Optimum speed for best MPG, Anybody? Options

[Legit94GT]

So every car has an optimum speed to achieve the best MPG, I am curious if anyone knows what the optimum speed is for our celicas?

Both 5SFE and 7AFE. I know there is an acutal formula to figure it all out, but i dont have it anymore:(

thanks

[FAQdaWorld]

This is something I've always wondered about...

Shouldnt we be achieving the best mpg when the engine is at the lowest possible rpms while in the highest gear without bogging the engine?

For example cruising at 40 - 45 mph in 5th gear I'm guessing is around 1.5k rpm and should in theory be burning less fuel per mile than it would at 55 mph at 2k rpm?

I have one buddy that insist that no that's not true but at least with my current understanding of how EFI works it makes sense to me and that the 55mph speed limit is actually pretty arbitrary and a generic 'efficiency point'.

Anyone want to correct / verify this?

[razor7]

Shouldnt we be achieving the best mpg when the engine is at the lowest possible rpms while in the highest gear without bogging the engine?

Yes,
But there are other factors at play. For instance, if you're doing 1.5k rpms in 5th, you can be doing a lot of detrimental harm to your engine. Causing it to turn over slower than it's supposed to, and keeping it well outside of your power band can harm your crank, rods, and cause some nasty build up on your pistons. A good rule of thumb is to keep RPMS above 2k, and under 3 while cruising. I prefer to shift around 3.5-4 since then I'll be using my power band and allowing my engine to breathe.

If you really want great mileage, accelerate (once) as fast as possible every other time you fill up. This will cause less carbon build ups. These build ups will eventually weigh down your pistons, and cause them to fire incorrectly (think about valving). Obviously, do this safely, and it should only be done once in a while rather than all the time.

After doing this, just cruise. You'll get the gas savings you want, and will also keep your engine healthier. Obviously this is a little bit counter intuitive, but lick up old Jaguars and Ferrari's, it's a method that's required to keep their engines purring.

As far as an equation, I have no idea. Honestly, I don't think that it'll do you much good either, since you would have to keep it at that exact MPH/KPH in order to get the savings. Those equations also don't take into account acceleration, which is one of the biggest forms of gas consumption. If you have a heavy foot when accelerating and a soft one when cruising, you'll likely cancel out any savings you'd make.

Not to mention that it's probably some absurdly low number that wouldn't be safe to drive at on a freeway. Even though most cars benefit around 50-55, you'd be pulled over if you attempted that on a freeway for blocking the flow of traffic.

-TC