Tires, gears and MPG.

I reciently changed my worn out 225-75-15 tires for some 30.9.5 -15's. About the same time I changed all my fluids to synthetic. Now my milage has gone to hell. I have not done any highway driving since the changes but driving aroung town I get poor milage. I would think the size 30 tires and 3.55 gears are a poor combination for the 4.0L. With nothing remaining in the piggy bank my dilema is this, should I get some smaller tires for driving around and use the 30's for wheeling (a not too expensive option)? Should I continue to spend the kings ransom on gas until I save enough to regear? Regear is the prefered option but the budget do not support it at this time. Maybe a good set of used 225's for now would help the regear budget along. :huh:

Did you verify the accuracy of your speedometer since the tires size change?

I'm not so sure you'll see much improvement after regearing, at least in town. I did not see much with mine. The highway figure improved a bit, but it seems like no matter what gearing you have, it's still requires more effort to get bigger tires rolling. I'm sure that tire type has alot to do with this also.

--ron

Yea I did and while I cannot give precise figures on the MPG I know I need to fill up more often now compeared to the past for driving the same route. Also the AW4 shifts down more often and stays there longer. The GPS show I am doing about three miles faster than the speedo indicates.

im not trying to be an ass here, but a thing to learn about jeeps is biger tires means bad gas mileage, you accept it once you get into the hobby. i just got 4.56's for my 33's and the gas mileage increased a little bit. but it still sucks. alot better to drive with the gears tho. perfect on the freeway and my jeep has a little kick again. maybe i should race some hondas, but anyways i dont think regearing is to big of a deal with only 30's. if you ever go that route tho know that you will eventually be diagnosed with inchitis and will want to go bigger. so dont just go to 3.73's.
go with the smaller tires see how it works.save some money for a regear.

Going from 225s to 30s isn't really a big jump. The jump from 215s to 30s calculates out to about 6%, so your jump should only be about 4% or so. Try driving around town in third gear (don't use the OD position on the gear selector) and see if that helps.

Eagle said:

Going from 225s to 30s isn't really a big jump. The jump from 215s to 30s calculates out to about 6%, so your jump should only be about 4% or so. Try driving around town in third gear (don't use the OD position on the gear selector) and see if that helps.

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Around here is hilly. What the XJ used to do in 4th now requires 3rd. Maybe if I select 3rd it will use 3rd and OD. She still takes off well and is almost perfect in the lower gears. On longer not too steep hills with the cruise on, it continuosly hunts between 3 and 4. It is as if 3 is too low and 4 is too high. I will try some long highway runs this weekend to see what my milage is. My family in NY would love to have me for a day. It is about a 400 mile return trip.

key element was that you switched from "worn out" tires. your true rolling radius with worn out tires probably put you closer to a 28" tire. stepping up to a 30 tire probably gives you about a 29.5" tire. some 215's or even 205's will help you out significatly. while it is the less desirable option over regearing... it could give you the gas milage you need to pay some principal on your Credit card to have the room for gears and install.