Fuel Tank Options

I've been looking around for some options to add a bit more fuel tank capacity to my 1989 90 (fuel tank under the drivers seat) since I only get maybe 110 miles out of a tankful (Because V8).
There's doesn't seem to me much choice that I can find - does anyone know of any good options?

Boy,that is low less than 12mpg. Also fuel is heavy and more weight results in lower mpg [ something you have to manage if you are a LRDG [ long range desert group] but most of UK has fuel stations well less than 110mls between. What model V8 does your truck have?

Yeah, it's not great. I will also do some work on the fuel & ignition map (I have it on a megasquirt), but to be fair, it's a V8. If I wanted MPG I'd have a diesel.
It's a 3.9 motor from a 97' discovery (what's called the "intermediate" motor), with the rimmer bros sports exhaust (tubular headers) and the classic plenum & trumpets intake on a megasquirt setup.
Would still like a bit more fuel capacity though.

The usual extra fuel is to fit a tank on the passenger side as done by the military but they were usually under seat filling at least on series models. A tap switched tanks and on some switched the gauge over as well. Another option is to have a second tank and fit a tidy sized cross feed pipe so second tank fills and empty's from the maim.
A mate had a V8 with the Megasquirt which caused us much headache. Symptoms were after running for a while and given some work it would start to over fuel. I mean a lot, it would be stinking of petrol at the exhaust and running badly. Cool down and would be fine again. Engine its self was not overheating. If I remember right we found an issue with the pressure in the main fuel rail [ after having swapped out every sensor on the engine].
Cutting a long story short in the end we found out the megasquirt has a system in it that protects the engine from overheating by sending in extra fuel into the motor [ fuel is cold ] But if temp sender is duff or the wrong one fitted the system messes up and delivers extra juice even at normal running temperatures. Hope this helps. Cheers.

Are you sure? I mean, I built (soldered the components on the boards) & programmed this megasquirt, along with a 2nd one on my boat, along with helped another friend with the specs and guidance on his one. All work really well. And if you don't want it to do something, you just change it. The issue on the LR is it was my first one and I got bored of frigging around with the map - I planned to do the ignition as well before further tweaking it.
The car's been off the road for a while now, I'll pop the laptop on it once it's running again and clean up the map a little. Perhaps even lock the distributor (or remove it) and moving the ignition map into the ECU.
Still would like a little more fuel capacity though.

In fact, here you go. Here's the page where you can control fuelling as a function of temperature. This is a fairly typical curve - actually quite conservative on the warm up side of things. But enough. You could, if you wanted, dump extra fuel. Quite why you would on a normal road engine though, I dunno. Maybe on something running methanol you'd do that.

If you wanted to give the engine temperature based protection, in the 2nd pic you can see there is an option for temperature related rev limiting though. I don't have it turned on the LR, although I do have it on on my boat (3rd pic) - if the engine overheats it has a fairly agressive 1500rpm rev limit with no override.

The one my mate had was an ex Comp Safari truck which is probably why it was set with the fuel dump. As he was just doing trials with it the fuel dump was a right pain. We eventually sorted it with a reprogram with the help of the original owner

Honestly still can’t think of a cooling need for over fuelling. Certainly you do want to over fuel slightly at 100%, but you’re not cooling the engine, you’re cooling the ignition mixture, plus, just by the way it works, for max power you very slightly over fuel too. And that’s not engine temp related, it’s the fuel map.

Anyway, if anyone here runs an MS and is stuck, I have some knowledge. This is one of the others I did. I built the whole engine, along with the ECU. It came out OK

110 miles on a tankfull is mad! Are you in low box!?

Yeah, I’ll hook up the laptop and fine tune it a bit once it’s all up and running again, I’ve learnt a lot since I wrote this map. But even so, the 3.9 V8 is always gonna be juicy.

they arent the most fuel efficient, but a decent cam will help, and decent compression. the original/cheap cams are like cheese and you soon lose power meaning a heavier foot is needed

Yeah. The motor’s in lovely nick, super clean under the rocker covers unlike the old days. But I can just very slightly hear the stock cam in it has one lobe a little noisier.
Am deciding whether to rebuild it as a 4.6 with some nice heads or just to do the cam in it “as is”. But unlikely to do either immediately. It’s such a shame nobody ever made roller lifters for it, at least as far as I can tell - that would allow for some much nicer cam profiles.

Actually, I lie. But not off the shelf and not cheap. https://thewedgeshopstore.com/roller-cam-setup-rover-v8/

Design and Development Engineering makes an extended range underseat tank but it's intended for a Series not a 90/110.
I think fitting another one on the other side with a corss-feed would be a good idea.

Regarding fuel economy, you should be getting more than 200 miles on a 55 litre tank fitted to your 90 - my twin carb 3.5 V8 110 County Station Wagon has done an average of 20.78mpg for the last 1,300 miles, so that's where I'd be looking first!

Yeah, I'll have to check my number there - car has been off the road for a while, but I was sure it gets just a sniff under 20mpg. So maybe I'm mis-remember how many miles I get to a tank.
Anyway, I'd still like a bit more range.

Me thinks with the power you have you should be running a disco transfer box, you'll loose some acceleration, but with your power it'll still be better than any normal Landrover.
You will gain a much better fuel consumption...

I was looking in to that the other day. As mine is a factory V8, I think I've already got the 1.2 ratio transfer box.
I was contemplating putting a roamerdrive on it (Because what's better than 10 forward gears and 2 gear levers - well, 20 & 3, clearly!)
But I thought with the 1.2 transfer box, 5th + Overdrive might start to be a bit tall even with the torque of the 3.9 V8 - so I decided to check the rpms at cruising speeds in 5th first.

I will certainly review all the above once it's (hopefully soon) back on the road. HUGE list of jobs done this time, but we're getting there and I hope it lays a good foundation for when I eventually re-chassis it (trying to get the original chassis to 40 years - but I can see a bit more welding will be needed soon).

Anyway, in all cases, I'd still just like a bit more fuel capacity - for example, when I tow my boat (let's assume it's a full 3.5 tonnes) to the coast and back, it'd be nice to be able to get there and back without refuelling the LR. It's only 110 miles.
Obviously fuel consumption is obscene when I'm towing that much weight & air resistance at 50mph - which might be my mpg error above ^^, I can't remember since I've had it off the road for some time now.