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Old 06-29-2008, 11:33 AM   #1
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How Accurate Are Your Mpg Figures?


An interesting discovery..
Just recently it was necessary for me to have the Burb checked at work with the G.M. Tech 2.(diag computer) for an air bag light.
I knew the PO had up-graded the tires, from a 245 to a 265 cross section, but it never occurred to me to re-check the tire height. Seems the new tires are 3 1/2 inches taller. Re-programed to 31 inches and gained 300rpm. Were running 2200@55mph, now 1900@55mph.

Plan a trip over the 4th, looking forward to knowing how much of a change there is.

Any math gurus out there??
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Old 06-29-2008, 11:50 AM   #2
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Looking at your numbers it looks like you were going faster then you thought. When you thought you were going 55 MPH you were most likely going a little over 63 MPH. That much of a difference could have been very costly. Did you ever wonder why you were passing so many people?

Speedometers are never right on, I use my GPS to determine what my exact speed is.

With real numbers I would expect to see around a 15% increase in calculated MPG. Since you will now be driving slower, it should be even better.
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Old 06-29-2008, 03:10 PM   #3
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My calcs confirm azfly's. and am surprised you never noticed passing everyone. I put 33's on my truck (not my tv) and at 60 pass all on the highway.
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Old 06-29-2008, 03:23 PM   #4
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With real numbers I would expect to see around a 15% increase in calculated MPG. Since you will now be driving slower, it should be even better.
R U saying higher tires will get more MPGs?
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Old 06-29-2008, 03:35 PM   #5
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R U saying higher tires will get more MPGs?
That is not what I am saying, but it might be true.

I said calculated MPGs. Since his odometer was reading incorrectly his numbers were wrong. Before he corrected the odometer every mile displayed was about 1.14 miles.
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Old 06-29-2008, 03:40 PM   #6
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Higher tires effectively lower your rear end ratio. Most likely will lower gas mileage, due to increased wind drag on the vehicle, because it has a higher profile. Some larger tires have more rolling resistance, which also lowers mileage. Next time you see someone driving a lifted 4X4 with monster tires, ask him what his mileage is. Lucky to get 10 or 11 mpg.
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Old 06-29-2008, 06:01 PM   #7
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Higher tires effectively lower your rear end ratio. Most likely will lower gas mileage, due to increased wind drag on the vehicle, because it has a higher profile. Some larger tires have more rolling resistance, which also lowers mileage. Next time you see someone driving a lifted 4X4 with monster tires, ask him what his mileage is. Lucky to get 10 or 11 mpg.
The way I understand it, taller tires lower your RPM by raising your gear ratio. A taller tire has more surface per revolution, engine doesn't spin as fast. A 30" tire covers more ground per revolution than a 10" tire.
Fuel mileage is really affected by the mass of the larger tires, most of which seem to be about 20" wide.
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Old 06-29-2008, 06:12 PM   #8
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looking forward to knowing how much of a change there is. ?
hi rc...

changes?

does the truck have larger tires AND wheels or just taller/wider tires on the stock rims?

IF you now drive SLOWER the mpg may go UP relative to your prediscovery mpg....

but the larger circumfrence tires/wheels effectively reduce hp/torque compared to stock...

also the larger tires/wheels generally weigh MORE which increases braking distances, stopping times and brake wear.

and other handling issues are impacted too (higher cog, changed slip angles and breakaway parameters),

depending on whether suspension changes were done FOR the bigger shoes.

generally the larger tires/wheels also reduce mpg relative to STOCK shoes...

while larger tires can wear longer, IF the alignment wasn't tweaked FOR the bigger tires, uneven wear is common.

so what differences YOU may notice depends on the reference point and driving style after discovery...

the only functional reason to put larger wheels (and tires) on a towing vehicle...

is after the tv has been fitted with larger brakes, which may not fit the original size rims...

the rest is bling and zing....

cheers
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Old 06-29-2008, 07:44 PM   #9
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My calcs confirm azfly's. and am surprised you never noticed passing everyone. I put 33's on my truck (not my tv) and at 60 pass all on the highway.

All our trips with the new rig have been on the NYS Thruway, any who have travelled there know you won't pass very many at anything less than 70mph.

The only difference,OEM 245/75/R16 replaced with 265/75/R16,
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Old 06-29-2008, 07:56 PM   #10
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I remember the Thruway well. Where the heck is that ticket...

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