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.

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.

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.

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.)

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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Originally Posted by strokeZ

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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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.

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.

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.

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.

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.

Yes I did get those numbers messed up sorry about that 275 not 295 oops. I have 32" tires and going to 33" tires only makes it 1/2 taller over all tire dimension. I looked based on my tire size during flex situation it appears to me that you can get 33" tires on if your willing to bend the pinch weld over and melt and form or cut some plastic out of the way. But if you have no plans of going off road I don't think you will have an issue an just rubbing every now and then. This is just my opinion thats it my next set will be at least 33" tires so I can tell you for sure then LOL. oh yes he is running BFG MT KM1 on MOABs with 1.5" spacers and HD OME lift. Just a side note I think with the spacers you would rub more then without.???????? what do you think

Well the problem is that without spacers the MOAB wheels wont clear the brake calipers. Also without spacers you will never get a tire larger then a 265/70 to clear the ball joint bolt while still running stock wheels. Most 265/70's wont even clear the ball joint bolt without spacers. My tires wouldn't have. Also, I realize that a 33 inch tire isnt much bigger (1/2 inch taller over all tire dimensions) but you'd be suprised how much that 1/2 inch makes a huge difference. Especially if the vehicle ever leaves the pavement.

I agree there would be some rubbing but it you get absolute no rubbing on a 32" tire how bad could it rub on the 33" tires with the modifications done.

Well just to get my 32's to clear, I had to bend over the pinch weld and remold the plastic inner fender well. I think with 33's there would be an issue with the rear AC/Heater lines. I know not all XK's have them but mine does. I guess there is only one way to find out. My XK will need tires soon so maybe ill jump up in size and make them fit!

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Originally Posted by 07JeepXK

Well just to get my 32's to clear, I had to bend over the pinch weld and remold the plastic inner fender well. I think with 33's there would be an issue with the rear AC/Heater lines. I know not all XK's have them but mine does. I guess there is only one way to find out. My XK will need tires soon so maybe ill jump up in size and make them fit!

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So if you purchase a 32" tire and install it add the weight of the vehicle it dosent sit at 32" it sits at like say 31" correct. Or am I way wrong on this one.

Yeah pretty much. When I measured my tires to recalibrate my speedometer they were right at 30.5 inches tall. They had about half tread on them at the time. But yes, they weight of the vehicle changes the rolling diameter of the tire.

another thing to keep in mind is that a 33" tire isn't. i had a brand new set of BFG A/T KO's that were 33/12.50/15 but they only measured about 31.75". you can go on most mfgr websites to get the true specs on their tires.

We've been torn between street and off road.. Feeling that we may lean more towards the off road and loose the MPG's in the long run. But lifting is out of the question. So we may considered getting a cross over tire similar to the Michelin LX A/T2 series. Although we are keeping the same general size, we are getting a bit wider tire.. at least to keep a bit more on the road. I just want to know that the tires we get are going to last as well as preform "if" we venture off road again.

I would go with an MT that is less agressive and has a more even tread pattern. This way you get a decent ride and still have a pretty capable tire. Something like the Hankooks I have or the Firestone Dest. M/T, or the Nitto Trail Grappler M/T. These are a bit quieter than something like a KM2 (which I love) or some others.

Not quite relavant to the last post but....

Earlier in the thread the topic of different size tires make a difference on the calculation of MPG. Just wondering if this is a proper way to calculate the difference.

The original tires did 682.75 revs/mile.
My Tires do 638.1 revs/mile.

My current MPG on the dash reads 15.1.

15.1 x 682,75 (what size tire the Computer thinks I have) = 10309.525 (Total revs for 15.1 miles the computer counts)

Now divide this by what I actually have : 10309.525 / 631.1 revs per mile = 16.15 MPG.

Does this sound like the correct procedure to the the Mathmatically inclined? I guess I could express it as a ratio as well which would be 682.75 / 638.1 = 1.069. I can use this number multiplied by MPG to get the correct reading I think.
I have the 4.7L Flex and a lift. Does this sound right for what I have?

The 16.15 is city driving in Houston. I get about 17.7(dash readout) which would be 18.5-19ish, according to the math, on the HWY. (Staying right on the speed limit.)

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Originally Posted by superacerc

I would go with an MT that is less agressive and has a more even tread pattern. This way you get a decent ride and still have a pretty capable tire. Something like the Hankooks I have or the Firestone Dest. M/T, or the Nitto Trail Grappler M/T. These are a bit quieter than something like a KM2 (which I love) or some others.

Not quite relavant to the last post but....

Earlier in the thread the topic of different size tires make a difference on the calculation of MPG. Just wondering if this is a proper way to calculate the difference.

The original tires did 682.75 revs/mile.
My Tires do 638.1 revs/mile.

My current MPG on the dash reads 15.1.

15.1 x 682,75 (what size tire the Computer thinks I have) = 10309.525 (Total revs for 15.1 miles the computer counts)

Now divide this by what I actually have : 10309.525 / 631.1 revs per mile = 16.15 MPG.

Does this sound like the correct procedure to the the Mathmatically inclined? I guess I could express it as a ratio as well which would be 682.75 / 638.1 = 1.069. I can use this number multiplied by MPG to get the correct reading I think.
I have the 4.7L Flex and a lift. Does this sound right for what I have?

The 16.15 is city driving in Houston. I get about 17.7(dash readout) which would be 18.5-19ish, according to the math, on the HWY. (Staying right on the speed limit.)

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Seems we have the same set up my math has my display 2mpg under what it really is. That being said I don't get know where near your numbers bro with the correction I'm looking at 14MPG country roads display says 12MPG.

I got the revs per mile from discount tire's tire size calculator which seems to be wrong for my tire (since the size isn't always exactly as stated). Hankook actually claims that it is not 631 revs per mile but actually 651 which actually puts my milage closer to only .5 off instead of 1. That's by my math which is actually more dissapointing because it means i'm only getting 15.6 in the city. Could be worse I suppose. My moms Yukon only gets 13 in the city and it has normal tires.

How did you figure your's out?