Happy 13BT Highway RPM

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Glenn-BJ74

Glenn-BJ74

10 Year 15 Year
Joined
May 7, 2006
Threads
279
Messages
3,363
Location
Kelowna, BC
What do 13BT engine owners (e.g BJ74) consider the ideal RPM range cruising in 5th gear on the highway? I am considering some tire/gearing options and I am interested in the opinions of others of what is the 'happy' RPM range around 105 kph (actual speed).

With the stock small tires LT215/70R15 tires, I calculate the engine would be spinning at about 2800 RPM at 105 kph, which would seem to use excessive fuel on the highway, but maybe they thought the truck would be generally cruising at lower speeds (say 90 kph).
 
Same. I'm running 33's and the cruiser sits @ 110Kph with the rpm a bit below 3000.
What sort of gearing do you need to drop the rpm 500 revs in 5th?
As I should be finished getting the cruiser up to A1 condition soon I can start spending money on the goodies.
Really want lockers. Gears sound interesting as I would like to go to 35" tires.
 
I'd have to say that at 105 - 110 kph you're going to want to be at around the torque peak. 2200 - 2400 RPM.

The other consideration is tire size, as 33x12.50R15s put a lot of extra drag on the engine and make it feel a little sluggish (and return comparatively poor fuel economy numbers).

For more lively performance, something like 255x85R16s will be better - but they are somewhat taller than 33x12.50R15s.

I have installed Mark's gears in my T'case and the change in low range is quite substantial. I left the high range alone, though a slightly lower gear may have given better performance off the line and on longer hills.

Keeping revs low is only part of the fuel economy equation, reducing the load on the engine is the other. In a diesel the air:fuel ratio is not stoichiometric as in a gasoline engine; generally speaking there should always be enough extra air present to burn the fuel that is delivered.

~John
 
I think the auto have a taller gear. A lot of people assume that since it is a diesel that they are good on fuel but on twenty year old vehicle that just ain't so. I think people back then wanted them because diesel was a lot cheaper then gas. At least in the last few months it is actually cheaper than gas now. Remember last year. I do not know whether it will be the best to run all year round. If I can make it to 25 years then it will get collectors plates and then I can insure two cars affordably.
 
I think my truck likes something in around 2000 - 2200 rpm for best fuel economy. 3.70 gearing and 33" tires in 5th should put you there. It'll maintain speed on the flats but not in the mountains.

I don't think age has that much effect on the fuel economy. My truck has 575000 kms and it's still return 20+mpg, not bad for a decent sized SUV.
 
I useally travel between 85-105km/h on the clock (100- 124km/h actual speed), ± 2650- 2800rpm. My Cruza feels best here, enough wooma for small hills etc. and keeps good momentum. Feul consumpt. ± 7- 7.5 km/l (14.3 - 13.3l/100km). If I calculate correctly, that is ± 21.2mpg. I say, leave it as is, or mount 33" for better rpm on highway's. When I bought it, on 31" tyres, got up to 10.5km/l ( 29+mpg) at average 100km/h (62mph). But then again the speedo would have been overreading a lot ?

January 15, 2010 update:
RPM clock GPS
2200 80kmh 92kmh
2400 90kmh 104kmh
2600 100kmh 115kmh
2800 110kmh 127kmh
33" MT, 4.111 ratio, 5th gear
Last two tanks on 2600rpm / 100kmh / 115GPS, 8.15 km/l and 8.3 km/l (12.3 & 12.0 l/100km).
This is a nice speed for me to travel on, looks good to hold speed on the dash at the speedo side aswell on the rpm side. Lots of up & downs en route ( some 850km travelled this week). Keep speed well, will drop from 100kmh to 80kmh before changing to 5th, only once.
 
Last edited:
Spalet, I've checked mine just a few hours ago, at 2,200rpm I was running 100kmh(63mph) GPS speed, I have the normal 4.111 ratio. You sure about that 3.73 ratio ? On that rpm it ran well, small inclines no problem, but climbing hills saw me quickly losing speed. I bet the 14b with turbo carries that momentum much better than the 13bt.
 
I am running my JDM BJ74 w/ 4.11's, H55F and 35" tires at:

5th Gear
2300RPM - 100km/h
2600RPM - 115km/h
2800RPM - 124km/h

4th Gear
 2500RPM - 92km/h
2600RPM - 97km/h
2800RPM - 105km/h
3000RPM - 112km/h

We all know to expect better mileage at a lower RPM but how does RPM effect EGT's. I don't yet have a pyrometer so I can't say, but under load won't the pyro temps stay down better at a higher RPM?
 
