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

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airlaird

airlaird

Joined
Jan 7, 2005
Threads
314
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Location
Dallas, Texas
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! Anyway my results are in:
The ongoing debate between 4:56 and 4:88 on 35' tires has plagued mankind for centuries....until now ;-) I have the answer so sit back and enjoy.

TEST VEHICLE:
I have had 4:56's for about 2 years now with my 315/70R17 BFG KO's. My set up is a Supercharged 1995 with 4:56's and the speedo is spot on (via truespeed calibrator). I live in Texas. Two weeks ago I decided to go to 4:88's on my 35's. Since no one to my knowledge has gone from 4:56's to 4:88's I decided to track my results.
I have heard that there is a 200 to 250 RPM difference between 4:56 and
4:88's. Therefore, I documented every speed in 10 MPH calibrations.

The RPM difference is (que drumroll please..remember this is for mankind and the never ending debate of gearing).....................................

100 RPM.

What?? 100 RPM difference?????
YES, 100 RPM IS THE VARIANCE BETWEEN 4:56 VS 4:88's. My results (again sitting in my Digital camera) show speed increments of 50, 60 70 and 80 MPH with 4:56's and two weeks later with 4:88's at the same 10 MPH increments. Needless to say I was VERY dissapointed.

At 50 MPH with 4:56's, my Speedo indicated 2,810 RPM.
at 50 MPH with 4:88's, my Speedo indicated 2,910 RPM.
Therefore my results indicate..I wasted my money...or did I?
After 2 tanks of gas my mileage is up 20 more miles per tank with 4:88's
(it is possible with 4.56's the engine was lugging and therefore got worse mileage)
I feel another SLIGHT bump in acceleration with the slight increase in RPMS.

Summary: Those with 4:56's and 35's..you not missing anything.
Those with 4:88's and 35's, you did the right thing as well.
For those that believe there is a 200 -250 RPM difference....no way...I documented it, I lived it and I'm here to tell the story ;-)
airlaird
 
Duly noted.

Thanks for the sacrifice.

maybe one day (many years from now) I will be facing a decision. I will use the magic search functioin. I will find this thread. I will send you a sixer for making my life easier.

Smit
 
if you want to post pics E-mail them to ______________ and I can compress them in photoshop "save for web" so they can be attached, seams that text gets the message across though.
 
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Thanks Raven..it turns out I "expired" last week from the forum!! Since I haven't posted in a while until today, I had no idea. That's why I could not get my pics through. I'll try again once I get back on...but I think I got the message across!
airlaird
 
Thanks for takin one for the team. 2 Questions...how did you measure your speed and how did you measure your RPMs in both cases? It would be nice to contrast your numbers with others that have similar tires and other gear ratio options. (unless you want to be a total masochist and do it yourself.) ;)
 
That can't be right. You would be getting worst gas mileage with 4:88 because the engine is spinning faster. You need to do the test several times to see if you are getting accurate results.
 
Not necesarily. I was doing some research when I was looking into buying a chevy pickup and the guys with the bigger engines were getting better fuel economy. Why? The engine didn't have to use as much energy to do the same amount of work. Here is another example. Say you have a box of something that weighs 100lbs. It takes a certain amount of energy to pick that box up off the floor. Now, if you lift that box with a lever, it is much easier and takes less energy, and takes increasingly less energy the longer the lever gets. Granted, at some point the RPMs would get to a level where they would be detrimental to fuel mileage, but I don't think that is the case here.
 
airlaird said:
My set up is a Supercharged 1995 with 4:56's and the speedo is spot on (via truespeed calibrator).
Click to expand...

Did you recalibrate the truespeed with a GPS after you did the gear change from 4.56 to 4.88?

-B-
 
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airlaird said:
At 50 MPH with 4:56's, my Speedo indicated 2,810 RPM.
at 50 MPH with 4:88's, my Speedo indicated 2,910 RPM.
Click to expand...

Something is not adding up.

There is a ~7% difference between 4.56 and 4.88 and a ~4% difference between 2,810 and 2,910.

7% does not equal 4%
 
Wrench said:
That can't be right. You would be getting worst gas mileage with 4:88 because the engine is spinning faster. You need to do the test several times to see if you are getting accurate results.
Click to expand...

That's not necessarily true. If the engine is being more efficient, i.e. not working as hard, it will burn less fuel. Think about it, when you floor it from a stop you are dumping a ton of fuel and air into the motor but it doesn't start spinning at 4000 rpms immediately, it has to work its way up. So it really depends on the load.

If the mechanical advantage produced by the deeper gears outweighs the slight increase if fuel consumption by spinning 100rpms faster, then fuel economy will go up.
 
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.
 
Well I'm glad I jumped straight to 4.88's. I didn't take any data, but there is significant improvement.
 
airlaird said:
at 50 MPH with 4:88's, my Speedo indicated 2,910 RPM.
Click to expand...

Ken,
Using your GPS for actual MPH, what is your RPM at a steady 50MPH? You are also running 35's so these numbers should be close if both vehicles are traveling at the same speed.

-B-
 
Hmm my rpm at 80mph is 3000 with 4.88s.
Sean
 
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