some of you may have seen my gas mileage post in the V6 forum, but I'm starting to wonder if the wheel/tire combo on my son's car is contributing to the 13.3mpg rating...

we bought the car about a month ago, and it has 245/35R20 tires on it

using tire calculator - it's about 4.9% too slow, so when the speed reads 60mph - we're actually going 63mph

he's looking to put some 18" rims on it with reasonable tires to better match the OEM height/revs per mile - but until then he's a little miffed getting 13.3mpg

is there some simple math that I could use to estimate the true MPG?

 

Do you have something to check the odometer against, ie is it running about 5% low too. If that is the case, then the 13.3 is probably closer to 14 in reality (not that that is good either)(13.3 + 5%).
But to answer your original question, bigger tires will give you lower mpg readings than you are really getting (as long as the odometer isn't corrected).

 

I agree with Flamingo here and something else may be in play. Mileage is easy to figure out, by just measuring the amount of fuel you purchase and mileage you get.

Zero the ODO at say a 1/4 tank and fill it up and make note of the amount of gallons you purchase. When it reaches 1/4 again, just do the math and see what the actuall mileage happens to be. Of course, amount of highway vs. city miles will come into play, but do this same calculation a 1/2 dozen times and will get a perfect average of MPG. Then do some research on what your sons exact make model of car gets for others

Jazzer

 

well, we did the SCT tune on it tonight (91 octane - thanks Cali)

adjusted the revs per mile (with the tuner) to 775 (actual tire specs are ~777) and we verified the speedo with the GPS on our phones, the speedo is within 1mph now @ 60mph

a little more oooph on the car as well so that may help move those heavier rims until we find something a little more gracious...

lots of vendors to shop

 

Although the wheels are heavier than some others and will save some money with 18" tires, I am not sure you should expect to see a noticable mileage change by just losing that weight. It can only help, but the money for new wheels will off-set quite a bit of the improved mileage you gain.

Not trying to be bearer of bad news, just want you to consider this and maybe see how the tune effects mileage over time.

Jazzer

 

yes that's understandable

we will also be selling the existing 20" rims and tires (which are brand new) -so that will recoup 'some' of that cost

we track our mileage on http://www.fuelly.com which shows MANY of the 1999-2004 Mustang V6s getting 20-22mpg pretty regularly

 

"Eggcelent"

Jazzer

 

Yeah, I had a 2000 V6 A/T back in high school and I had no problem getting 22-24. My mom actually got 26 a couple times. But with my typical driving style and the fact that most of my miles were "cruising" in town I often saw mid-high teens. If you drive nice, you should easily get into the low 20's though.