Street tires vs. off road tires

[WanderingTs]

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.

[LWM]

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.

[GPintheMitten]

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.

[Doc in AZ]

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.)

[strokeZ]

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.

[Sal-XK]

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.