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Old 01-19-2003, 04:07 PM   #85
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Look at the formula's I posted at 10/16..

At 50 MPH with a 5.13 gear and 30" dia tires you will be pulling 2872.8 RPM. At 65 MPH you will be pulling in a wopping 3734.64 RPM!! These formula's are figured on a 1:1 output (Drive). Oscar
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Old 01-19-2003, 04:13 PM   #86
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Too Low Ratio ???

Thank you all for your knowledge, one thing I will add, is the vehicle I have, has overdrive and 225/70R19.5 tires,
Thanks again
Jason
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Old 01-19-2003, 04:16 PM   #87
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Formula

Oscar,

Would you mind sharing this formula?

I'm just interested in figuring out different speed/rpm.

Tks-John
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Old 01-19-2003, 04:20 PM   #88
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Hey John,

Just scroll up to 10/16 on this thread, I have 4 formula's posted. Oscar
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Old 01-19-2003, 04:29 PM   #89
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Thanks!

Oscar,

Thanks!

John
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Old 01-19-2003, 04:30 PM   #90
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too low ratio

O.k

Do you think my 5.13 ratio, is too low, or could I live with it ?
How easy is it, to alter if not.
The truck is 1997 3500 hd Chevy, 5.7l turbo diesel.
It was a Florida ambulance, so I guess it would have gotten to some speed on occasions ??
Jason
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Old 01-19-2003, 04:34 PM   #91
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One more dump question

Would a LT245/75R16 = a 32" dia tire?
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Old 01-19-2003, 04:47 PM   #92
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Best fuel mileage

My horsepower and torque curves intersect @ 2500rpm, would this be my optimum rpm for fuel mileage?

According to the forumal, 58mph is my speed @ 2500rpm with a 4:10 grears and 32" dia tires.

Is my thinking correct?

Tks-John
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Old 01-19-2003, 04:54 PM   #93
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Closer to 30.5" I'll give these formula's in the mornin'. Gotta catch some sleep. Oscar
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Old 01-19-2003, 05:13 PM   #94
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Lightbulb Sounds right!

Oscar,

30.5 sounds right because I get my best gas mileage(13mpg) while towing at 55mph. I forgot the exact rpm, but after doing your forumal with a 30.5" dia tire, it came out to be 55mph @ 2500rpm w/4:10.

The only question I have now is: am I correct in thinking that where the horsepower & torque curves intersect, is your optimum rpm for best fuel mileage?

Tks-John
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Old 01-19-2003, 07:46 PM   #95
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tow vehicle

For what it is worth, I have owned several airstreams and two tow vehicles. in addition i live at 4600 ft in western Colorado and haved towed over 10000ft passes.
First was a 1987 suburban 350 1/2 ton w 3:73 gears. This was adequete for a 1974 31ft Sovereign and a 1974 25' Caravaner. they each weighed less than 6500 lbs full. The only thing a little light was braking.
Second rig is a 1995 3/4 ton suburban w the 454 motor. with this rig I towed the 31ft Sov all over in the west and even up to Alberta. Noted better braking and power on steep grades. Bought a 1985 25' Sovereign (w rear full bed) and it towed w ease, it's weight was about the same as the 74 31' was about 6700 lbs.
Oddly enough the fuel economy was about the same. i could get up to 13 mpg on level streches if I stayed under 60. One memorable trip of 3000 miles to Canada and back got 11 mpg total and at times I was driving pretty fast.
In summation, be over equiped when towing, a Suburban esp the 3/4 ton has everything bigger and stronger and you just might need it.
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Old 01-19-2003, 10:11 PM   #96
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Formula

According to my figures:

(width (in mm) X aspect ratio (in mm) divided by 2540) X 2 + wheel diameter = tire diameter

(245 X 75 / 2540) X 2 + 16 = 30.468503"

(265 X 70 / 2540) X 2 + 16 = 30.606299" and so on.

(265 X 75 / 2540) X 2 + 16 = 31.649

Tire width can be figured by size in mm / 25.4 = width

245 / 25.4 = 9.645" wide

For a gear ratio guide for larger tires, go to www.4wheelparts.com/Tire1_Gear_Ratio.asp?id
This site will show a tire diameter / gear ratio combination and the resulting RPMs at 65 MPH in a 1:1 ratio with manual transmission. An auto will have a slightly higher rpm due to slippage in the transmission and torque converter. With an overdrive trans., rpm will be 15-30 % less than indicated according to the chart.
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Old 01-19-2003, 10:59 PM   #97
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Jason,
I forgot to mention that the rear end ratio is only one part of the equation and cannot be considered by itself. The size engine, it's torque capability and the powerband it functions in must be taken into consideration in addition to the tire size. For instance, my '01 2500HD with 8.1 litre engine and 5 sp. auto transmission came with a 3.73 rear end ratio. I could have had an almost identical truck with a 4.10 rear end but wanted to save on gas while traveling the interstate without the trailer in tow. My truck came with LT245-75-16 tires. If I wanted to put on taller tires, say LT285-75-16, then I would have purchased the 4.10 rear end to keep the engine in a powerband that the engine could live with. While not available, if I went with 3.08 gears, the engine would be in such a low RPM range that it would have struggled on takeoff and any hills it managed to come across. Gas mileage also would have suffered unless I was on the interstate going 100 mph all the time. On the other side of the coin, 5.13 gears with my heigth tires would have run the engine somewhere around 1,000 rpms higher at 65 mph over the 3.73 gears. That's gas you're burning. Smaller engines like to rev higher when you are comparing 4 cyl and 6 cyl engines to some of the 8 cyl engines but that is opening another can of worms.

My '77 K5 fullsize Blazer with 350 V8 was a dog with 32" tires and a 3.08 rear end. It could tow a boat but would have problems with my 31' Excella if I still had the truck. Outfit the Blazer with 4.10 or 4.56 gears and she would have been a stump puller with her Edelbrock Performer alum. manifold and camshaft combination. I'd say your ambulance would pull the trailer without problem but if you find that you want to lower your rpms, go to a slightly taller tire and have your speedometer recalibrated.
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