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Discussion Starter · #1 ·
My 1998 Olds Bravada has 235/75-15 tires. The door stick read that they should be 235/70-15.

Will the MPG be lower or will it increase if I change back to 235/70-15?
:cool:
 

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The speedo and odo will turn faster telling you your mpg got better. But it'll actually be your "true" mpg if you go back to stock.
 

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shorter tire (smaller diameter) will increase rpms and reduce mpg at the same speed as a taller tire (larger diameter ) will.
 

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Discussion Starter · #4 ·
Here something to think about. Will the transmission shift at a late time keeping the Motor rpm higher for a longer time lowering the MPGs.
 

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jonsommer said:
Here something to think about. Will the transmission shift at a late time keeping the Motor rpm higher for a longer time lowering the MPGs.

Tranny won't be affected but your mileage will, any time you increase rpms you will lose mileage. It's that simple.
 

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Another thing to think about is a bigger tire is heavier, you can't slap a 38" tire on that thing and expect better mpg. Also rpm's are not directly related to mpg. Of course you will get bad mpg if you run at 5k rpm's all the time but you'll also get bad mpg when the motor is being lugged. It will be wasting fuel. The car was built from the factory to take a certain sized tire and geared accordingly. However I don't think you'll see much of a difference, the difference in diameter is ~3%. Things are not so simple since your mpg is ultimately determined by the amount of fuel it burns and you don't know which size your motor/trans prefers. On the plus side your odometer will read 3% fewer miles (3k miles per 100k) :p
 

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Your MPG may be greater at higher speeds, but from an acceleration standpoint, it won't be as good. But if you really notice a difference in MPG i'd be very suprised. Theyre not that much different in size.
 

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03dynasty said:
Another thing to think about is a bigger tire is heavier, you can't slap a 38" tire on that thing and expect better mpg. Also rpm's are not directly related to mpg. Of course you will get bad mpg if you run at 5k rpm's all the time but you'll also get bad mpg when the motor is being lugged. It will be wasting fuel. The car was built from the factory to take a certain sized tire and geared accordingly. However I don't think you'll see much of a difference, the difference in diameter is ~3%. Things are not so simple since your mpg is ultimately determined by the amount of fuel it burns and you don't know which size your motor/trans prefers. On the plus side your odometer will read 3% fewer miles (3k miles per 100k) :p
The more revolutions your motor makes the more fuel it requires, actually it is that simple.
 

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Discussion Starter · #10 ·
This is interesting there seems to be two trains of thought.

If RPMs Lower MPGs how do you explain why my 1985 S10 pickup got better MPGs running at 65MPH then 55MPH. The average increase was about 3MPGs.
I did alot of long distance runs for the company I worked for. It was a 2.8 V6
4 speed with eagle GT tires. The 4 speed was rare for a S10. Average MPGs was about 23.

Question : What air pressure should I be running in the 235/75-15?
 

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jonsommer said:
This is interesting there seems to be two trains of thought.

If RPMs Lower MPGs how do you explain why my 1985 S10 pickup got better MPGs running at 65MPH then 55MPH. The average increase was about 3MPGs.
I did alot of long distance runs for the company I worked for. It was a 2.8 V6
4 speed with eagle GT tires. The 4 speed was rare for a S10. Average MPGs was about 23.

Question : What air pressure should I be running in the 235/75-15?

Unless you are running an aftermarket cam of some kind that changed the factory power curve
you won't get better mileage at 65 than 55. Run the tire pressure as stated on the sticker inside the door jamb of your vehicle unless these are not the factory size tire.

Some of you can be hard headed at times, but in it's simplest terms. Do any of you really think a vehicle
that has a higher numerical gear installed or a shorter (smaller diameter) tire put on it will get better mileage ? Each of these cases will increase the vehicles RPM at any given point as compared to factory.
 

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Discussion Starter · #13 ·
I tracked mpg for years This S10 just had better MGPs at 65mph. No cam! I do believe the sweet spot had something to do with it. Air dynamics possible play a big roll in getting better MPGs at high speeds.

Let rethink about the transmission and its roll in MPGs. If the tires are larger the sensors count less pulses per rev of the tires travel. If the computer is programmed to shift at a set number of pulses/sec the engine will rev higher before it shifts. this go for each gear. Add to this that the larger tire will require more torque to start to turning from a stop(more rpm). This could lower MPGs.
 

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Jetmeck said:
Unless you are running an aftermarket cam of some kind that changed the factory power curve
you won't get better mileage at 65 than 55. Run the tire pressure as stated on the sticker inside the door jamb of your vehicle unless these are not the factory size tire.

Some of you can be hard headed at times, but in it's simplest terms. Do any of you really think a vehicle
that has a higher numerical gear installed or a shorter (smaller diameter) tire put on it will get better mileage ? Each of these cases will increase the vehicles RPM at any given point as compared to factory.
The first reason to believe of dismiss an argument is because of personal experience. if this guy said he got better mileage at 65 as opposed to 55 and he is talking from experience then you cant say he wont.

i am not saying this is always the case because i agree if you are running the engine faster you may use more fuel, IF you are not lugging the engine. i can run my truck in 5th gear and 15mph @ 600rpm but its not going to get as good of mileage as it running 2000rpm @ 55 on the highway.

another factor to consider is if you are doing a lot of stop and go traffic, having smaller tires would be beneficial because it wouldnt be as much of a load on the engine to get the truck going. the fact is, the only way to know for sure if to test it out with both tires
 
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