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Old 01-24-2013
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Odd gas mileage with new tires

Hi,
I own a 97 ford ranger xlt 2.3L 2wd. The original tire size is 225/70R14 but when I bought (10 years ago) the car, the previous owner swap those with new 225/75R15 tire size.

I thought at the time the tire size was too big meaning the overall diameter was too much high than the original size.
Never mind that, my gas mileage was not the best but I could manage to do 24-26 mpg last year.
Recently, I change all of my tire and put new one 235/60R15 which overall diameter come closer to the original size.

And this is where it becomes very odd: my car seems to use more gas then before. I don't understand that fact. I'd have think I'll get at least the same gas mileage as before.

I recently changed my spark plugs, oil filter, air filter, IAC valve, EGR and DPFE (last year).

So i suspect the tire pressure may be the culprit.

I inflate to 35 PSI cold like the manual says but I'm thinking that this pressure is good for a 225mm tire (old tire) and not for a 235mm tire (new tire).

Or maybe i'm overthinking this. Honestly I don't know so any suggest would be helpful.
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Old 01-26-2013
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bigger tires require less RPM's to maintain the same speed as smaller tires. less rpms means better milage. now there is a friction point where tire size to HP/torque plays a big part and the vehicle has trouble turning the bigger tires. at that point you will lose milage and power also...

my 1998 ranger gets 32+ with 235/70/16 and only got 22-25 with the 215/70/14's * given my stock rear end was 3.45 and my new one is 3.73 * but my end tire ratio is still better then the stock ratio/tire..
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Old 01-27-2013
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It sounds like, if anything, you're new tires are slightly less diameter than the 225's (too lazy to open calc.exe). So they're a little wider (potentially a little more friction) but a little less diameter (slightly easier to rotate) which should be a wash basically.

1.) How much less gas mileage? Have you calculated by hand, driving most of a tank?

2.) Did you change those other things the same time you put on tires?

3.) Is the truck throwing any codes? (Never hurts to ask )

My truck is basically the same as yours. With my 225-70R14's and the 4.10 gears, I usually get about 25 mpg in the summer and 20-21 in the winter.
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Old 01-27-2013
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Quote:
Originally Posted by turbo98ranger View Post
bigger tires require less RPM's to maintain the same speed as smaller tires. less rpms means better milage. now there is a friction point where tire size to HP/torque plays a big part and the vehicle has trouble turning the bigger tires. at that point you will lose milage and power also...

my 1998 ranger gets 32+ with 235/70/16 and only got 22-25 with the 215/70/14's * given my stock rear end was 3.45 and my new one is 3.73 * but my end tire ratio is still better then the stock ratio/tire..
It didn't cross my mind at the time but now it may be logical if you're looking that way.
So the solution will be regearing the ratio/tire but it's too late now but I'll think about the next time I change my tires ( in 5 years lol)

Thanks !
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Old 01-27-2013
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Originally Posted by red_rider View Post
It sounds like, if anything, you're new tires are slightly less diameter than the 225's (too lazy to open calc.exe). So they're a little wider (potentially a little more friction) but a little less diameter (slightly easier to rotate) which should be a wash basically.

1.) How much less gas mileage? Have you calculated by hand, driving most of a tank?

2.) Did you change those other things the same time you put on tires?

3.) Is the truck throwing any codes? (Never hurts to ask )

My truck is basically the same as yours. With my 225-70R14's and the 4.10 gears, I usually get about 25 mpg in the summer and 20-21 in the winter.
1) My odometer broke 5 years ago, so I calculated by hand most of the time.

2) No. I change those things way before the tires.

3) the truck was throwing only P1443 last year (I fixed the other codes long time ago) and fixed it last december. But just after fixing it, my gas mileage was very good (+25-26 mpg).

I filled up my tank last monday and I keep record of my driving. I'll see how much mpg I loose. Hope not too much.

Do you know what is the original ratio gear ?

After all, it may be normal the mpg I'll get now.
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Old 01-27-2013
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What about the tire pressure ?

For the stock tire (225/70R14) they recommend 35 PSI cold air but for 235/60R15, does it gonna be a little more ?
The contact surface is a litlle wider than the stock tire, so if we apply 35 PSI on the new ones, they're gonna be underinflate ?
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Old 01-27-2013
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Just curious, how are you hand-calculating your gas mileage without an odometer reading?

Also, for your truck, reg cab should be 3.73 gears and ext cab are 4.10. That's assuming it's a stick, I'm not sure about autos.
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Old 01-28-2013
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Originally Posted by red_rider View Post
Just curious, how are you hand-calculating your gas mileage without an odometer reading?

Also, for your truck, reg cab should be 3.73 gears and ext cab are 4.10. That's assuming it's a stick, I'm not sure about autos.
I know exactly how much gas i filled at the gas station and I know the distance I made everyday from home to work and back home. So for a full tank, I have my mpg.
It is still approximate but reliable at some point.
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Old 01-29-2013
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Gotcha. How much worse gas mileage and does your truck vary much based on the temperature? Sorry just trying to get an idea.
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Old 01-31-2013
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Originally Posted by red_rider View Post
Gotcha. How much worse gas mileage and does your truck vary much based on the temperature? Sorry just trying to get an idea.
I filled up my tank last week...and there's still some gas into it so when it'll be empty I'll know if it goes worse.
Temperature ? Well I never notice any change..so I guess not really. I live in French Polynesia and we only have 2 seasons here: it is "summer all the time" and a period of rain.
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Old 01-31-2013
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larger tires do require less rotations of the motor, which save fuel. but too little rotations and too much load use more fuel too. its a middle-ground. id run a 235/75/15 on it. It would give you good mpg.

I run 33x12.5x15 tires (in inches) and i can get 23mpg on the highway. its all about setup and driving habbit,.

anything over 20mpg your flying high, trust us
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