety issue having tires aired up that high that arent meant to be....:twak:
 
33x10.5x15 BFG A/T and they say 50psi on the tire the guy at the shop filled and told me to adjust as i need,
And as far as weight goes the 33x10.5 is lighter the my 32x11.5 will try to put them on a scale but moving the around there is an obvious diff in the weight.
 
I run my 33x10.50's at 28psi,seems to ride best and the best wear!
 
Local tire guy whom I trust suggested ignoring the recommendations on the tire and START with the factory tire pressures recommended for the vehicle either on the door jamb or owner's manual and work from there.
 
cruiser54 said:
Local tire guy whom I trust suggested ignoring the recommendations on the tire and START with the factory tire pressures recommended for the vehicle either on the door jamb or owner's manual and work from there.
Click to expand...

Your always supposed to do that!
 
I run my 265/75's at about 30 psi on average, slightly higher when loaded.

50 psi is WAY too much, and tread wear will suffer.
 
My 96 which weighs 4100 lbs, I,m running 32 psi. Tires have over 60,000 miles and are wearing smooth, they have been rotated and balanced about every 10,000 miles. Also 8" rims,33x12.50 -15 BFG ATs.

MPG on my 300,000 mile motor will vary between 13-17 mpg depending on brand of gas and weight of foot--

Wayne
 
50 psi is more then a load range c tire should be filled to. you will definately wear the middle of the tires out much faster then the edges. i rune 25 psi in my 33s i ran about 20 psi is in my toyotas 37s and i ran about 30 psi in my prerunner rangers 33s. it kinda is personal prefference on the pressure but 50 is much to high for sure
 
Mickey Thompson MTZ 35X12.50-15 I am at 26 lbs, need to drop ~2psi yet to have the right pressure for my xj and tire combo.
 
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