My 37 Chevy p/u that has a 235 and a T-5 will keep up with traffic but the engine is not in its sweet spot and the gas mileage is not what it could be,I think it could use 3.73 or 4.11 gears but since I have the speedo about right I will keep the 3.55 gears for now and deal with the mileage it gets.
All the vehicles I had or have that had an o/d unit in them where for gas mileage AND lower engine rpms......but I could keep up with or pass other cars comfortably if I wanted to. All are trucks with either 3.73 or 4.11 rears, way to much for regular freeway driving without overdrive.
I've been in the old car hobby for many years and never has gas mileage been a concern. My major goal was to make my cars go faster. Who in their right mind would engage an overdrive at low speeds? My 60 Pontiac's torque converter won't lock up until its over 45 MPH. It appears like a lot of the negative comments about 4 speed automatics are by those who know nothing about them. I find theres nothing negative about a 2004R. If you don't want it in overdrive it can be driven the same as a 3 speed auto. If its gas mileage your interested in buy a Prius.
My 40 year old O/T cruiser car has a big block and 2.70 rear gears and will cruise all day at 80 getting about 15 mpg and I would drive it coast to coast I'm hunting for a OD trans and I'm switching from 3.90 to 3.55 gears on my 53 Dodge so I can do the same . I pulled off a 1200 mile trip over the summer over the summer using 2 lane highways my little 95 hp flat head needs a little help to run 60 mph while turning 2800 rpm
My '40 Ford coupe has a 302 HO, a T5 trans with a .66:1 ratio in 5th gear, and a 3.73 rear gear with 30" rear tires. 80 mph is 2200 rpm and is my cruising speed on 4 lane or more Xways with limited access when no cops are around Car is quite comfortable above there @ 90-100 mph mph, but most all of the states down here in SE have a law that drastically increases speeding fines above 80 on 4 or more lanes and 75 on 2 lanes. Sort of makes the gain in time traveling not enough to offset the penalty if caughtI don't think I've ever actually done more than mental estimations about gas mileage But I do agree that you aren't gonna gain ANY on gas mileage around town with a change to OD or a taller rear gear, and out on the road you can lose mpg if you get the engine in a lugging condition and/or cause the power valve to open.
One sure way to tell if the engine is running economically or not in overdrive, would be to attach or install a vacuum gauge. Using the vacuum gauge can tell you if you are in the correct gear for you engine and the driving situation. If the vacuum is in the range where the power valve is open, then the lower gear will save you money. I found a vacuum gauge to be particularly useful for saving fuel when operating boats I have owned. The reason I use a boat as an example, is that the engine is loaded all the time, and a couple inches difference in vacuum, say between 6 and 8 inches, can make a considerable difference in fuel consumption with very little loss in speed. Bob
Remember too that many overdrive trannys give you lower first and second gear ratios in addition to the overdrive on top. That lets you run a lower numerical rear gear if you wish , or stay with the stock ratio and get a little more low end grunt.
I regret not putting an overdrive trans in the ‘58, it could use one more gear on the interstate. The sweet spot is 2000 rpms with the RV cam in my 454, unfortunately it’s only doing 50mph. I’d like it to run 65-70 at 2 grand, there’s plenty of torque to do it. Long road trips would be even more enjoyable...
With a 454 you can drop the rear end gear ratio into the mid 2s and still have half decent performance, as well as low rpm cruising. Overdrive does make it nicer, though. I have a TH400 and a Gear Vendors in my 59 truck now, behind a 454, with 3.54 gears.
I would say that 80 here in the west is a moderate speed on the major freeways. Interstate 80 speed limit is 85 some places. If it matters I do get 19 mpg at 80. Hamb friendly 64 327.
THIS IS THE KEY ^^^^^ I installed both a tachometer and a vacuum gauge and I pay attention to using the gear that achieves the best vacuum NOT the lowest RPM. That NOS vacuum gauge paid for itself by giving me optimum (well, as best I can drive) performance.
I used to drive faster, now, I just drive a little slower than the fastest cars. Yes, the vacuum gauge is handy. Bob
interesting....the thing about high vacuum, is that it means you're not using the engine very efficiently. It does mean there's less load on the engine. You get best vacuum at idle, right? and how much work are you getting out of the engine? ZERO.
