While I do agree that the best BSFC ratio for a 12v does occur at 1600 rpm, I think it must be taken with a grain of salt. What the BSFC # means is the ratio of hp made as compared to fuel consumed but, this # is calculated @ WOT. Since we don't cruise @ WOT, just how important is the BSFC ratio? Since fueling levels and boost are much lower during cruise at 1600 rpm as compared to WOT at 1600 rpm, is the BSFC ratio relevant? Not to nit pick - I do agree with what you are saying......I'm just not as sure as used to be that BSFC is important as I thought when it comes to the ideal cruising rpm......Your thoughts?

 

i go anywhere between 1700 and 1800rpms on the highway that only gets me somewhere around 60mph or 100-105km/h...so i park myself in the right hand lane and take it easy....id even go a lil slower if the highway is empty outside the city.....but i agree with Bill now that i have the 3kgsks i tried cruising on the highway at like 2200rpms and i felt it was sucking fuel

 

I got 3.54 gears and want to run a 2in leveling kit and some 315 BFG AT. Would my fuel milage improve on the interstate. 90% of miles are on interstate. Would this be wise to do?

 

You will lose mileage with 315's and 3.54's. With the higher spinning weight and much higher rolling resistance of the larger 315 tire, it will require more torque to cruise down the highway than with the stock tire - you will consume more fuel even if you did regear for the 315's to put the engine back into it's best cruising rpm. Just how much more fuel you would consume to cruise with 315's will not be clear but you can be certain that they will NOT help improve mileage.

 

for a 96 you don't have to change the speedo gear at the transfer case because it is electric single.superlift makes a TruSpeed Speed Signal Recalibrator. i get better mpg with larger tires because of the 4.10.now when i cruse at 70 my rpm are around 1850.

 

Speedometer error is NOT constant. So you can't just subtract X amount of mph.

Speedo error is a percentage. So if it's off by 10%, then it's 7mph off @ 70, but only 3mph off @ 30.

 

Boost is NOT a measure of fueling. It's a measure of engine load, which is related to fuel being used in a way. If boost was a measure of fueling, then I'd be able to hit 35psi in every gear. But I can only hit 35psi in 6th gear. I can barely hit 28psi in 4th, and 30psi in 5th.

In your above example, there's no way to know which is actually using more fuel, because we don't know engine RPM.

10psi of boost at 2500rpm is a LOT more fuel being burned than that same 10psi of boost at 1700rpm. Because at higher rpm, the engine is consuming more air. Therefore, the turbo compressor must spin a lot faster to generate the SAME BOOST PRESSURE.


It's quite possible that in your example, the 10psi @55mph is actually LESS fuel than the 2psi at 80mph is. In my truck 55mph is about 1550 rpm, while 80mph is about 2200 rpm. So 80mph is 42% higher RPM than 55mph. Yet the air pressure into the engine (in PSIA) is only 32.4% lower.

So rpm is running up faster than boost is falling, I'd say we could reasonably conclude that you are actually using LESS fuel at 10psi boost at 55mph than you are at 2psi of boost at 80mph-- because 2psi of boost at 2200 rpm is MORE air than 10psi is at 1550rpm.

This example is based on my truck (265 tires, 3.54 axles, 6-sp trans) and assumes sea level air pressure.

Different combos would produce different calculations, but probably the same conclusion.
Because the turbo is spinning faster, it takes more shaft hp to drive it. This extra shaft (or turbine) hp comes from FUEL being burned.

 

Yeah, I understand what you mean. I guess what I should've said was if you're going the same RPM in the same gear, then you're MPG would vary with boost level. For example, if you push the pedal harder to get up a hill but keep the RPM's constant.


Anyway, I was also wondering... Just curious as to what kind of MPG's are you guys with the lifted trucks and 36-40" wheels seeing?