2.5L Petrol daily driver mpg - Defender Source
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  #1  
Old January 21st, 2016, 06:10 PM
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David
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2.5L Petrol daily driver mpg

Ciao everybody. Did a quick search and couldn't find anything so hopefully someone can help. I've got a 1989 D110 3-door ex-mil that I brought back from Italy. Basically I daily drive this bad boy to work about 55 miles round trip, all highway. I've been doing some fuel tracking lately and it seems that I'm getting around 8.5-9.5 mpg. I know that these can be "thirsty" engines, especially in a 110, but it just seems too low. I'm looking to figure out a few ways to raise the mpg a little to save some cash that can then be repurposed into new fun things for the truck!

Right now it's siting on Italian military steel wheels at 16x7 with 205r16s, I think that "actual" tire size is around 205/75r16. Trying to use maths, it seems like larger tires would increase the highway mpg, right? My reasoning behind this is basically that the increased circumference of the tire would cause a lower engine RPM to move the vehicle the same speed. Help me out with that.

This leads me to my next and main question, engine RPMs. I usually travel 55-65 mph on the highway to work, and I don't have an issue getting up to or maintaining that speed. However, with no tach it seems like (by ear) the engine/transmission rpms are VERY high at that speed. Seems like I'm redlining after 45-50mph. Doing any sort of engine work wouldn't reduce that without changing the gearing of the drivetrain right? Even if I drop in a burly v8 or a hybrid motor from a prius, the gas mileage could change, maybe the engine rpms, but the transmission rpms would still be redlining, right?

Trying to apply logic but that doesn't always work with Defenders lol
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  #2  
Old January 21st, 2016, 06:43 PM
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That's about what my 109 with a 2.25 petrol gets. No experience with a 2.5 in a 110.
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Old January 21st, 2016, 06:48 PM
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I've been searching around and most everything that I've read, people complain about their 2.5L petrols "ONLY" getting 15-19, some 12-15. But nothing as low as what I'm getting. I've yet to city drive it, and keep the RPMs a little lower, but I'd be interested to see what I would get out of a full tank without going 55mph+
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  #4  
Old January 21st, 2016, 06:56 PM
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whats trans do you have? does it have an overdrive?
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  #5  
Old January 21st, 2016, 07:12 PM
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I had a 110 2.5 petrol pickup a couple years ago and that got high teens around VT. Hell even my 109 Ambulance still gets 10mpg
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  #6  
Old January 21st, 2016, 07:30 PM
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Typical...

With the small tires and the 1.6 T-case and the engine revving high, you'll get terrible mileage with a 2.5.
Pierce manifolds in CA has a replacement carb that may help some.
Best to install a different engine.

I get low 30s with the Isuzu 4JB1-T Trooper 2.8 Turbo Diesel.
Also got in the low 20s with a 200 TDI and our 300 TDI.
The OM617 also got about 25 MPG with diesel, a few miles less per gallon with veg oil.
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  #7  
Old January 21st, 2016, 08:16 PM
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Quote:
Originally Posted by phoenix37 View Post
whats trans do you have? does it have an overdrive?
LT77 I think, whatever comes stock on the 2.5 petrol. No overdrive, and from shopping around they seem to go for $2k new, haven't found a used one yet
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  #8  
Old January 21st, 2016, 11:25 PM
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I can't tell you what your problem is but I got 13mpg at 60-70mph driving in a 2.25 petrol 90 and 15+mpg in a 110 pickup with 2.5 petrol that I thought was in rough shape.
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  #9  
Old January 22nd, 2016, 05:02 AM
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Had the same discussion with the new owner of one of the cars I sold. Very high RPM and high fuel consumption. After some emailing back and forth, we found out he was driving it in low range all the time.....
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Old January 22nd, 2016, 05:31 AM
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Quickest path to improve cruising comfort and lower engine revs would be to replace the tires with 235/85-16.
I wouldn't be surprised to learn that you have a 1.6 transfer case either.
But do the tires first, plan on a tune up unless you know everything is good.
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  #11  
Old January 22nd, 2016, 09:38 AM
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Yeah, try that. If you have a tach, what kind of RPMs are you turning?

