Shift Happens: HDJ80/81 Gear Ratios & Highway RPMs - Want to hear from other Mud members

[GreenPump]

After a half-dozen 1HD-T and 1HD-FT powered 80s, I am chasing after the perfect highway RPMs to maximize fuel efficiency and drivability on roads in the American west, where the speed limit is 80 MPH.

"Just roll bigger tires!"
This subject is up to personal preference, and for what I do, 33" tires are the sweet spot. My reasons are the following:

• I can still run factory brake lines & brakes
• I have the choice between 10.5" wide or 12.5" wide tires from several prominent manufacturers
• Less modification to the factory steering & associated hardware (sway bars, linkages, casters)
• Smaller lift means minimal body roll & better on-road handling is achieved with less work
• Smaller rotating mass (I still do not fully understand how this relates to fuel economy; see questions)
• In my personal, totally subjective opinion, 33s look the best
• Still fits on factory spare tire mount locations, both underneath the vehicle and on the swing-out carrier

Now, with that out of the way, lets talk about peak torque on the 1HD-T and 1HD-FT. I believe peak torque is achieved between 1800-2000rpm on both engines. From what I have observed in my own trucks, they seem very happy cruising around 2200 RPM. As cruising RPMs creep up, the fuel economy suffers significantly. It seems that the 1HD-FT might be happier in a slightly higher RPM range, but I do not have empirical data to support this claim.

Here is one of my 1993 80s (1HD-T engine) cruising along on the highway in Idaho. This speedometer is correct and calibrated via GPS. Factory final drive ratio is 4.10 and tire size is 33". Of course, if the speed limit is 80mph, you'll drive 85... right? At 80mph, the speedometer readout is almost exactly 2700 RPM.

Some math:

	1HD-T w/ A442F Factory Spec	1HD-T w/ H151F Factory Spec	1HD-T w/ H152F Modified Gearing (Proposed)
Overdrive / 5th gear ratio	0.765	0.881	0.75
3rd gear A442F / 4th gear on the manual - gear ratio	1.00	1.00	1.00
10% T-Case gearing Yes/No	No	No	Yes
Final drive ratio	4.10	3.73	3.73
Engine RPM @65mph (4th Gear / 3rd Gear)	2868 (will shift up to O.D. at this RPM)	2609	2348
Engine RPM @80mph (5th gear / Overdrive)	2700 [OBSERVED]	2829	2167

Based on this quick math, re-gearing my transfer case with the Sumo 10% overdrive gear, running a H152F gearbox, and a final drive ratio of 3.73, my theoretical RPMs look very favorable for American highways at both 65mph and 80mph. Now is the part where I need some advice.

QUESTIONS:

• How much does tire width/tire weight (steel vs. alloy rims, etc) effect fuel economy?
• What is the "ideal" RPM for these engines in terms of fuel economy? High 20s miles/gallon seems common at low engine RPMs, while closer to 3000rpm and mixed driving commonly yields under 20 mpg, even as low as 16mpg with a heavy foot on the pedal
• How does gross vehicle weight effect fuel economy? From my experience, it doesn't seem to matter much with the diesels
• Considerations for towing? Assume lower RPM is better?
• Creating an awkward spot in 1st, 2nd or 3rd gears - Running the H1F1F first gear could combat laggy starts, as it is a shorter 1st gear
• 1HD-FTs seem to return a few extra MPGs over the 1HD-T; but, I've only owned one. It seemed to get about 4-5mpg better on the highway.

I would love to hear from the iH8mud community, and if there is anything that I have missed, please chime in. Thanks in advance!

 

[rick_d]

Icky has close to 350,000 miles on 1hdt.
33” tires. H55. Stock t case. 3.70:1 gears (note not 3.73’s). I don’t think he has seen 23 mpg since conversion in 1996! He also drives like old lady and babies his engine and egts.

You could always calc in imperial gallons and get 37 mpg (the joke canadians use w 3b mpg).

 

[GreenPump]

Icky has close to 350,000 miles on 1hdt.
33” tires. H55. Stock t case. 3.70:1 gears (note not 3.73’s). I don’t think he has seen 23 mpg since conversion in 1996!

That would make sense, the H55 5th gear is 0.845:1 - right?

Transfer case gearing I don't know how it compares. At 65mph in A442F, I've seen mid 20s in 1HD-T (RPMs under 2200).

I see comments all over mud from both camps, but I have a hunch most people seeing higher numbers have a local 65mph limit or more favorable gears.

Although, with the 0.881 and 0.845 5th ratios, autos will be cruising a much lower RPM than early manuals.

