Gas Mileage

[Original_Ranger84]

OK, So I was wondering if bigger tires changed gas mileage a ton on carburated vehicles. My truck got 20 mpgs mixed driving up 3 big hills a day, no stops. It has a 2.8L carbed vehicle (soon to be the FI convert. I hope) and now I have 31" tires others were 29"s, they only weigh 10lb more per tire at the most. Any way I was thinking because a FI vehicle changes the amount of fuel if the vehicle needs more power plus there is individual injectors for each cylinder, but my 2 barrel carb would send in the same amount of fuel depending on where the pedal is right... so if I keep the gas pedal where I usually had it before I changed the tires it would send in the same amount of fuel as before therefore same if not better gas mileage with a bigger tire (the perimeter of the tire is bigger so farther with each roll).

I was just bored today, but would be nice to hear what you think.

 

[Wicked_Sludge]

fuel is fed to an engine depending on load, regardless of fuel system type (carb. or FI). the more you push the pedal down, the more fuel flows to the cylinders in all cases.

larger tires decrease mileage by raising your vehicle (less aerodynamic), increasing rotational mass, and increasing final gearing. you can eliminate the gearing by re-gearing your axles, but the other two your stuck with.

in short, your mileage will be impacted the same regardless of fuel delivery systems.

 

[Original_Ranger84]

I don't push the pedal down any more than I used to so I guess it should stay about the same.

 

[Wicked_Sludge]

theres no way you can tell while driving. you'll have to figure your mileage for a few tanks and see what happends.

 

[martin]

Just as a side note. Most carbs have a power valve which richens up the mixture when the manifold vacuum drops below a certain figure (dependent on the valve) regardles of the position of the pedal.

 

[kunar]

dont forget that changing your tire diameter will make your speedometer and odometer readings inaccurate unless you recalibrate....

 

[Will]

Just as a side note. Most carbs have a power valve which richens up the mixture when the manifold vacuum drops below a certain figure (dependent on the valve) regardles of the position of the pedal.

Excellent point. But it's not irregardless of the pedal since manifold pressure is directly effected by the pedal. With either system you are going to slow down if you hold your foot in the same exact position. So you aren't going to do that and when you touch the throttle the pressure will drop and the fuel enrichment crutch (power valve, air valve metering rods, whatever) will allow an imprecise amount of fuel in to cover the lag before the main jets become active again.

Fuel injection, unless you push the pedal wide open, never gives an imprecise pulse.

This isn't about fuel injection vs. carb, I know, but since a stab was taken at fuel injection I feel inclined to parry.

 

[Original_Ranger84]

dont forget that changing your tire diameter will make your speedometer and odometer readings inaccurate unless you recalibrate....

yeah, i figured out that 300 miles on a tank with the stock tire setting will actually be 36 miles less than with 31"s, so when it hits 300 i've really gone 336 miles.

If it does lose mileage with the carb then I'm gonna start my FI conversion and get more power and mileage.

 

[dogboy]

If your milage does go down, I honestly don't think it will be tooo much.

I went from 28's to 33's with 3.73 gears and a 4.0L and just using rough guess numbers, I might get 14-15mpg instead of 15.5 mpg like I did. And your tire differance is smaller and you probably didn't go from a fairly street-ish style tire to a mud terrian either.

Also, tire width and pressure is going to play a big part in what kind of milage you get with the 31's too.

I had my 28' at 45 psi (don't know why the shop aired them to that) I took them down to 32 front/ 35 rear and my, and my milage went from almost 16.5mpg down to 15.5. Mind you it is about 50/50 city and highway.

 

[MAKG]

Do keep in mind that 2.8Ls have computer-controlled metering rods. They are supposed to adapt to conditions as well.

It's not impossible to calculate fuel mileage instantaneously with a carb. It's just less accurate than on EFI. With EFI, you can measure the injector pulse width and multiply it by RPM. With carbs, you have to measure manifold airflow somehow (with a differential heated wire sensor or with absolute pressure and temperature) and infer a mixture. A wideband oxygen sensor can help with the latter.