4:56 VS 4:88...been there..done that

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Arya Ebrahimi said:
Recalibration of the truspeed would be absolutely crucial to the accuracy of his observations.

If he has not recalibrated the motor is spinning faster to obtain the same speed. This obviously will affect the speedometer since the speedometer runs off the T-case. By going with deeper gears you are throwing off the speedometer to the high side(says you're going faster than you are). If he recalibrates the speedo it will say he is going slower than he originally thought, therefore requiring additional throttle to achieve the previous test speed. Clear as mud, right?

In other words, if he hasn't calibrated the speedometer with the new gears, there will be a bigger RPM difference than seen from the initial tests.
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Explain to me how speedo calibrations are affecting RPM, i.e. how the engine can spin the gears faster or slower based on speedometer calibation?

FWIW, I had the same exact experience with my old XJ. I put in 4.56 gears (3.55 stock) for 31" tires (which was great) and then went to 32's and 33's. Each time it was just about 100 RPM difference.

I have never found the paper math for regearing/tires to be accurate.
 
Recalibrating the speedo isn't affecting RPM's. Speed is being used as a baseline from which to determine the difference in RPM's at said speed.

Something isn't right from the standpoint that no matter what the gearing is 4.10, 4.56, or 4.88 that RPM is too high at 50mph.

Buck Buchanan
 
OK, now that I think about it the truespeed calibration is only for the tyres and he is still running the same tyre.

It would be interesting to get the results of others running 4.88s with 35" tyres at a corrected (GPS) speed of 50 MPH.

-B-
 
Buckru said:
Recalibrating the speedo isn't affecting RPM's. Speed is being used as a baseline from which to determine the difference in RPM's at said speed.

Something isn't right from the standpoint that no matter what the gearing is 4.10, 4.56, or 4.88 that RPM is too high at 50mph.

Buck Buchanan
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Got it :idea: . Just wasn't reading it right. The clear answer here is forget both 4.56 and 4.88 and head straight to the 5.29's :D

Nay
 
Beowulf said:
OK, now that I think about it the truespeed calibration is only for the tyres and he is still running the same tyre.

It would be interesting to get the results of others running 4.88s with 35" tyres at a corrected (GPS) speed of 50 MPH.

-B-
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Are you saying that because the tires didn't change the calibration would still be correct, despite the gear change?

If so, you are mistaken. The speed sensor is connected through the diffs to the tires, so when you change the gear ratio in the diffs, you are affecting the reading at the speed sensor. If the speed sensor were at the wheels, it would be a different story.

Ary
 
Arya Ebrahimi said:
Are you saying that because the tires didn't change the calibration would still be correct, despite the gear change?

If so, you are mistaken. The speed sensor is connected through the diffs to the tires, so when you change the gear ratio in the diffs, you are affecting the reading at the speed sensor. If the speed sensor were at the wheels, it would be a different story.

Ary
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My take on it agrees with yours.

Higher ratio R&P's make the drive shaft spin further for each rotation of the axle, no? For any given wheel speed, the drive shaft is spinning faster for higher R&P ratios.

Since the speedo sensor is at the T-case, the speedometer interprets the faster drive shaft as higher road speeds.

Bigger tires and higher gear ratios cancel each other out to some degree.

Right?

Hayes
 
Arya Ebrahimi said:
Are you saying that because the tires didn't change the calibration would still be correct, despite the gear change?

If so, you are mistaken.
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OK, I'll retract my retraction. :D

I'm back to agreeing he needs to recalibrate the Truespeed for the 4.88 gears, then report his RPM at a true 50MPH.

-B-
 
Beowulf said:
OK, I'll retract my retraction. :D

I'm back to agreeing he needs to recalibrate the Truespeed for the 4.88 gears, then report his RPM at a true 50MPH.

-B-
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Yep those rpm do not correlate with 50mph. I am going out right now and see what my rpm is at 50mph. I have gps calibrated truespeed with 4.88s. Back in 5.

315/75R16 tires, 4.88 gears = 2000 rpm at 50 mph.
Cheers,
Sean
 
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Wow he is way off in his numbers. I thought it looked not right. Great to have people here that no there stuff.
 
Guys of course I calibrated using a GPS, so yes it was calibrated.
Well, I stand behind my results. My post is a COMPARISON between RPM's using 4:56's VS 4:88's. Please note several of you are going to run out and see your own results please do. Bear in mind some 35's are TRUE 35's and some are not. My KO is NOT a true 35...this must be considered. "Way off on my numbers"?...no way.., that's why I photographed it. And it figures I have issues posting since my subscription expired last week. In any event, the seat of the pants was very minimal going to 4:88's.
airlaird
 
1FZFE_RPM.jpg
 
airlaird said:
Well, this thread may be worthless without pics but I tried to shrink my pics down and it was STILL too many pixels...the pics were literally down to the tips of the speedo and tach and STILL too big!
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I feel your pain everytime I post a pic as well. It would be better if they were automatically resized (hint,hint,wink,wink)

I plan on doing 4.88's soon.
 
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