I have a pre-turbo pyro and RPMs don't seem to have that much effect on EGTs under load, rather it seems to more related to the amount of the fuel burn and engine load. If you are pouring the coals to her on a hill, downshifting at the same same throttle position does little to reduce pyro temperatures. BI think anly bigger exhaust and cooler intake air will lower EGTs, at any given throttle and engine load situation

By the way, what size rim and tires are you running?
 
Glenn-BJ74 said:
By the way, what size rim and tires are you running?
Click to expand...

I am running 15x8 rims with 2 1/2" backspacing, 1" wheel spacers and 35x12.5x15 Mickey Thompson MTZ's
 
yes, rpms on a hill does make a difference to the EGTs. of course if you are underfueled then the difference is minimal.
if you are running larger than stock tires with the fuel cranked up to where you are comfortable then pull Rogers Pass at 90-100 with 33s or 35s in 5th will run the EGTs throught he roof, if you drop to 4th the rpms go up and the exhaust gasses are expelled quicker thus lowering the reading.

as for fuel economy, what better fuel economy? slow down.
 
In most cases when down shifting going up hills say 5-4th and and the revs going up , I am also backing off the throttle too.
 
Mine sits nicely and sounds right to me between 2200 and 2400 in the highway. I am typically slower then most traffic, as I have the Marks 1:08-3:05 gears in the t-case, and run either 34"-35" tires. Max boost is set to 13-14 psi, and I've 2.5" exhaust. It's undergeared for the highway. I would have been fine with the 1:1-3:05. However, on the upside, when it goes SOA or coil on 36" tires (with the marlin H55F toybox) it should be all round just about right in any number of situations.

GPS corrected numbers with 315-75R16

2200=89 kmph
2400=98 kmph
2600=107kmph

gb
 
Last edited:
crushers said:
yes, rpms on a hill does make a difference to the EGTs. of course if you are underfueled then the difference is minimal.
if you are running larger than stock tires with the fuel cranked up to where you are comfortable then pull Rogers Pass at 90-100 with 33s or 35s in 5th will run the EGTs throught he roof, if you drop to 4th the rpms go up and the exhaust gasses are expelled quicker thus lowering the reading.

as for fuel economy, what better fuel economy? slow down.
Click to expand...

I agree with this been there and tested a lot. Low revs and the foot down upohill definately brings up the EGT very quickly.

I am getting 9.5-10k per liter or 27mpg from the highly modified LC II 13B-T with 35" sneaakers and yes I verified the speedo to the GPS. I usually cruise at 95k-100k on the highway. In fact I get better mpg than I do from the 1KZ in my other LC II and the TROOPY gets about the same as the 13B-T.

However if I bury the foot in 2WD or 4WD offroad it will burn through 23-30+l/100k very rapidly but that is a very fast trip through the jungle tracks.

We did a test last week LC II running modified 7M donk, 1KZ in a LC II and RED with its 13B-T. 165K and I just used 16liters, the 1KZ 21 liters and the 7M a massive 25.8 liters. We all travelled together in a convoy with the 7M in the middle to save fuel hahaha. On the return trip the 7M knockeed the 165k trip of in 75 minutes and used 60+ liters of fuel runnign at 150+k on 36" XT sneakers. I didn't even think of trying to keep up.

I sit on about 2200 revs and one little stomp on the pedal and im at 3000 revs rapidly and the LCII is really cooking the 21psi turbo definately lets you know it likes action.
 
Last edited:
Glenn-BJ74 said:
What do 13BT engine owners (e.g BJ74) consider the ideal RPM range cruising in 5th gear on the highway? I am considering some tire/gearing options and I am interested in the opinions of others of what is the 'happy' RPM range around 105 kph (actual speed).

With the stock small tires LT215/70R15 tires, I calculate the engine would be spinning at about 2800 RPM at 105 kph, which would seem to use excessive fuel on the highway, but maybe they thought the truck would be generally cruising at lower speeds (say 90 kph).
Click to expand...

90kph is very economical for this gearing.
 
On a recent trip I took some fuel ecomony figures. With the gps I found that my speedo is out 10 to 15kph. So when I thought I was doing 110kph I was closer to 125 - 130kph:)

So sitting between 95 - 110kp on this trip I returned fuel figures of:
21 US MPG
27 Imperial MPG
11 Liters per 100k's

RPM are @ 2400 at 100 - 105kph

Pretty standard. I was thinking of dropping the turbo boost on longer trips to save fuel. Cruising at 100kph the turbo is at full boost of 12-13psi on the flat and up hill. Would dropping the boost to 9-10 psi save much fuel?

Cheers.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom