2.5/1 isn't a bad conversion, it is in fact ludicrously OP.
2.5/1 is the linear rate based on Coal in a Generator or Stirling Engine, which frankly is just wrong. The power sources scale wildly differently from there, and the fact that IC2 power is outright trivial to produce means that conversion at any rate is absurd.
Let me explain the reason why coal was used as the model of energy conversion... (since I was one of the earlier members who helped champion the idea)...
Back when the first official release of Minecraft was done in 2011, Forge was still very young and fresh and a tad bit limited in some of its functions. Power systems from BC had just been developed not very long ago, and IC2 was just getting off the ground as people were raging over the initial castings of Yogcast with Technic. Energy conversion was hypothesized when people began to truly expand into the two systems because, let's face it, BC pipes suck for energy transfer, and oil is highly lucrative when it comes to fuel and fuel based energy. For many, this prospect was actually ignored since solar panels were being spammed en-mass on worlds, and CASUC's were being exploited due to the nature of infinite water in Minecraft. Yet for those of us who needed energy who were unwilling to follow the 'current metagame' looked for solutions within Buildcraft to use the long burning solutions of hydro carbons for an energy source. Thus, between Power Crystals's Power Converters and Sykne's Transformers, a standard was formed in order to satisfy the initial early game of Minecraft play...
Now granted, this meant that BC Fuel could generate about a Million EU per bucket, but when you are competing with CASUC reactors that output 1800 EU/t and HV Compact Solars that make UU-Matter instant access, it wasn't a big deal. The conversion made Oil competitive with other energy systems, and made those types of add-ons worth having in your /mods folder as you played...
Fast forward to IC2 1.106 when the new nuclear system was brought along, and hard distaste for solar energy was being thrown (again) at that type of renewable energy. Players had evolved from simple energy setups with mostly LV based power, to highly complex energy hungry systems that mass fabricated materials on the fly, and rendered enormous domes of forcefields in order to shield against hostile threats. The nerf came hard for nuclear operators, and killed many setups (possibly servers) that were using it.
So thus the meta-game shifted, and people began exploring the Nether for Geothermal energy (something
I predicted back in 1.8 beta). People wanted energy because they needed it, fast. So the old conversion was discovered, and people swarmed to it immediately. Suddenly, a black gold rush began on updated servers, as that form of energy became the new god-send for mass energy need, and in many ways, it was exploited just the same as nuclear and solar before. The linchpin in that, however, was that it used external conversion to decide how powerful Oil and Fuel could be. On one end, conservative server owners wanted something balanced and challenging, while others wished to return to the golden age of having billions of EU at their direct disposal...
So thus the flaws of the 2:5 ratio were quickly discovered by developers. I'm not saying it's the 'right' ratio, I'm just saying that it was something needed as a quick fix for the conditions at the time. Saying that the ratio was just a hideous flaw to begin with discards the evolution that this game has been under during the past year or so. We've changed quite a bit. Even vanilla Minecraft is different from when it first launched officially. So the ratio is a relic of the meta-game, and the needs that people had in order to stay competitive with their energy generating peers...
Personally, I think a 3 MJ:4 EU ratio is better. This works under the guise of the nature of horsepower (1 hp = 746 watts, or
.75 Killowatts), and in a realistic sense balances out fairly well. This would mean that fuel would produce about 800k EU ( 200k under a uranium ingot at 1 Million EU by itself), and would only lend a small (about 4k EU) gain in Geothermal energy when using an engine instead of a geothermal generator. That's basically an extra coal of energy, which you can gain from a sawmill for about the same price either or. This means that, when scaled, Fossil fuel energy is still being outperformed by nuclear, but more attractive short term by those who aren't worried by fuel renew-ability. I think that, since we can look at real-life issues about out own energy generation as an example, BC Oil/Fuel should have a higher output than the other forms of energy as they show the 'trap' that players can get in if they rely on nonrenewable energy sources for too long. If Oil/Fuel values are too low, people will simple ignore them. If they are too high, you run the risk of admin duping and 'Oil fabrication' (be it by add-on or other exploits) and people crutch themselves on that type of energy. BC Oil/Fuel needs to be both attractive and balanced, hence the need for such energy conversion as it enables Oil/Fuel to be taken advantage of, while demonstrating the Minecraft/real-life pitfalls of that type of energy...
(tl;dr)
But I can agree, the 2:5 ratio (atm) is bad... however, don't shoot it apart on the simple premise that it was a 'designer flaw'. There's a meta-game history that goes with it...