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Old 01-18-2010, 12:46 AM   #1
TwoFiftyShifter
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Best RPM for highway driving? Tires too?

I just installed a tach on my '87 K20, and I was wondering what the best rpm for mileage is on the highway. Now I know 1800 rpm is usually best but the highway near me is really hilly, and I need lots of throttel below 2krpm to climb hills, resulting in worse mpg. Is 2500 rpm too high for good mpg?

Also, I currently have 245/75r16 tires, and am soon due for new ones, I'm wanting something a little taller looking to fill the wheel wells just a little better, what tire size? 235/85 looks ok on other trucks... what do you think?

The truck in question is an 87 TBI350, 3.42 gears and 4 speed granny transmission, seems to run 2200rpm or so at 60mph, who knows how accurate the speedo is, thanks for the input.

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Old 01-18-2010, 12:50 AM   #2
Stitch
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Re: Best RPM for highway driving? Tires too?

My C20 crewcab has 235/85/16s on it, don't have any pics with the motor in it or the bed on it or I'd post it...the tire size is a nice size...
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Old 01-18-2010, 12:59 AM   #3
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Re: Best RPM for highway driving? Tires too?

i would say between 2200 to 2500 is good but what do i know BTW that is a nice truck!!
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Old 01-18-2010, 01:15 AM   #4
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Re: Best RPM for highway driving? Tires too?

i did 3k rpms for 4hours straight (well 2 stops for gas) didnt have a problem
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Old 01-18-2010, 01:34 AM   #5
bikes&bowties
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Re: Best RPM for highway driving? Tires too?

285/75r16 FTW.
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Old 01-18-2010, 11:04 AM   #6
Benelli
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Re: Best RPM for highway driving? Tires too?

I like the 32x10.5R15's

2300 at 55 for an hour last night........gas suckin.
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Old 01-18-2010, 01:08 PM   #7
dan76
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Re: Best RPM for highway driving? Tires too?

Bear in mind if you install "taller" tires, the overall gear ratio will change.

Best way to determine the engine's most efficient rpm (without using a dyno) is to determine the cam specs and find out where the torque curve is. Hopefully you'll be into the front half of the curve at the road speed you'll most often drive.

Last edited by dan76; 01-18-2010 at 01:10 PM.
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