For benchmark, my 109 at highway cruising speeds, is turning around 2700~3000 RPM (rough estimate). I'm getting about 15mpg. I'm running a worked over 2.25 gasser, series transmission and transfer case, Roamerdrive, series diffs (4.7), and 235/85/16 tires. I try to cruise between 60 and 65 mph.

Also what carb do you have? A quick research indicates you've got a Weber 32/34. What kind of condition is it in? When was the last time you had a tune up?

There is a device called a "Color Tune". Helps with jetting the carb. This is a great tool.

If you are wanting to play around with some calculations, below is a link to a handy calculator:

Ashcroft Transmissions - Ratio Calculator
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  #12  
Old January 22nd, 2016, 09:56 AM
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A 110 with a 2.5 petrol would have come stock with 1.66 transfer case gearing.

Your 205r16 is a 29 inch diam tire when new

Believe stock size was 7.50 r 16 or a 265 75 16 both which are a 31.5 diam tire. 235/85 is also 31.5

Your tires are 2.15 inches shorter than stock which significantly changes gearing for the worse if your use is highway speeds.

Congratulations on having a 2.5 petrol in California. Most (easily discouraged/inside the box thinkers) would tell you that is impossible.
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  #13  
Old January 22nd, 2016, 10:03 AM
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I would switch to a 1.4 t. case. Obviously make sure it is tuned well. Air filter, timing.
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  #14  
Old January 22nd, 2016, 10:21 AM
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I have worked on 6 or 7 110s with the 2.5 petrol.
All three of the ones we owned arrived with a 4 cylinder petrol, 2 were 2.5s and one was a 2.25.

In every case the ignition timing was way advanced to try and get a little more power.
All ran much better after the timing was set correctly and as John suggests via the above list, you need to check everything else if you haven't already.
In some cases the POs had the ignition timing so advanced that the engine was suffering from cylinder ping caused by pre-detonation which will cause broken rings, melted pistons and generally wreck the engine.
A result of timing too advanced is poor fuel mileage.
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  #15  
Old January 22nd, 2016, 12:34 PM
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Ok, so it seems that swapping to larger tires would be a very good option, for performance and looks as well. The pizza cutters that I have now look like go-cart tires on my 110 lol. I'm currently saving up for a set of 5 cooper discoverers in 235/85r16. I ran sizes through a calculator (photo attached) and there's a pretty big difference in circumference and rotations per mile, which should reduce the engine RPMs at cruising by about 13%.

As for the transmission, I'd be willing to bet that it has 1.6 gearing in the t-box considering that it's ex-mil and Italian. Anything that I can do about that? A total t-box swap should be pretty pricey. Maybe re-gear the internals?
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  #16  
Old January 22nd, 2016, 12:38 PM
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It will be a 1.6. That is the factory stock for 110s with four cylinder non-turbo engines. Have a look at the serial number and post up to confirm. Swapping the box is easy and a used one should be quite cheap. You can also change the gears as well.
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  #17  
Old January 22nd, 2016, 12:44 PM
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Quote:
Originally Posted by Red90 View Post
It will be a 1.6. That is the factory stock for 110s with four cylinder non-turbo engines. Have a look at the serial number and post up to confirm. Swapping the box is easy and a used one should be quite cheap. You can also change the gears as well.
The VIN or the serial on the t-box? I ran a vin decoder and it just said "manual transmission".
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Old January 22nd, 2016, 12:46 PM
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Serial number on the transfer case.
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  #19  
Old January 22nd, 2016, 01:17 PM
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Quote:
Originally Posted by Red90 View Post
Serial number on the transfer case.
I'll have to look when I get home. Where is it located?
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  #20  
Old January 22nd, 2016, 01:41 PM
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