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Old 07-10-2015, 03:00 PM   #11
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Actually, the gear ratio of the larger tire to the ground will give you slightly better highway fuel mileage that is impossible to measure since your odometer was calibrated for the smaller tires.
That's what I was thinking. If the tire is taller, the circumference is greater, so every revolution of the wheel will take you just a little further down the road, which would improve your fuel economy. But that may be offset by the decrease in torque, making your engine work a little harder during acceleration and on hills.

Automotive engineers discovered long ago that increasing tire diameter helped them achieve greater fuel economy. My 1989 truck had 15" wheels. My 2007 truck has 16" wheels. And the newer ones have 17" and even 18" wheels. That is by design so that pickup truck manufacturers can meet the CAFE regulations.
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Old 07-10-2015, 03:30 PM   #12
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That's what I was thinking. If the tire is taller, the circumference is greater, so every revolution of the wheel will take you just a little further down the road, which would improve your fuel economy. But that may be offset by the decrease in torque, making your engine work a little harder during acceleration and on hills.

Automotive engineers discovered long ago that increasing tire diameter helped them achieve greater fuel economy. My 1989 truck had 15" wheels. My 2007 truck has 16" wheels. And the newer ones have 17" and even 18" wheels. That is by design so that pickup truck manufacturers can meet the CAFE regulations.
Buuuuuut, if the engineers were really after better fuel economy, they would be tweaking the transmission and differential gear ratios to get fewer engine revs per mile. Which only helps gasoline fueled vehicles.

Because Diesel engines don't use a constant mixture, small changes in engine revs per mile don't impact fuel usage. Gears that allow a Diesel to run with clear exhaust use the least fuel per mile.

(Should I add 50 MPG VW Jetta TDi to my signature?)
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Old 07-10-2015, 04:25 PM   #13
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Buuuuuut, if the engineers were really after better fuel economy, they would be tweaking the transmission and differential gear ratios to get fewer engine revs per mile. Which only helps gasoline fueled vehicles.

Because Diesel engines don't use a constant mixture, small changes in engine revs per mile don't impact fuel usage. Gears that allow a Diesel to run with clear exhaust use the least fuel per mile.

(Should I add 50 MPG VW Jetta TDi to my signature?)
I beg to differ. I am a service training instructor for hydraulic excavators, so I teach this stuff for a living. Our engineers have devised many strategies in recent years to help maintain constant engine RPM, in an effort to increase fuel economy. The hydraulic hybrid excavator released two years ago will dig all day long, with the engine at a constant 1600 rpm. That machine, along with an accumulator to start swing movement, boasts a 20% increase in fuel economy over the standard machine of the same size, using the same engine.

Any fluctuations in engine rpm, due to load on the engine, will decrease fuel economy - gasoline or diesel. But you are correct about gearing and rear end ratios. A standard 3:73 rear end gets good gas mileage, but doesn't pull very well due to loss of torque in that gear range. My 4:10 rear end pulls great, however, but my fuel economy suffers. You can have one or the other.

A diesel engine gets better fuel economy because the peak torque band is at a much lower rpm than a gas engine. That's why your VW does so well, along with 6-speed transmissions, and is also the reason most new cars in the EU these days are diesel powered. I've often wondered why American car manufacturers don't start putting diesel powered cars out there. They can get great fuel economy with smaller engines, as well as low speed power.
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Old 07-10-2015, 04:35 PM   #14
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What you all are telling me absolutely makes sense. According to the website I came across, the larger tire make 47.63 less revolutions per mile than does the smaller one.
(285 vs. 245). And it makes sense that it takes more torque to turn those revs.

Sailor, I did think about the gps just this week and found a free one for iphone that seems to be accurate. I did compare it to my speedo, and the chart I found on the website. BTW, the website I keep refering to is, tacomaworld.com. I'm sure there are others, but this one caught my eye first.

Mike, I don't recall the mess with the NYC taxi cabs. I think I was still a little wet behind the ears then and didn't care about much of anything.

This has been an interesting thread, and I've learned much. So many thanks goes out to everyone that's responded to my inquiries.

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Old 07-10-2015, 04:55 PM   #15
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I beg to differ. I am a service training instructor for hydraulic excavators, so I teach this stuff for a living. Our engineers have devised many strategies in recent years to help maintain constant engine RPM, in an effort to increase fuel economy. The hydraulic hybrid excavator released two years ago will dig all day long, with the engine at a constant 1600 rpm. That machine, along with an accumulator to start swing movement, boasts a 20% increase in fuel economy over the standard machine of the same size, using the same engine.

Any fluctuations in engine rpm, due to load on the engine, will decrease fuel economy - gasoline or diesel. But you are correct about gearing and rear end ratios. A standard 3:73 rear end gets good gas mileage, but doesn't pull very well due to loss of torque in that gear range. My 4:10 rear end pulls great, however, but my fuel economy suffers. You can have one or the other.

A diesel engine gets better fuel economy because the peak torque band is at a much lower rpm than a gas engine. That's why your VW does so well, along with 6-speed transmissions, and is also the reason most new cars in the EU these days are diesel powered. I've often wondered why American car manufacturers don't start putting diesel powered cars out there. They can get great fuel economy with smaller engines, as well as low speed power.


You are correct. That is why CVT's are coming to be used more frequently. Keep the engine in its most efficient range. Fewer gears, and you go above and drop below the efficient range. More gears are better, and CVT is like infinite gears, keeping the RPMs in a very tight band.
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Old 07-10-2015, 07:13 PM   #16
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Well I can't speak for the whole world but every time I go to a bigger tire I loose fuel economy. I have a 12 f250 with the stock tires I was getting 15.5 to 17 ave. I went to bigger tires and now ave around 13 just my 2 cents I have tried it on 3 pickups and a jeep and always loose 20 to 30% but boy do they look better
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Old 07-10-2015, 07:16 PM   #17
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Doesn't the taller tire also create increased side to side flex in the sidewall, compounded by the weight of the trailer ?
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Old 07-10-2015, 07:52 PM   #18
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Yes it does create side to side flex from the larger sidewall. A 15in rim with 35in rubber will not be a stable tow, regardless if they are LT tires. You want a bit of sidewall to protect the rims over bumps and soften the ride, but the small rim/large sidewall tires are for the rock crawlers and mud boggers... not towers!
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Old 07-10-2015, 07:55 PM   #19
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I was pretty sure, but wanted to get this important fact into the thread
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Old 07-10-2015, 09:47 PM   #20
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IdahoBoy, I curious as to how you are figuring mpg. When you went to the larger tire, did you have the speedometer recalibrated?
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