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Discussion Starter #1 (Edited)
Hey! Thought I'd start a thread about a subject I'm very curious about. It will help me determine what lift/tires I end up with. Please post your lit / tire setup and what kind of MPG you are seeing highway, in-town, and mixed. Maybe a picture to keep it interesting ;)

Just in case you don't know how:
http://www.wikihow.com/Calculate-Your-Car's-Fuel-Efficiency-(MPG)

Me first:

1987 Samurai stock suspension and 255/70r15 highway tires Seeing 25 MPG highway.


Let's see those numbers!
 

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Same here on pavement. Better at 40mph on the back country roads. Same off road on the trail in lo range 1st or 2nd.
 

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1987 Samurai stock suspension and 255/70r15 highway tires Seeing 25 MPG highway...... that sucks..... i got stock everything i get 30 or more in the city 40 or more on the highway.still havent install k&n stock filter or high performance coil annd some alloy wheels im waiting on some money. 87 convertible sammy.35 psi air pressure in tires new o2 sensor,tweaked air fuel mixture screw on stock carb. full tune up replaced all vaccum hoses.oh and i wanna put a pacesetter header and a msd ignition box too.and a g.m. alternator so i can run a electric fan and a better stereo and fog lights.
 

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Haha so I'm a bit crazy, I reset my trip counter and fill up my tank every time. I do the math every time I get gas to calculate my mpg to 2 decimals. I see from 25.5 mpg, up to 26.6 consistently. Depending on how much traffic I sit in that week etc... from a 93, fuel injected 1.3. All stock less for the 225/70/15's on 8 inch rims and the sprint top. A/C removed. 149,000 miles. Catalytic converter removed. Looks like 1 7/8" exhaust with stock header. And yep, those tires rub a little on a deep bumpy left turn.
 

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Almost stock except a Thorley header and 2" exhaust, stock size tires. I average 23 mpg. Goes up to 25 if I take it easy, and down to 19 if I put my foot in it and try to keep up with interstate traffic.

Keep in mind that bigger tires will make your fuel economy seem better than it really is unless you regear the diffs to compensate.

EPA estimate is 24/27.

I think the exhaust, mainly the header, may be pulling mine down, making it run richer, since the O2 is only looking at the middle two cylinders. I feel that may also be killing my cats prematurely.
 

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87 with stock 1.3 and 31" x 10.5 and been getting 28 mpg. Today I put on a new set of 225/75 and they are running 10% over the gps but haven't checked the mpg yet. It sure drives better and I hope gets better mpg.
 

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I get right around 20. Weber carb, Pacesetter long tube header, 2" exhaust, no cat and a turbo muffler. 215/75/15 BFG Mud Terrains. Mostly short trips on rough dirt roads, with the occasional 15 mile trip to town. Stock engine burns oil, is down on compression and I drive it like I stole it. Foot to the floor and no mercy. I just bent one of my OME Dakar springs and all the shocks are leaking. Took a bad hop into the ditch on the main dirt road and hit a large rock with the left front tire. Tire survived. Spring is not looking too good. Still drives straight.
 

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87 with stock 1.3 and 31" x 10.5 and been getting 28 mpg. Today I put on a new set of 225/75 and they are running 10% over the gps but haven't checked the mpg yet. It sure drives better and I hope gets better mpg.
This is a great example of the false fuel economy numbers bigger tires give. The stock tires are 26" The 31" tires are 19% taller so the fuel economy numbers are 19% higher than the actual numbers. With 28 indicated, you were actually getting about 23 mpg.The 225/75s are 28" and so your fuel economy numbers will be 8% higher than reality.
 

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Discussion Starter #10
Actually, if you think about it, a larger tire causes the axle to rotate less often than a smaller tire. Therefore showing fewer miles on the odometer than are actually traveled. So, larger tires have the opposite effect by creating "lost miles" and actually cause calculations to show poorer fuel economy than in reality. Although I'm not sure how larger tires could cause you to get better than EPA estimates haha.
 

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Here is another wrench to throw into your calculations... a 31" tire is heavier than a 26" tire (especially offroad tires), so it takes more energy (fuel) to turn them...

Fill the tank.
Use a GPS to measure a 20 mile trip.
Refill the tank.
Base your calculations off the amount of gas you put back in and the 20 miles you drove.

Real math -vs- theoretical math...
;)
 

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Also, a full tank of gas is heavy, so your MPG actually increases as you drive along!

Just messin wit y'all, I know it's Monday, but for me it's already been a looong week IYKWIM!
 

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Wayne I'm a 53 year old mechanical engineer, I know how to figure mpg. The miles were determined using a Garmin gps which I'm pretty sure are accurate.
 

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Use your GPS

Actually, if you think about it, a larger tire causes the axle to rotate less often than a smaller tire. Therefore showing fewer miles on the odometer than are actually traveled. So, larger tires have the opposite effect by creating "lost miles" and actually cause calculations to show poorer fuel economy than in reality.
True and if you try to use your odometer you'll go crazy trying to figure it out as you really need to recalibrate the speedo to match the tires but you can use a gps and track your miles that way for a much more accurate figure.
Around town and in the trails mileage will stay fairly the same because you will still be keeping the rpm's at the optimal spot. But when you get on the highway and steadily drive say 65mph in top gear the bigger tires at that scenario will give you better mileage(unless it's extrememly aggressive tread) than the smaller tires because the rpm's will be lower at the 65 mph speed with the bigger tires as you gain more distance with each turn of the wheel.
Right now on 215-75-15 driving around town on county roads at about 45mph i'm getting about 30+mpg and i drive with a very light foot and in top gear as much as possible. 3.1 motor. Less on the Interstate as my rpm's are much higher at 68mph
Joe
 

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Mpg

1987 Samurai Hardtop stock suspension and 255/70r15. I reset mine evertime being that my gas gage is trippy. i calculate it everytime and get from 26 to 27 mpg all around average speed probaby 35
 

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Permanlink - MPG

When you start putting bigger diameter tires on a Samurai or whatever, remember the wheel/tire does not have to rotate as man times to go the same distance. With that said, a wheel that used to turn a hundred times this distance, now only turns 88 times which is 12% less turns. Now take that 12% and add it to the 25.5 mpg you were getting which should show you at 28.5+ or better. You will loose some mileage due to resistance, but as much as you first thought.

Remember you haven't had your speedometer calibrated to the newer tire set up. What I said in the first paragraph is how to figure it out without calibrating you speedometer.

Hope this gives you idea, as to why the MPG looks bad.

Merry Christmas and keep on Zuk'in.
 

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hummerwannabee.....
I like the way you explained that and wasn't quite sure how to calculate it so if i was going from stock tires to a 235x75x15" (29inch tire) how would i calculate that say i was getting for an example of 30mpg with the 29"tires.
Joe
 

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I am driving a 92, Problem is I drive on the freeway a lot and drive between 65-80mph. 205/70/15 tires and I average only 25 mpg. I am sure I need as full tune up and such to get more.
 
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