Re-gearing Help

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trinitron12

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NOOBIE HELP!!! 17 JKU just regeared to 4:88 gears but still seeing elevated RPMs at 70 MPH. currently seeing 2700-2800 RPMs at 70MPH

Current Set up
3.6 auto
37x13.5x17 wheels
4:88 gears

I see the chart all says should be in the 2500 range and was wondering why i would be little higher and if there is anything to be concerned with.
 
trinitron12 said:
Yes calibrated with Flashcal. and validated GPS speed is good. Speedo checks out just elevated RPMs nothing crazy but still higher than expected
Click to expand...

Have you measured the actual wheel diameter as installed on the vehicle? Most (not all) tires are smaller than the listed diameter. If, for instance, your nominal 37" tire measures out to be 36" that could explain some of it... The calculated 2576rpm from the fear ratio chart would actually be 2648rpm
 
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tJKrider said:
Have you measured the actual wheel diameter as installed on the vehicle? Most (not all) tires are smaller than the listed diameter. If, for instance, your nominal 37" tire measures out to be 36" that could explain some of it... The calculated 2576rpm from the fear ratio chart would actually be 2648rpm
Click to expand...

Yes they measure out at 35 1/4. Which is crazy all by itself

Can't figure out what would cause the elevated rpms. It's just odd
 
trinitron12 said:
That's where I'm leaning but no idea how to know that for sure.
Click to expand...
Grab your multimeter and set it to AC.
Start the engine and connect the meter to the tach wire.
Run the RPM up to 1K, 2K, 3K and record the meter reading at each RPM. You may need to get a Lovely Assistant from the audience. If they don't match, thank your assistant and repair or replace the tach.
 
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Dirty Dog said:
Grab your multimeter and set it to AC.
Start the engine and connect the meter to the tach wire.
Run the RPM up to 1K, 2K, 3K and record the meter reading at each RPM. You may need to get a Lovely Assistant from the audience. If they don't match, thank your assistant and repair or replace the tach.
Click to expand...

I’ve never thought about this before but, that’s good info.

Thank you.
 
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trinitron12 said:
Can't figure out what would cause the elevated rpms. It's just odd
Click to expand...

Not odd at all really, it's a factor of the connected mechanical components. The smaller the tire diameter, the faster the engine has to turn to maintain any given linear speed, all other things held equal.

Since you verified the vehicle speed is accurate via GPS, a 35.25" actual diameter will require 2704rpm to maintain 70mph vs. the 2576rpm on the calculated gear ratio chart for a 37" tire.
 
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the chart tire size. is that actual size or is that the tire size? Example my 37s actually measure out at 35.25

This may explain my confusion if i look at 488 gears and 35s the rpm is matching my actual tires. but its high if compared to 37s
 
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Snowbird Tom said:
RPM calculations are done with actual tire size in real life and on the gearing chart.
Click to expand...

Thanks i think this is my confusion. I put in the flashcal the 35.25 size and speedo is perfect. but looking at the chart i compared to 37in tires. if i look at 35 RPMs are spot on.

thanks for the help gents!!! sorry about the noobiness!!! :}
 
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trinitron12 said:
Thanks i think this is my confusion. I put in the flashcal the 35.25 size and speedo is perfect. but looking at the chart i compared to 37in tires. if i look at 35 RPMs are spot on.

thanks for the help gents!!! sorry about the noobiness!!! :}
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No problem! If you don't know you ask and then learn. It's always preferable when the outcome is that there isn't any issue that has to be rectified!
 
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You're getting exactly as calculated...

70 miles/hr x 1hr/60min x 63360 in/mile = 73920 in/min
35.25" diameter = 110.58" circumference
73920 in/min x 1rot/110.58 in = 668.5 rot/min
668.5 rotations/min x 4.88 revolution/rotation = 3262 revolution/min x 0.083 OD ratio = 2707 RPM
 
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