I will be running a T-85 3-speed overdrive from a '65 Ford truck in my '57 Chevy sedan. Adapting it to the Chevy with '62 Pontiac T-85 (non-OD) gearcase and early T-10 4-speed input shaft and front bearing retainer. Did similar conversion to T-85 OD in my big block powered '55 Chevy in the early '70s. Ran a 9" Ford rearend with 4.11 gears behind it. Tranny and rearend held up fine behind a strong running 450 HP LS6 454. The '57 I am building just now will have a 292 inline 6 though...... And like flathead Fords, these are relatively small bore, long stroke engines so will build it for low to midrange power and keep it below 5,000 RPMs. Should be a stump puller making plenty of torque from 2,000 RPMs up. Might consider 3.31 rear gears with this engine. That would pull 2,100 RPMs cruising at 70 MPH in 3rd gear OD. Can always lock out OD to keep engine higher in powerband if towing the boat trailer up in the Texas hill country. 70 MPH without OD is right at 3,000 RPMs and cammed as it is, the 292 should pull like a freight train at those revs. Happy Motoring, Harry
Jim, I started writing the answer, and realized that after 1/2 hour, that I was only half way through the explanation. I decided that it might be better to just look on line for an explanation that explains the theory and use of the vacuum gauge, and just cut the part about highway driving from my own explanation.. I think the two links below will be very helpful for using a vacuum gauge to diagnose engine problems, and using the gauge for more economical driving. https://www.gregsengine.com/using-a-vacuum-gauge.html https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/ One thing I thought I would add in addition to the article, is that on the highway, each engine car combination will have a slightly different peak vacuum reading to indicate the most economical cruise. This will depend on the altitude, weight of the vehicle, size of engine, and stock or high performance modifications. There are other considerations, but the amount of difference caused by temperature and pressure altitude don't really figure in unless you are changing your altitude by a significant amount. In simple terms, the vacuum gauge at cruise, is an indication of your power setting. The lower the indicated vacuum, the higher the power setting, and the more fuel you are burning. So, if you are driving on a level highway at 60 mph in overdrive, and your vacuum is 10 inches, and you take it out of overdrive, continue driving at the same speed, and the vacuum decreases to 9, then the conclusion you can make is that your car is more economical in overdrive for that condition. (In piston powered aircraft, we used a manifold pressure gauge for the same purpose. A manifold pressure gauge, is basically nothing more than differently calibrated vacuum gauge, . The reason for this change, is that it is used to set the power for supercharged or turbocharged engines, and a reading above atmospheric pressure is required. (In piston engine fighters, up to 65 inches) There is a lot of information available from such a simple gauge. Bob
But in real life, it works the opposite...in overdrive you have LESS vacuum at a specific speed, yet you are getting better mileage. Why? Because the engine is turning fewer RPM, but it is operating more efficiently since it is not throttled as much. Look at it this way...when an engine is idling, doing no work, it is consuming fuel, and it has a relatively high vacuum reading. Where is all that energy from the burning fuel going? It's going to overcome internal engine friction. None of it is going to do useful work. Now, rev the engine, but still leave the transmission in neutral, and watch the vacuum gauge. It goes up even more. The engine is using more fuel. It's still not doing any useful work. It is more efficient, or less efficient? Efficiency is work out divided by fuel in. Same work out, more fuel in, higher vacuum = less efficient. Now, think about what happens when you put a load on the engine. If the engine is at low RPM, but the throttle is open quite a ways, how much of the fuel burned is going to overcome internal friction? How much is going to moving the load? The more the throttle is open, the higher the percentage of work is getting done, vs overcoming internal friction. I agree that looking at a vacuum gauge to see if an engine is working well at one specific load, can be helpful. But when you are trying to pick a gear ratio, using a vacuum gauge might steer you in the wrong direction.
"I agree that looking at a vacuum gauge to see if an engine is working well at one specific load, can be helpful. But when you are trying to pick a gear ratio, using a vacuum gauge might steer you in the wrong direction." This^^^^^has not been my experience. Since the topic is OVERDRIVE and the conversation continues with its merits I will relay my personal real life experience with a '49 Chevy 1/2 ton, dd/only vehicle the past 21 years, presently at 115 k miles. The engine is a 1956 235, Fenton headers, 2x2 Edelbrock intake with 2 Carter-Weber carbs, 1969 Saginaw/Borg-Warner 3sp+od transmission, 1957 3.90 truck differential. The overdrive as I have it connected to it's governor, is essentially a 5 speed: 1st, 2nd, 2nd overdrive, 3rd, 3rd overdrive. Since the overdrive is .70 and the differential 3.90, .7 x 3.90 = 2.73 final drive ratio. Since my installation of a vacuum gauge several years ago I realized my shift points here in the piedmont region of central Virginia have drastically changed. I first took note of this soon after vacuum gauge installation with a trip over Afton mountain, approximately 2000 ft at the pass. I live at 500 ft above sea level so that the climb is 1500 ft in roughly 20 miles. The gauge is only a guide, but IMHO, a great guide to better efficiency. Instead of realizing that the engine/transmission in overdrive is lugging (w/o a V gauge) and then down shifting to maintain speed going up the grade, I have a visual reminder that helps me shift, in my own estimation, earlier, with greater efficiency. On a recent 2000 mile round trip mostly interstate travel, I did a random fuel mileage at 17.8 mpg. I drive at the prevailing speed limit but really prefer 72 mph as the truck seems to be in a sweet spot, I think it's my own perception of harmonics.
question: when you're cruising on level roadway, at 72 mph, what is the vacuum reading when you are in OD? what is the vacuum reading under the same conditions, but not in OD? What is the mileage in both cases?
