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Thread: Street tires vs. off road tires

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    Lifetime Member Getting Dirty WanderingTs's Avatar
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    Street tires vs. off road tires

    When I lifted my XK before Camp Commander this year and installed my new 33" tires, I found a significant drop in my MPG. I went from about 15 mpg to 10 mpg. (Based on mileage and speed from GPS data, not MPG calculator in the XK.) There were also some other mods that may have changed aerodynamics a little, skidplates changes, winches, extra weight in cargo, etc. I am guessing the majority of this change is due to the tires though.

    How many of you have an extra set of road tires for use when traveling vs. just running the off-road tires? How much difference do you see in MPG or other factors? Have you seen it to be cost effective in increased tire wear?

    What I am hoping to do is figure out how much MPG I might gain getting a second set of street tires, to see if it is cost effective in the long run.
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    I had a set of street wheels & tires that I sold Monday, I did get a little better mileage when they were on but the overall comparison was minimal. When I considered things like storage space, tire life (even when not in use) time to swap back and forth and the cost to buy four more street tires when they wore out I figured it would be cheaper to just pay a little more for the gas.

    If you get even 5 mpg better mileage with the street tires you are only saving about $5.00 per fill up, and if the street tires run $120.00 per tire ($480.00 for a set of 4) it would take almost 100 fill ups to pay for the extra set of tires.
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    Senior Member Getting Dirty
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    I took a hit on mpg with my larger tires and probably also due to extra rotational weight of the spydertrax wheel spacers. But it would not be worth it to me to buy extra tires/wheels and the effort and time of swapping them out.
    2007 XK Limited, 5.7L Hemi, QDII, Rusty's 2" lift, Spidertrax 1.5 inch wheel spacers, GetLost4x4's front hitch mount for portable winch.GY Duratracs 265/70R17s, Rola Roof Rack.

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    Senior Member Getting Dirty Doc in AZ's Avatar
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    keep in mind that if the tires are larger than stock and you don't correct the speedo you will get a false MPG reading.

    here's my example.
    my tires are 24% larger than stock (215/75/15 up to 35/12.50/15)
    my gear ratio is 30% deeper - i had to re gear from 3.73's to 4.88's to turn the tires.
    that is a net difference of 6% to the engine - this compensates for the larger circumferance (also known as roll out) and the greater rotating mass.

    when my speedo reads 50 mph i am actually running at 47 mph. not much of a difference, but it adds up.
    off a tank of gas if it shows 300 miles for the tank i actually only got 282. which changes my mpg average from an indicated 20 mpg to 18.8 mpg.

    in your case, if you put larger tires and didn't change gears or recalibrate the speedo, then you will show a slower speedo reading and less miles per tank.

    not knowing your drivetrain specs this is just an arbitrary example.

    tire size change becomes a 10% net difference
    at 60 mph indicated you are actually traveling 66 mph.
    at 300 miles for a fill up you calculate 15 gallons and come up with 20 miles per gallon (yeah i know it's a dream)
    you actually went 330 miles which calculates to 22 mpg.

    when i was hot-rodding dodge trucks with the cummins diesel engines there was a list on the forum that had real world numbers showing the MPG loss by upgrade
    1mpg per 1" of lift
    2mpg for 4wd vs. 2wd
    etc

    changing the load, tire size & weight of the vehicle will certainly change the fuel economy but you also have to factor in driving habits (quick acceleration), speed, terrain (lots of hills?), weather (vehicles always get better fuel economy at sea level in cooler temps) & time of year (the gas stations sell different mixes of fuel in different seasons.)
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    Lifetime Member Getting Dirty strokeZ's Avatar
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    I agree w Doc. Stock tires are around 29" in diameter and if you put 33's on you are running 70 when your speedo reads 65 (or thereabouts). You would need a programmer to correct this and get the calculated mpg correct for the 'puter. Have you done the manual calculation? You would need to use a GPS to get actual miles travelled since the odometer would be off as well (I am assuming you have not re-programmed the vehicle).

    Personally, I did not see a very big drop when I installed my 32's. I see huge variances with outside temp, especially over 95 deg F - so make sure that is not impacting your results as well.
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    Lifetime Member Getting Dirty Sal-XK's Avatar
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    Quote Originally Posted by strokeZ View Post
    Personally, I did not see a very big drop when I installed my 32's. I see huge variances with outside temp, especially over 95 deg F - so make sure that is not impacting your results as well.
    I agree the tempt here in NC finally dropped below 90 and the humidity dropped below 100% and my AVG started climbing almost immediately. 32" your running are about 5% error. I did the math once correcting the mileage and such so for me I add 2mpg to my average read out and thats my correction so my dash is displaying 12.6 right now so I add 2 which gives me 14.6MPG. Now that is running with no cargo on slightly hilly roads but mostly open with maybe a couple of stop lights once and a while.

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    Lifetime Member Getting Dirty strokeZ's Avatar
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    Here in TX the temp is still way up there, however I did a road trip last year and averaged 17.8 mpg calculated (gps mileage and actually gallons used) - temp was around 65 deg. Did the same trip a few weeks ago in 95 plus degree heat and averaged 14.6 mpg. Somewhere in the 90's the 4.7L becomes terribly inefficient.
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    Super Moderator Getting Dirty 07JeepXK's Avatar
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    Do you have any pictures off the 33 inch tires installed. Also, what did you have to do to make the 33's fit on a 2 inch lift? I know some of the guys with 4 inch lifts had problems getting 33's to fit properly with some wheel/tire combo's that they went with.
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    Lifetime Member Getting Dirty Sal-XK's Avatar
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    I don't have pictures but I have a video of Romeo at flagpole on my you tube page his are 33's I believe. 295/75/17's I could be wrong but I think thats what size he is running.
    Last edited by 07JeepXK; 08-30-2010 at 06:19 PM.

  10. #10
    Super Moderator Getting Dirty 07JeepXK's Avatar
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    I think you got your numbers mixed up a little. A 295/75R17's is equivalent to 34.4 inches tall.

    Is romeo still running the 2 inch Old Man Emu lift? I talked to him a while back and he mentioned possibly swapping it out for a 4 inch lift.



    BTW.....I accidently hit edit post under your post instead of reply to post, thats why it says your post was edited by me.
    Last edited by 07JeepXK; 08-30-2010 at 06:25 PM.
    2007 4.7 Liter XK | Front Modified OME HD Lift | Rear 4" Superlift | JBA UCA's | Rusty's Rear Adjustable Track Bar | Rysty’s Adjustable Upper and Lower Rear Control Arms w/Heim Joints | Mickey Thompson Classic III 17x9 Wheels | 315/70R17 Goodyear Duratracs | Airflow Snorkel | Flowmaster 40 Series Muffler | Superchips Flashpaq | Mopar Skids | 4xGuard Belly Guard | American Rebel Rear Diff Cover | Rear Powertrax No-Slip | Front Electronic Locker | Rear Heat/AC Delete

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