I recently checked the fuel mileage of my 91 Montero with 33x12.5 tires (14.7 mpg). I was pretty bummed until I realized that this is not accurate since the number of miles traveled with 33ö tires is not accurately reflected by the odometer.

Has anyone devised a conversion factor to correct the fuel mileage when driving with 33s? Would the same conversion work on the speedometer? IÆm assuming it would.

Thanks

I think going from stock tires to 33" is a 15% change in speedo. So if your odometer says 100 miles, you actually went 115 miles. And watch the radar traps, because your speed is showing low!

What don said. On 31" tires, my speedo is 7% off, therefore, at 65mph displayed on my speedo, I'm actually going more like 70mph. Can the same percentage be directly applied to mpg though? ...becuase if it does then I'm happy. <img src="/forums/images/graemlins/smile.gif" alt="" /> (16.7mpg is actually 17.9mpg)

The larger tires are much heavier, especially the thicker sidewalls of an M/T.
This weight must be turned , and as this weight increases, it consumes HP, and thus decreases MPG.

Decreases mpg, yes. But actual mpg is something other than what you'd figure out with displayed miles traveled and gallons consumed since the mileage is off due to larger diameter tires. So yes - the larger tire will be harder to turn and increase fuel consumption and decrease mileage, but the mpg figure is off due to the larger diameter. So what you'd figure out from the numbers, though it be lower than what it would be with stock tires, is incorrect. Shouldn't it actually equate to a higher number than what you'd figure from pump and odo numbers?

Well, I've got 32's on mine, and my MPH at around 65-70 is only off by a max of 3 mph (GPS verified).

When I've run the GPS for mileage, I get just about 290-310 miles per tank on the GPS, when the odometer reads about 270-290 miles (going off of memory here, but actual mileage is slightly higher than odo mileage).

Not sure how this relates to a stock truck though with OE tires, or 33's. I only kept track after the 32's. <img src="/forums/images/graemlins/confused.gif" alt="" /> Hope it helps!

I added the 2nd post just to cover the concept that the altered milage from larger tires will also be hampered by HP loss due to the energy required to spin the larger mass of the larger tires.

It gets more complex: At city speeds the larger tire will not challenge the engine as much, so larger tires will mostly cause you to travel further than you thought, so adjust the mpg by (tire diamter/28.5) to correct.
At freeway speed, and especially hills or any other heavy work at speed, then the tire mass will beging to work against you by increasing fuel consumption. But the mpg calculation remains as above.

Added tidbit:
Factory speedmeters are deliberately set to read ~3% to 5% higher than your actual (GPS tested) speed. This automotive detail is provided by Dr K.
I tested my trailering tires (215/75 15, or 27.7") on the Colorado trip, and the speedo was out -5%. Similar testing on my 31's (when new) showed the speedo out +3%. So doing the math, it looks like our speedometers will read exact on 30" tires, and be out +7.7% with 33's. This is compared to ACTUAL distance covered with a GPS, tested for minimum 100 mile (160km) distances, uninterupted constant speed, constant elevation, and carefully measured.

So:
If you want "REAL" MPG #'s, you need to know the speedo correction for each tire size (YOUR tire size), and remember that the factory odometer is out by -3% to -5% on the `87 A/T (and that this is an industry-wide standard).

Gentlemen, start your calculators!


edit:

Ok, Ok.

215's -5.0%
225's -4.0%
235's -3.0%
29's -1.8%
30's +0.6%
31's +3.0%
32's +4.5%
33's +7.7%