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Topic: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270 (Read 5312 times) previous topic - next topic

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #100 –
Quote from: krush –
I think mileage may have improved slightly. But, it's not a fair comparison because before I was driving 62-64mph and now I drive 70-75. So any increase in mpg was lost due to increase in average speed.

I seem to be getting 8.5 to 9mpg. But again, I cruise 70-75 when able. Top speed was about 95mph, for those that are curious.
Wind drag from 62-64 to 70-75 decreases my mileage more than 1 mpg
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #101 –
Quote from: craneman –
Wind drag from 62-64 to 70-75 decreases my mileage more than 1 mpg

It may be the same too. Really, I didn't notice a change in fuel mileage--if there was it was small. But I do drive much faster now. So the takeaway is that I can't truly speak if there was an improvement in fuel mileage. But, all else being equal, it appears mileage improved with the gear change.
1998 U270 34'

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #102 –
Also, I forgot to put in my update the following information: A main reason for this gear change was to reduce/eliminate vibration that I had over 65mph. Also, it was to prevent failure of the p3 bearing that many FT owners have had replaced in the transmission. I feel that the vibration and failure is caused by the drive-shaft spinning too fast while going 65mph or faster. The higher rear end ratio has eliminated this problem and hopefully I will never have the p3 bearing problems that others have experienced.
1998 U270 34'

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #103 –
An update: Been almost two years and another 10-15k miles (nearly 30,000 total). All is great, no issues.  Any slight increase gear whine at highway speed is negligible and is covered by wind noise. On rolling hills I manual select 5th gear. On flat, I cruise ~1700rpm at 70. If no rush, I tend to cruise around 68mph.

This gear change really shines on the flat land. But it also puts the power range in better use for hills too.
1998 U270 34'

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #104 –
Thanks for the update,  seems logical.  Does it seem that 5th (overdrive) gear puts you back at 5.13?  Does it act like you have a shiftable rear axle, with overdrive 5 & 6th gear.

Re: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Reply #105 –
With the 4.33 the 5th gear is basically just like 6th was with the 5.13.  Somewhere in this thread or another I have my excel spreadsheet where I did all the calcs. It kicks down to 5th as needed, but I usually run in eco mode so it doesn't shift as much. The problem with eco mode is that by the time it shifts down to 5th, it may be too late and RPM will be much below peak power.  If there is going up/down in shifting a lot, I just put it in 5th.  I treat 6th as a true overdrive for flat land cruising.

Kicking down to 4th gives max HP RPM (2200RPM, I think) right at 59mph. This works well for most hills.  The only downside is that if it is really steep, the jump from 4th to 3rd is pretty large....if can't keep speed in 4th going up the hill, I may be climbing at 42mph at 2200RPM in 3rd.  A 10 speed would really make the 8.3 pull a lot better on various grades.
1998 U270 34'