28.5 MPG in a 93 Jeep Cherokee!

[eco-jeeper]

Hey y'all,

First off I want to say thanks to you folks, I've been gleaning ideas from this website since last summer and you are an inspiration to me! This is my first post though, I created account just to share my success story with you!

On a 68 mile interstate drive last night I got a calculated 28.5 MPG using the fill up, drive, fill back up, divide miles driven by gallons used. For reference I got 21.8 MPG on my last full tank fill up after just driving around town ~340 miles.

This was on my 93 Jeep Cherokee. Last week I purchased this Jeep from someone who had done several mods on it, but they were mostly intended for offroad use, though several have helped fuel economy. Some engine components were damaged from mud and replaced.

How was 28.5 MPG possible?

- The engine is a four cylinder 2.5L
- The top half of the engine was rebuilt
- Two new belt pulleys, rebuilt alternator, new water pump
- Installed aftermarket electric fan (from a semi truck haha)
- The engine supposedly had a "full tune-up"
- Manual transmission
- High flow aftermarket exhaust with shortened exhaust pipe
- Tires were oversized from stock 215mm (27.7 inches tall) to 31 inches
- Gears were not reset, so the Jeep is traveling faster for the same engine speed (speedo reads 60, actually moving 65, from calculations and empirical tests)
- Tires were inflated to 44 PSI (max sidewall 50 PSI)
- I never exceed 65 MPH (well for the second half lol)
- I used a vacuum gauge
- If I wasn't going uphill I never opened up the throttle to less than 10 PSI
- If I was climbing I allowed 2 PSI less vacuum per MPH I dropped below 65 MPH
- In other words, I stayed at 10 PSI at 65, 8 at 64, 6 at 63, etc..
- If I got down to 60 MPH I opened the throttle fully (0 PSI). I did this because it takes a LOT more time/fuel/concentration to get back up to 65 MPH while staying above 10 PSI vacuum

It seems that in my particular setup, 65 MPH is the sweet spot. My Jeep vibrates heavily at 70, and at 60 on hills I have to open the throttle fully just to keep an even speed.


So what do y'all think?? Installing a tach today!

[oldtamiyaphile]

Originally Posted by eco-jeeper

- Tires were oversized from stock 215mm (27.7 inches tall) to 31 inches
- Gears were not reset, so the Jeep is traveling faster for the same engine speed (speedo reads 60, actually moving 65, from calculations and empirical tests)

Have you checked it against GPS? I have basically the same setup and the bigger tyres have just corrected the usual speedo under-reading, now reads 101 at a GPS 100.

Note too that the odometer is calibrated differently from the speedo. From the factory a speedo typically reads +5% while the odo reads accurately usually within +/-1%.

[eco-jeeper]

I don't believe in using GPS. I'm like a non-violent Neoluddite - I don't even have a smart phone.

31 in. tires / 27.7 in. tires = 1.12 (times faster than speedo)
60 MPH * 1.12 = 67.2 MPH

I drove a mile on the interstate at exactly 55 MPH speedo (55*1.12 = 61.6 MPH), it took me 59 seconds between mile marker signs.

61.6 MPH = 1.027 miles / minute

59 seconds / 60 = 0.983 minutes

1.027 miles / minute * 0.983 minutes = 1.0095 miles

Looks like my variance is less than one percent or so. I realize there will be some discrepancy but that is inevitable. -1% MPG would still be higher than 28 MPG.

[elhigh]

I wonder if there's an aftermarket gear to put in your transmission for the speedo.

I just dated myself, didn't I? They haven't used speedo gears in decades, have they?

Beating the 20s in a Cherokee is damned good no matter how you measure it. I'm a big fan of the AMC 4, a big fan of big fours in general and that one in particular even though there isn't a lot of mod support in the aftermarket.

Start a page for your ride. I'm curious to see how it goes along.

[eco-jeeper]

Originally Posted by elhigh

I wonder if there's an aftermarket gear to put in your transmission for the speedo.

I just dated myself, didn't I? They haven't used speedo gears in decades, have they?

Beating the 20s in a Cherokee is damned good no matter how you measure it. I'm a big fan of the AMC 4, a big fan of big fours in general and that one in particular even though there isn't a lot of mod support in the aftermarket.

Start a page for your ride. I'm curious to see how it goes along.

Thanks elhigh!

Never heard of a speedo gear haha but a quick search just found one for the Jeep. I'd rather put that money elsewhere in the Jeep tho, just drive a little slower haha.

Yeah I was amazed at the mileage. I think it's only possible from the combination of all the mods it does have, and I have to point out this was a one time 70 mile interstate-only drive from 300 ft elevation to 100 ft, though there were lots of hills. It's kind of a pain in the ass to have the original gearing ratio though, as hills are really hard to climb. I would never want to tow anything with it for now, too little torque which would put undue strain on the transmission. I'll bump up the ratio when I get a limited slip rear differential.

Thanks for the support, how do I start a page?

[elhigh]

Scroll up to the top of this page and while signed in, see where it says "Garage & Tools." Click "Garage."

Look for the "Add a new vehicle" option, click that and fill in your info. Then at every fillup, update the Garage page with how many miles and how many gallons. The Garage automatically generates a graph so you can easily see how this fill did compared to other fills.

You can't add a flag to the graph to call out when mods were added (Ultarc in particular is lobbying for this) but it's easy enough to note it in the comment slot for the fill.

[oldtamiyaphile]

Originally Posted by eco-jeeper

I don't believe in using GPS. I'm like a non-violent Neoluddite - I don't even have a smart phone.

31 in. tires / 27.7 in. tires = 1.12 (times faster than speedo)
60 MPH * 1.12 = 67.2 MPH

I drove a mile on the interstate at exactly 55 MPH speedo (55*1.12 = 61.6 MPH), it took me 59 seconds between mile marker signs.

Looks like my variance is less than one percent or so. I realize there will be some discrepancy but that is inevitable. -1% MPG would still be higher than 28 MPG.

You assume that the speedo and odo are calibrated the same, they're not on any vehicle I own.

Your odometer was accurate from the factory to about 1%, it probably now under reads by ~10%.

When you put 31's on a standard Jeep XJ/TJ, you end up with an accurate speedo (about 1% over reading), but the odo now under reads by around 10%.

[eco-jeeper]

Thanks elhgih, will do!

Originally Posted by oldtamiyaphile

You assume that the speedo and odo are calibrated the same, they're not on any vehicle I own.

Your odometer was accurate from the factory to about 1%, it probably now under reads by ~10%.

When you put 31's on a standard Jeep XJ/TJ, you end up with an accurate speedo (about 1% over reading), but the odo now under reads by around 10%.

Like I have shown, my theoretical speed (speedo x 1.12) was equal to my ACTUAL speed in a one-mile time test, to within 1%. Your speedo might have been 5% over, mine is apparently less.

As far as the odometer output I'll have to test that separately over a longer stretch of interstate, but for now I simply calculated my MPG as miles * 1.12 / gallons. No calculation like this is going to be perfect, but the variance between the speedometer and odometer is likely negligible, if the odometer was indeed factory accurate within 1%, and the speedo*1.12 at this time is accurate within 1%.