 

[rick_d]

…..i run 3.9:1 vdj79 gears w h152 and 12 valve hdt w 33’s. I drive fast.

 

[VADUS]

lets talk about peak torque on the 1HD-T and 1HD-FT. I believe peak torque is achieved between 1800-2000rpm on both engines.

It is not correct for both, but just for 1HD-T. 1HD-FT has a peak torque at 2500 rpm. So the range between 2200-2600 is the best for cruising. With 33" BFG AT tires, H151F MT and 3.7 FGR I have 80 mph = 2800 rpm. 70 mph = 2450 rpm. And IMHO it is very nice and it doesn't make any sense to run for 5th gear from H152 with this particular configuration. The reasons are: 1) the power output at lower rpms is much lower and it is a heavy job for the engine to maintain a high speed; 2) the oil pressure is lower at low rpm; 3) if you need to overtake someone, it will be much easier to do with H151 5th gear.

How much does tire width/tire weight (steel vs. alloy rims, etc) effect fuel economy?

The weight does, especially during acceleration.

What is the "ideal" RPM for these engines in terms of fuel economy?

For cars like TLC80, I would pay more attention to the speed, if you are really concerned about the fuel economy. The aerodynamic of the "brick" does NOT help at all. That is why after 65-70 mph, the wind drag force starts to grow up too fast. For your config, 70 mph is 2400 rpm, which is more, than nice. Faster you go - lower MPG you get.

 

[VADUS]

Considerations for towing? Assume lower RPM is better?

No, it is not.

How does gross vehicle weight effect fuel economy? From my experience, it doesn't seem to matter much with the diesels

However it does matter, when the load is about 1200-1500 lbs. It is important where this load is located at. If you load your roof rack much, then you increase your wind resistance and wind drag force.

 

[GreenPump]

Thank you @VADUS for the great info and detailed feedback.

It seems I have conflated air resistance with engine rpm; from my perspective, I attributed all of the fuel efficiency loses at high speed to RPM. I have no control dataset driving slower at high RPM... so, you can see why I made such an assumption.

Of course, as we all know, you aren't driving an 80 series because it's good on fuel. They are indeed shaped like a brick! I am simply concerned with pinning down what is going on between 5-10mpg difference between rigs, and most importantly, what does the most efficient gearing in my specific application.

In theory, with an 1HD-FT, if I used H152 5th gear, leave transfer case untouched, and run 3.73 final drive, my RPM at 80mph will be similar to what you achieve at 70mph (2400-2500rpm). Passing I have the option of 4th gear, as well as cruising in 4th at 65mph running 2500-2600rpm.

Alternatively, I could leave the transmission unmodified and run the taller transfer case gear (10%), which will be about 100 or so RPM higher.

My preference is to modify the H151 5th gear, because I prefer a larger gap between the 1.0:1 4th gear and the 5th gear ratio.

My understanding is that 1HDxx engines do not require a particularly high oil pressure during operation. Can engine wear be accelerated in a particular RPM band due to oil pressure?

 

[VADUS]

Can engine wear be accelerated in a particular RPM band due to oil pressure?

Not only the oil pressure, but two factors simultaneously: low rpms + lower oil pressure. At high load (speed or/and towing), these factors increase crank bearings stress, especially together with reduced hot oil viscosity. You need to keep in mind, that H152 was made for much more powerful V8 engine with common rail, which does a power stroke every 90 degrees of crank turn. Our 1HD does it every 120 degrees - feel the difference. Also, H152 goes with 3.9 FGR, as far as I remember. That is why I do not recommend to run after H152 5th gear, while using 3.7 FGR and 1HD engine with 33" tires.

 

[TheBussman]

Just put some overdrive gears in my transfercase, trying to get down to 2500 RPM at 70mph (about stock) Feels like it stops making any more power at 3k. I'm on 35s with 4.88s

 

[The Machinist]

Slightly different setup in my truck, but I'm pretty happy with where it is right now. 1HZ-T, H55, 10% underdrive in the tcase, 3.73 ring and pinion, 35's. At 75mph its dead on at 2500rpm which seems to be perfect for cruising. When I had 33's it was slightly more fun to drive around town but it would start to run a little too hot after a while on the interstate if I tried to stay at 75-85.

 

[gilmorneau]

Another different setup but maybe useful for reference. Mine is a 1997 HDJ80, Euro-spec, 1HD-FT, H151, K374 axles (3.583), 235/85-R16 tires (31.7" diameter). (By my math, using 33's and 3.7 FGR would be more or less equivalent to what I have, for engine rpm at least.)

