Transmission choice for best fuel economy

[Matt Timion]

As many of you know, I'm building a setup for my car to get max MPG. I'm putting in a vtec-e engine, which is honda's most fuel efficient engine. I am also going to put in a CRX HF transmission due to the longer gears.

Recently someone on another forum said that the DX transmission is actually better than the VX/CX transmission (the VX is the car the vtec-e engine came from. The CX transmission is exactly the same). They have said that using the VX/CX transmission you have to use more gas to get up to speed, so it actually hurts fuel economy.

What they said:

Originally Posted by one guy

The reason a Dx trans would be better than the HF or VX is simply because of traffic and hills; the super economy tranny's would lug the engine really hard in stop and go and climbing big hills. Provided if you live in the central midwest with no hills or neighbors, then I would say go for the H/V box.

You are going about this in a 88-91 civ chassis right? The gearing in the transmission is only a single part of this equation, and a very easy to anticipate the results of any change to it, ie. gear swaps, final drive changes. I think that using a DX 1-4gearset and FD (3.83) with a HF fifth(.690) or a hydro Ex,Dx,Lx fifth gear(.702) will give you a very streetable trans with great freeway cruising.

Originally Posted by another guy

Exactly. The cx/vx trans are geared SO tall that it usually requires additional throttle input from the driver just to get the car moving at a reasonable rate of acceleration, more than what would be required with the dx gears. Plus to keep the engine rpms where fuel consumption would normally be at its most efficient with the cx/vx final drive, in 5th gear you end up travelling at speeds where wind resistance becomes a major drawback (and well over the posted speed limit on most roads in most countries).

So once again I'm torn as to what I am to do. Do they make good points? Should I not worry b/c the d15z1 has enough HP (92HP at 4500RPM) to overcome the tall gears?

Will my HF transmission end up hurting me more than helping me?

[GasSavers_BIBI]

I'm b20b16 NA from superhonda, and just sign up to ur site after watching ur topic on SHO, and I tell u one think, even if they are tall, ur rpm zone for power start at 1500 rpm, so u dont even go up to 3000 even if u have to climb hill, and even if u have to switch to 4 or 3 when u climb, u still damn low rpm cause considering in 3rd gear at 80km/h u are at 2400rpm, so the final ratio is sooooo long that i top 180km/h in 3, I'M serious...

I get 600km city and 700km highway with my 92 hb with 35L
if u go with the dx, at least get a 5 gear of a VX because on highway, there is no question about it, dx suck and will get u way higher in rpm, at 110km/h (great cruisin speed) i'm like 2100-2200rpm.

Forget about dx, just get a 2inch exhaust with header and u will no more care about hill.

[Matt Timion]

Thanks BIBI. I've always understood the HF tranny to have a very peppy 1st and 2nd gear. I'd much rather just stick with the HF tranny than the DX one. I really have no desire to make a hybrid transmission, as I'm hoping to get the engine all together and just drop it in and have as little down time as possible. If I had to take apart transmissions and such, that would essentially take me an extra week or two as I have no experience with rebuilding transmissions.

I'm not really too worried about hills and such. while I live in the mountains, most of my driving is city. We do go up in the mountains every so often, but I'm certain that a HF 3rd or 4th gear will be enough for this. Besides, with the vtec-e, I'll get a HP boost going up hills (assuming I get above 3000rpm or so).

[SVOboy]

I agree with going with the longer gearing, lower rpms is the best, you won't even have to accelerate past 2k rpms so it's not like you'll be high in the rpm range while accelerating.

[Flatland2D]

This is one thing I've been thinking about. I have a B16 (S4) transmission on my B18C1, which is geared kind of short for a Honda. That really limits the number of ways I can increase mpg without a transmission swap. The only major things I could do is reduce the fuel mix (and/or supplement with H2) or cut off cylinders from firing. I'm at 3500rpm at 65mph, so any other mods won't help much.

Ideally I'd like to buy an old beater Honda someday and do some crazy stuff on it. That way if I screw anything up I'm not in the hole $2500 for a new B18C1 and install.

[SVOboy]

Wow, in my 4 speed auto I'm still at 3k at 65, so your 35 on the highway is pretty good considering you were at 3500. You could always just go find a fifth gear from a vx or cx or whatev else shares that .702 gear (look for my post on gear calculation) and crack the tranny open and swap it just for cruising. You'd be at 2000-2200 on the highway and then have kick *** acceleration all the rest of the time. Plus a much higher top speed, not that you'd need to go that fast.

[GasSavers_DaX]

Here is a fun website where you can calculate speeds per gear:

http://www.f-body.org/gears/

One funny thing I found out is that negating wind resistance, with a set of 205/50/15 tires and a VX transmission, if you rev'd your engine to 8500 RPM in 5th gear you would attain a speed over 250 mph. LOL!