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Miles Per Gallon


b_mckendry

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Right now I'm getting 10mpg on my 86 bronco 2. Everything is stock no aftermarket parts - v6 2.9

I am running a 265 75 r16 tire but I feel that I shouldn't be that restricted on my mpg.

I have checked fuel pressure and am getting what the Haynes manual says at 40.

What is everyone averaging and what's the best thing I can do to maximize my mpg?
 


fastpakr

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Assuming you've done a full tune up, the next step is to check the computer for trouble codes. Instructions are on the main TRS page. Your 86 doesn't have a check engine light in the cluster but the ECU still generates codes and they're very easy to get.

Both of my B2's regularly got 20-24mpg and only rarely went below 18.
 
adsm08

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I get a consistent 17. Frankly, I think it's likely your huge tires that's doing the damage. Especially if the gears are still high and/or you haven't corrected for the odometer being off.
 

jmburton1993

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I have 30x9.50x15 on 8 inch rims and am getting roughly 15 mpg, and running super rich, but i have 3.73 gears and its a 5spd
 
Slammin Sam

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Most people don't realize how much tires can affect gas mileage. Even changing from one tread design or maker to another in the same size tire. Going to much larger tires not only result in more unsprung weight but induce speedometer error that needs to be considered when calculating mpg. I run 225 75 R15s vs the stock 205s. The 225s are about 5/8 inch taller than stock, but result in an 8% speedo error. BTW, I get 20 to 21 mpg mostly in town with 3.45 gears and a 5 speed.
 
alwaysFlOoReD

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Did you change gearing? Auto or standard? If you are still using stock gears that will throw off your calcs. I get a 17% tire diameter difference between a stock tire [205/75-15][27.1"] and your new ones [265/75-16][31.6"].

I have a b-II with a 4.0l/5-spd/4.10 gears/31"x8.5"x15" and get ~17 mpg winter driving in town. With theses gears and tire size I'm within 5% of correct speedo. I'm in Canada and our gallon is 20% bigger so converted is ~14 mpg[US]. The motor is old and worn out so that seems to me to be acceptable for in town winter driving, in 4x4 delivering pizza.

You're mileage does seem to be low but it's hard to judge without all the facts.

Richard
 

b_mckendry

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Thanks for all of the info.

How do I know what my gearing should be with a 2.9 v6 automatic on 265x75xr16 tires?

Is there a calculation to figure this out?

Also what is the factory gearing on a stock B2?
 

PetesPonies

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265 x .75 x 2 = 397.5 mm / 25.4 = 15.65 + 16 = 31.65" of tire.
a stock 205/75/15 is 205 x .75 x 2 /25.4 + 15 = =27.1 " tall. You have increased the tire height 4 1/2 ". That 4.55" change is 4.55/27.1 = 16.7% difference. Therefore your speedometer and you fuel mileage calculation are off by that much as well. It's linear.
 
alwaysFlOoReD

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b_mckendry said:
Thanks for all of the info.

How do I know what my gearing should be with a 2.9 v6 automatic on 265x75xr16 tires?

Is there a calculation to figure this out?

Also what is the factory gearing on a stock B2?
Click to expand...
You should have a sticker on your drivers side door post or on the door. It will have a code that will tell you what the factory used for gears in your axles. Use the "library" or "tech section" and find the code [breaker - can't think of right term] and that will tell you the factory ratio.
There is another way, the factory used a metal tag attached to the rear axle cover bolt. It will have a bunch of numbers on it, usually the one you want will be the bottom left of the tag, starting at the bolt side. Possible numbers being 3.08, 3.45, 3.73, 4.10. If there is an L in between the first and second letter that indicates a limited slip.
And finally you can jack up the rear and count turns of the wheel compared to turns of the drive shaft. It's been too long since I've done the last so for proper procedure please use the search as there is a possibility of harm if done incorrectly.
Good luck,

Richard
 
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