BabyGT

That is going to make a big difference in your calculations.

Toadster

i agree, I have an SCT programmer and need to recalibrate the tire size and tune the car - but need to find some time, will have to get the ECU code in the footwell panel since the one on the passenger door is worn off...

anyone know a rough conversion for the mileage before I can retune?

darcane

That just means to odometer is off by 4.9% too.

161.4 x 1.049 = 169.3 actual miles.

divide by 12.13 gallons and get 13.9mpg.

The 20" wheels typically weigh significantly more than stock and put most of that weight far away from the center of the wheel, which means a much larger moment of inertia. This means it takes a lot more fuel just to accelerate them up to speed. Big wheels typically translate to lower city mileage (lots of starting and stopping, when high moment of inertia hurts you), constant speed highway mileage shouldn't be affected much.

What do other people get with these convertibles? We haven't driven ours enough to know.

Derf00

The 20" wheels is the biggest problem. They are probably heavier than stock and with the slightly larger diameter you are exponentially increasing rotational mass. Rotational mass is part of your drivetrain loss/resistance. When you step on the gas the engine has to overcome the drivetrain loss/resistance to get the car moving. This uses more fuel.

Almost everyone notices a drop in mpg when they switch to 20" wheels, especially when stock is 15"! With stop and go driving (90% city) that extra rotational mass is like dragging around an anchor. if you still have the stock wheels, swap them for a tank and see the difference!

Derf00

Not really. 5% difference is height is probably less than 5% difference in MPG. The wheel size and driving conditions in this case are the biggest negative factors to MPG.

mustangman02232

I would put the stockers on and see what you get as mentioned, city my 02 never did that good, like 16-17.... My GT actually gets better fuel milage then my v6 ever did

The grey beast

These V6's do not really get that good of mpg's anyhow, let alone much larger wheels. GT or other V8 model guys sometimes will say well you got a v6 I bet you at least get decent fuel mileage and I have to reply no, in honesty the v8's do get just as good if not better fuel mileage in many cases. I think I average between 15-17 in town/city, and about 24-25 highway. I don't drive conservative really though.

Toadster

I wish we could, the 20's came on the car...

so we're shopping around for some smaller/lighter wheel/tire combos

darcane

I like putting newer, OEM wheels on. I bought these for our '01:



They are from an '05 GT (?). They fit pretty well with no spacers, but there is a little rubbing on sharp turns.

They changed the backspacing for the '05+ Mustangs, so you have to watch that if you get newer wheels.

Daehawk

I am dense sometimes its true...but I dont see how a coil pack could affect mpg. I mean a coil pack either works and fires the plugs or it doesn't and you get nowhere.