Interesting discussion, following along. I've had some experience with ODs and see some merit in both observations. Long ago when the auto industry was just getting onboard with max MPG efforts I read an article discussing the impact of high cruise vacuum on MPG. I've forgotten the numbers of course but was really surprised at the HP used just to overcome the 'drag' pulling the pistons down against high vacuum. The gist of the article was the balancing act required to lower the intake vacuum to reduce that, yet not so low as to trigger the carb's power enrichment circuit. I;ve never had much success with ODs behind a Holley carbed engine, probably for the above reason as they are an all or nothing power circuit. Carter's with stepped metering rods and the correct springs seem to work much better. JMNSHO
I don't get very good MPG with big blocks in go fast vehicles, with overdrive....my Chevy II with a 3.89 gear and Gear Vendors OD gets 10 mpg on road trips. It has a blown 427. My old 59 pickup with a 454 and TH400 and GV overdrive with 3.54 gears out back probably never does better than 13 on the road. But one or two MPG improvement when you are only getting around 10 to start with, makes a bit of a dent in the gas bill. I just did a little experiment with a 20 year old car (off topic), which has 2.86 final drive, 0.7 overdrive ratio, 26" tall tires, and 231 cubes. It gets over 25 mph in normal highway driving. With the cruise control on, steady speed of 65 mph, on level ground, it pulls 9" vacuum in OD. Shift down, same vehicle speed but higher RPM, it goes up to 11" vacuum. But yeah, getting better mileage, with LESS vacuum, because of overdrive, is a real thing.
A person should think of the vacuum gauge in two separate, distinct situations, when the engine is not under load, versus when it is under load moving the car. Your first statement is correct, and at that point, you will be getting better mileage in overdrive because the engine is turning approximately 30 percent faster in direct, and the lower vacuum is indicating a higher load. Steady state situation, under load, - a higher vacuum = less power being used. explanation. It is a somewhat complicated comparison, but not difficult to understand. It is important to think of a steady state situation when you are making the observations. (In a boat, this is easier, because the amount of power required increases as the third power of the increase in speed). In overdrive, the engine is maintaining a speed that yields a certain vacuum, say 9 inches, and is also lowering the engine rpm by approximately 30%. Because the engine is turning 30% faster in the lower gear, if the vacuum is lower at the same speed, you can automatically conclude the engine is using more fuel. (The vacuum gauge is indirectly working as a load indicator) If you are in direct at the same speed, and the vacuum is higher than in overdrive, you engine will be under less load, and you "may" be running more economically, but you must allow for the fact that the engine is turning 30% more to cover the same distance, so you might not necessarily be saving fuel, because the higher loaded engine in overdrive could still be yielding the same overall mileage. It just works. Bob
Since 1972, I have run an overdrive on the same truck using a variety of ratios, 4.11, 4.57, and now 3.73. This was a 396 with a Holley carb, and although it is too long ago to remember the numbers, I got better mileage after installing the overdrive, even while carrying a 1000+ lb camper, than I did before I had the overdrive. I made many long 2000 mile trips, and it was easy to confirm that my costs were significantly lower after installing the overdrive. ( I kept records back then). I was happy with the Holley carb, and had it set up for the way I drove the car. I can't remember for certain, but I think I was running with a 5 inch power valve. That meant that I had to really be in the throttle before the mileage started to suffer. Yes, I believe that a Holley can can be less efficient on the highway, but that making certain that the power valve is functioning correctly and that you have a power valve that is suited to your driving needs, will make a significant difference. For routine, daily driving, I actually prefer the Quadrajet over the Holley for economy. I haven't used a Carter (Edelbrock) since the 60s. Bob
Vacuum isn't necessarily good indicator in this instance. Think of it this way: At 60 mph a specific amount of hp is needed to overcome the rolling drag and aerodynamic resistance. In or out of OD, makes no difference. Since the engine is an air pump, there is less air being pumped through the engine when in OD. If the air fuel mix control is correct for the load (remember same load, mileage has to be higher even though the indicated vacuum would be lower in OD. Lower vacuum, same load. At least that's where my logic points me. The key is matching the rpm, spark advance, AF ratio to the load. JMHO
"I;ve never had much success with ODs behind a Holley carbed engine, probably for the above reason as they are an all or nothing power circuit." There was a two stage power valve available from Holley in the past, but I haven't checked to see if it is still available.
I do when I don't want to own a lame new car and drive them daily..........and it's WAY more then a "hobby" for me. Again there's that word "hobby". It's no hobby when you drive them daily, rain or shine (I have driven to snow in the mountains, but doesn't snow where I live) and don't own a Prius or any O/T vehicle. Trying to go fast to work here is pointless and with $4.00+ a gallon is dumb. I understand some peoples needs to race, but it's not my main concern. I choose to drive a lot, sometimes far and I have a few cars (instead of one good one). I don't judge the stoplight racers or the car show chair campers, so don't judge the MPG crowd.
If you're looking for mileage, OD can be a big help....if you start with a relatively light car, with decent aerodynamics, and a mildly cammed, smaller sized engine. That seems to be the formula for success. We just got a very OT car that fits this formula, and it easily gets 25+ mpg, and performance is quite acceptable for what you'd expect from a traditional hot rod. I have plenty of experience with getting far worse mileage in vehicles that fail to meet at least one of those elements, even though they have OD.
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