My M1165A1 is around 2900 rpm at 55 mph. This vehicle has a new red river 6.5 td and I put less than 1000 miles on it. The trans is a rebuilt 4l80 and the drive train was professionaly installed by Mike Hoffman. Everything is running as it should, and the trans shifts through all the gears including overdrive but the rpm be is around 2900 at 55 mph. Does this sound normal for this model truck? I have been told that it should be around 2200 at 55 mph and some guys are posting that they can do 70-75mph but I wouldn't even consider pushing my engine above 3000 rpm. My differentials and t-case are the same that came on the truck.
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M1165A1 high RPM at 55mph
- Thread starter jim1131
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2900 at 55 mph is roughly what you would see if the 4L80E is in 3rd (direct). Pull codes.
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Do you have an alternator with the "high speed" pulley?
Some alternators had a smaller pulley so they would make more power at lower RPMs, throws the tach off as that is what the tach reads.
EDIT, I think they were used mostly on the 400A alternators.
Some alternators had a smaller pulley so they would make more power at lower RPMs, throws the tach off as that is what the tach reads.
EDIT, I think they were used mostly on the 400A alternators.
Not sure which pully I have
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Someone should chime in with the info you need.
Autonomy_Lost
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Great thought. Either way, it seems like a good idea to verify that the tach is accurate before doing anything else.
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This has come up several times in the past, and the usual culprit was a tachometer that was reading incorrectly.
GuntherRommel
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We have a M1152 that runs like this with the big 400 amp alternator. I thought this was normal for all the turbos? high rpm, lower geared gas wasters. Checked the rpms with the Super Flo Shifter, shifts fine and overdrive.
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The simplest way to check is get a cheap optical/photo tachometer.
All you need to do it point it at a spinning object that has one reflective or bright marking on one spot , and read the numbers.
If the numbers don't match, then it's the tach that's off.
I've done this math in other threads, so here goes
With 3.08 diffs, 1.9 hub gearing, and .75 overdrive, at 55 mph and 37" tires you should be turning right at 2200 (2190) rpms.
(the tires don't actually matter in any of the math here because the tach and speedo both get their readings before them in the system)
In the threads where the tachometer being off due to having the smaller pulley on the 400 amp generator, the usual discrepancy is instead of idling at 700rpms, the engine is reading 950rpms That puts it at approx 1.35-1 difference.
Another posted that the tach reading 2000 rpms compared to an optical tachometer on the harmonic balancer/crankshaft showing 1500 rpms, comes up with a 1.33-1 difference.
Going by the above 55mph being 2190 rpms, multiplying that by 1.33 gives you 2920 rpms at 55mph, which is pretty darned close to the numbers you are saying, and lines up pretty well with the other threads discussing this.
All you need to do it point it at a spinning object that has one reflective or bright marking on one spot , and read the numbers.
If the numbers don't match, then it's the tach that's off.
I've done this math in other threads, so here goes
With 3.08 diffs, 1.9 hub gearing, and .75 overdrive, at 55 mph and 37" tires you should be turning right at 2200 (2190) rpms.
(the tires don't actually matter in any of the math here because the tach and speedo both get their readings before them in the system)
In the threads where the tachometer being off due to having the smaller pulley on the 400 amp generator, the usual discrepancy is instead of idling at 700rpms, the engine is reading 950rpms That puts it at approx 1.35-1 difference.
Another posted that the tach reading 2000 rpms compared to an optical tachometer on the harmonic balancer/crankshaft showing 1500 rpms, comes up with a 1.33-1 difference.
Going by the above 55mph being 2190 rpms, multiplying that by 1.33 gives you 2920 rpms at 55mph, which is pretty darned close to the numbers you are saying, and lines up pretty well with the other threads discussing this.
Tachometer Inaccurate?
I have a 2006 M1165, 6.5 turbo, 60mph speedo and a tachometer. My problem is that the tach rpm reading is wrong. The tach shows idle at 950rpm, but my handheld rpm meter shows 700rpm. And with the engine speed set and showing as 2000rpm on the tach, my handheld meter is reading 1500rpm. I...
steelsoldiers.com
M1152 Low Top Speed
Did any of the m1152’s come with really low gearing? I’m having trouble with my new 52, I’m at 3300rpms at 63-65 when multiple other people say theirs is 2700-2900 at 70-75. I believe I feel lockup, either way, it wouldn’t drop that much. I have shifted from 3rd to 4th to make sure it actually...
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I can’t say this enough till I’m blue in the face on all the forums, these ECV’s run 3.08 Diffs, so your not going to get rpm’s that you see in A2 trucks running 2.73’s, I have more ECV’s than I can count, drive them daily, I don’t care want my tachometer says, I’m only concerned that I have all 4 gears. The next issue is indicated rpm’s, there are 2 different pulleys used, fixed and clutching, fixed is smaller and will indicate higher rpm’s because the tachometer gets its signal from the Generator, lastly…make sure your TCC brake switch is working correctly and your TC is locking and unlocking as it should. There is nothing bad that will happen to your engine above 3k rpm.
if you want lower rpm’s? Change your diffs to 2:56 or 2:73.
Autonomy_Lost
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What are your thoughts on downshifting while driving down a mountain road? Okay to shift into 3rd to save the brakes? Asking specifically for an ECV truck.
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any Car or truck is best served using engine braking on long grades than riding the brakes, riding the brakes ultimately overheats the brakes and warps your rotors, not uncommon for brake fires on big rigs that don’t use exhaust brakes on downhill grades.
Autonomy_Lost
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Yep thats typically how I do it in normal vehicles, just didnt know if the humvee was different. Back when I had a 6.2/3 speed I remember people warning about "overspinning" the engine.
GuntherRommel
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Will say the 40" tires worked to bring the rpm's down at higher speeds but still not as good as my 2:73 M1123
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I am pretty sure I have the smaller pulley on the generator and most likely will get the 2.73 diff installed. Do you know if the factory tach is adjustable?
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If you have the smaller pulley you should change it, there is no need to use more of what little HP you have over spinning the alternator.
Your truck was basically turned into a mobile power supply/transportation for battle ground electronics, its main purpose was to sit for days at high idle in one spot.
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Hummer Guy
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Random question, what ratio does the H1s have at the diff?
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Random answer, usually 2.73 or 2.56, or whatever the owner decided to put in there.
EDIT: Changed 3.08 to2.56 because unlike military HMMWVs, they went to a taller gear in later years for lower cruising RPMs as opposed to lower gearing for higher torque in the military ECV/REV trucks.
92-97 had 2.73 gearing
98+ had 2.56
1998 Hummer H1 Model Year Changes and Buying Guide.
www.lynchhummer.com
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