I'm not an expert and haven't even run this race, but I'd say that it isn't that much of a relevant factor.
I assume you're going to use an LMP/Group C racecar, so the gasoline engines of the Group C cars will deliver more hp, and allow a slightly higher top speed, whereas the diesel-powered 908 HDi and R10 TDI will give you massive torque, and, in my opinion, a wider powerband, which will allow you to usually run a gear higher than normal.
On the other hand, this massive torque will usually derive in wheelspin in the lower gears, and more wheelspin means your tires will last less, and you'll need more pitstops.
After all my explanation, I'll tell you that when I get the oportunity to run the 24 hours of Le Mans, I'll run it with either the XJR-9 or the 787B, but that's only my personal choice, so in the end it's all up to your preferences

Good luck with the race