Cruising at 75mph I can usually get 25-26 mpg. Maybe 2500 +/- rpm's at that speed. That's Colorado driving with a couple of mountain passes thrown in. I don't usually cruise at 80 (I don't see that speed limit until I get to Utah), although the truck can do it all day long if needed.

The 235's naturally have smaller frontal area and arguably lower rolling resistance than larger tires, so that might help a little. Also, my 80 is about as light as an 80 can be: no roof rack, no aftermarket bumpers, no jerry cans or hi-lift jack, no flares or anything. Completely unmodified from stock other than the 4Runner steel wheels and 235's. Probably about as aerodynamic as an 80 gets, too, although that's not setting the bar very high.

I'd guess that weight is more of a factor for mileage if the terrain is hilly. Less so on flat ground. Speed and wind resistance are the big economy killers generally. It takes a lot more work to push all that air out of the way at higher speeds.

 

[GreenPump]

Mine is a 1997 HDJ80, Euro-spec, 1HD-FT, H151, K374 axles

Wow, I didn't even realize this was a factory ratio for 80s! How are 1st/2nd around town?

If I can find a set of gears, this is a GREAT option! How is RPMs at 65mph in 4th?

Great info from those who have commented so far.

 

[gilmorneau]

How are 1st/2nd around town?

Not sure what you're asking. They seem "normal" to me. 1st is really only for starting from a complete stop. If I'm rolling even only a little, I'll use 2nd and it's fine.

How is RPMs at 65mph in 4th?

Normally I'd be in 5th by that time, but today on the way home from work I took it to 65mph in 4th. About 2600 rpm.

 

[SNLC]

Another different setup but maybe useful for reference. Mine is a 1997 HDJ80, Euro-spec, 1HD-FT, H151, K374 axles (3.583), 235/85-R16 tires (31.7" diameter). (By my math, using 33's and 3.7 FGR would be more or less equivalent to what I have, for engine rpm at least.)

Cruising at 75mph I can usually get 25-26 mpg. Maybe 2500 +/- rpm's at that speed. That's Colorado driving with a couple of mountain passes thrown in. I don't usually cruise at 80 (I don't see that speed limit until I get to Utah), although the truck can do it all day long if needed.

The 235's naturally have smaller frontal area and arguably lower rolling resistance than larger tires, so that might help a little. Also, my 80 is about as light as an 80 can be: no roof rack, no aftermarket bumpers, no jerry cans or hi-lift jack, no flares or anything. Completely unmodified from stock other than the 4Runner steel wheels and 235's. Probably about as aerodynamic as an 80 gets, too, although that's not setting the bar very high.

I'd guess that weight is more of a factor for mileage if the terrain is hilly. Less so on flat ground. Speed and wind resistance are the big economy killers generally. It takes a lot more work to push all that air out of the way at higher speeds.

The “smaller” tires help more than a little.



Cheers

 

[IanB]

With respect to "How much does tire width/tire weight (steel vs. alloy rims, etc) effect fuel economy?", rotating mass counts for far more than static mass. Depending on where you look, it seems somewhere around 8X the effect. So adding 10lbs to your wheel/tires setup has the effect of adding 80 lbs to the truck, in the worst spot possible, the very thing you are accelerating/decelerating while driving.

 

[TheBussman]

Just got back from a trip NC to Ohio though wv mountains 72ish average speed right at 2500 RPM. 35s, 4.88 gears, 8.8%OD transfercase gears, ECT in "normal" mode 17.5MPG I was hoping for a little better, I still have some tuning to do. Plenty of power never a need to downshift for any hill, gearing feels good. Next set of tires may end up being 35x11.50x17. Should cut down on the weight and width a little but the current set is at about full tread. I've made trips on slower roads in the past and was able to get into the 19s

 

[GreenPump]

I became recently aware that PDI is selling swap kits for the 6R80 transmission, which is widely available in the U.S.

This appears to be a very strong and favorable option, with a 0.69 top gear ratio, and lots of good feedback from 1HDFTE owners.

Their market appears to be mainly 100 and 105 owners. Does anyone have more info on performing this install on a diesel 80?

 

[duece71]

1-HDT, OEM turbo/pump/injectors, A442F, tires are Falken 285/75/16 WPAT4W, afaik, stock gearing. 75 mph on the interstate, level grade, 2600 rpm’s, 9-9.5 psi (10 psi max, no tuning) EGT at 750-800F (non intercooled). Mpg, depending on my right foot….~16 give or take. Elevation is ~900 ASL. Intercooler needs to happen, flooring the pedal for on ramp/merging/up a grade quickly reaches 1200/1300F!!!