To get a good rough check on your current mpg. if you have not re-calibrated your speedometer/odometer, do the following:
1. Measure the circumference of both your old tires and your new tires.
2. Compute the percentage of increase in circumference from the old tire to the new tire. For example, if the old circumference was 100" and the new circumference is 110", then that represents a 10 percent increase.
3. The next time you do a mpg check, multiply the distance traveled by the percent increase in circumference before you divide by the gallons of fuel. For example, if your odometer indicates that you traveled 150 miles on 10 gallons of gas, that would seem to indicate that you're getting 15 mpg. However, if you first multiply the 150 miles by 10 percent, you will find that you traveled an additional 15 miles (150 x 0.10). Add that 15 miles to the 150 miles indicated by the odometer, and you find that you actually traveled 165 miles. Divide that 165 miles by the 10 gallons of gas consumed, and you will find that your gas mileage is actually 16.5 mpg, not 15.0 mpg.
Your increase in tire size appears to be significantly less than 10 percent, so I wouldn't expect this correction factor to result in a huge increase in calculaed mileage. FWIW, when checking gas mileage, I'd start with a full tank and drive until the tank is only 1/4 full before checking mileage.
Also, if you're only getting 6-8 mpg with a stock engine, even with your small increase in tire size, I strongly suspect that something is wrong/worn/misadjusted on your engine.