How far are you getting on a full tank?


freedom_in_4low

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Hey Jerry,

This is interesting. Can you say more about when higher RPMs are actually better for your MPG? I'm asking because I've heard it said that the 2.5L isn't actually more efficient than the 4.0, because you have to ride it in higher RPMs in order to keep up with traffic, or when going up hills, etc.

You're looking for brake specific horsepower, which is a measurement of how much power you get per unit of fuel. It peaks at a certain rpm.

Or in other words, 75% throttle at 2000rpm burns more gas than making the same power as you could with 30% throttle at 3000rpm.
 
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Mark K

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I like the OP's poetic statement about how it feels to drive a TJ in the Texas Hill Country. My 2000 4.0 3550 3.73 with BFG 31 KO2s (proper speedo gear) has the GenRight long range fuel tank, which they rate at 24.5 gallons. I usually fill at about 200 miles, as I did with the OEM tank, but I can go 350 miles on hill country roads (25 to 65 mph, no lights, a few stops signs) and another 35 when carrying the 2 gallon RotoPak on the Morryde tailgate hinge. At 350 such miles, there's about 4 gallons left in the tank.
 
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CharlesHS

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On a recent trip from Punta Gorda to Jacksonville driving 70-75 mph with A/C most of the trip; I filled the tank with 14 gallons and the odometer said I drove 283 miles.
Even though there was fuel remaining; I wasn't going to see how much before the fuel pump lost suction.

2001 Jeep 4.0L 5 spd 3.73 with 2" lift and 255/75x17 ATs.
 

sab

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You're looking for brake specific horsepower, which is a measurement of how much power you get per unit of fuel. It peaks at a certain rpm.

Or in other words, 75% throttle at 2000rpm burns more gas than making the same power as you could with 30% throttle at 3000rpm.

To be technically correct, it's actually Brake Specific Fuel Consumption (BSFC), which is the mass flow rate of fuel used divided by the brake horsepower (brake horsepower is the amount measured by the dynamometer). The BSFC is different at each operating point of the engine (meaning for each throttle position and engine speed). By mapping the BSFC for various operating points running the engine on a dynamometer, the most efficient operating point can be determined (where the BSFC is lowest). There's no easy way to determine that point other than creating the BSFC map, and creating that map is pretty time-consuming.
 
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srimes

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You're looking for brake specific horsepower, which is a measurement of how much power you get per unit of fuel. It peaks at a certain rpm.

Or in other words, 75% throttle at 2000rpm burns more gas than making the same power as you could with 30% throttle at 3000rpm.

The main issues at lower rpms is the efi going into open loop. If you get an obd2 gauge you can watch when this happens.
 

sab

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On a recent trip from Punta Gorda to Jacksonville driving 70-75 mph with A/C most of the trip; I filled the tank with 14 gallons and the odometer said I drove 283 miles.
Even though there was fuel remaining; I wasn't going to see how much before the fuel pump lost suction.

2001 Jeep 4.0L 5 spd 3.73 with 2" lift and 255/75x17 ATs.

Wow, that's over 20MPG. I'm jealous...
 

JKP

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On a recent trip from Punta Gorda to Jacksonville driving 70-75 mph with A/C most of the trip; I filled the tank with 14 gallons and the odometer said I drove 283 miles.
Even though there was fuel remaining; I wasn't going to see how much before the fuel pump lost suction.

2001 Jeep 4.0L 5 spd 3.73 with 2" lift and 255/75x17 ATs.
About 4.9 gal before that would have happened assuming you were flat and level and not cornering hard.
 

Sancho

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Hey Jerry,

This is interesting. Can you say more about when higher RPMs are actually better for your MPG? I'm asking because I've heard it said that the 2.5L isn't actually more efficient than the 4.0, because you have to ride it in higher RPMs in order to keep up with traffic, or when going up hills, etc.

Lugging your engine kills mpg.

There is an optimal/efficient rpm for the 4.0 consider the variables.
 

SkylinesSuck

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I track my mileage using an app. I get 11.2 mpg driving almost exclusively the rural back roads too and from work (and I flog the piss out of it pretty much the entire time). If I do significant highway driving during a tank, it goes up to 15-16mpg. 4.7L stroker on 33's.

I consider miles per tank super inaccurate for a bunch of reasons, but I generally go get gas when the light comes on and get like 160-180 normal driving miles per tank. Highway road trips I remember 230+.
 
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jimstj

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I get about 230 with the 19 gal tank that holds 15 gal🤦‍♂️

4:56 pushing 33s
2.5 inch lift
I usually calc with every tank, but my speedo is accurate with the right gear.
 

freedom_in_4low

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I track my mileage using an app. I get 11.2 mpg driving almost exclusively the rural back roads too and from work (and I flog the piss out of it pretty much the entire time). If I do significant highway driving during a tank, it goes up to 15-18mpg. 4.7L stroker on 33's.

I consider miles per tank super inaccurate for a bunch of reasons, but I generally go get gas when the light comes on and get like 160-180 normal driving miles per tank. Highway road trips I remember 230+.
Weird. Mine is the opposite. I get 11ish on 75-80mph interstate runs but I can get 13-14 if I keep to the 65mph US and state highways.
 

JDM

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2002 TJ automatic with 4” lift and 33x12.5 AT tires . I get 16-16.5 on highway trips running 70mph . Not sure how many miles I can push it since I haven’t gotten below a 1/4 tank yet . The math says I can go about 300 .
 

SkylinesSuck

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Weird. Mine is the opposite. I get 11ish on 75-80mph interstate runs but I can get 13-14 if I keep to the 65mph US and state highways.
I guess I should have specified. My "back roads" drive to work is 5 miles each way of twisty country roads with about 7-8 stops. When I was thinking highway I meant 65-70mph so fairly in line with you, although I can hold 85 effortlessly if I wanted to. Just feels a little squirrelly.

I also just reread my post and meant to say 15-16, not 15-18mpg on the highway if that matters. It's just a guesstimate but I'll make a mental note to track the highway specific mileage next trip.
 

LarryTJ

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1997 - 18mpg, 33",3.5"lift, Manual, 4.56 gears.
1997 - 15mpg (went to 35"), 35",3.5"lift, Manual, 4.56 gears.
Other TJ:
2004 Rubicon: 13mpg,33",4"lift, Auto, 4.10 gears (stock).
 

Jerry Bransford

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Huh.... Jerry if you think that way, why the Hell did you buy a TJ with .69:1 O.D. ratio ? :ROFLMAO:
:ROFLMAO:
First, my previous 97 TJ I had installed an automatic with no overdrive into was stolen. The local used TJ I bought had the required automatic but it was an 04 so it had the .69 overdrive. I compensated for its overdrive with 5.38 gears which did that nicely. That's what boosted its mpg by nearly 3 mpg.
 

freedom_in_4low

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I guess I should have specified. My "back roads" drive to work is 5 miles each way of twisty country roads with about 7-8 stops. When I was thinking highway I meant 65-70mph so fairly in line with you, although I can hold 85 effortlessly if I wanted to. Just feels a little squirrelly.

I also just reread my post and meant to say 15-16, not 15-18mpg on the highway if that matters. It's just a guesstimate but I'll make a mental note to track the highway specific mileage next trip.

ah, ok. My engine doesn't seem to be working hard to do 85 either, and the Jeep itself feels pretty planted at that speed, but knowing how quickly I'm burning through the fuel, as well as wind noise, encourages an upper limit of 70 or so.