The Stovebolt Site Forums The Shop Area The HiPo Shop gas mileage

i have a 1956 1/2 ton with a 305/350 set up with the stock rear end,390-4?? gears, i am replacing the transmission with an 700r4 and the rear end from a 86 chevy 1/2 ton, i think around 338 gears, was getting 10-12 mpg, hoping to see 15-20 when done, any one has any idea if that is wishful thing or possible

Keep the 3.90 gears. I doubt you will get better mileage with the higher gears, but that all depends on peak torque rpm of the 305. My guess is with 3.38 gears you will be lugging an underpowered motor at cruising speeds, costing you fuel economy.

You got good advice there. The 700R4 has a 30% overdrive so with 3.90 gears you have a 2.73 final gearing when in OD. Can't stand much shallower gearing than 2.73 if your ever going to have any load or pull a trailer. A vacuum gauge is great for a tell tale on gearing. If it drops to under 15 or even 16 then your geared too shallow and costing yourself fuel. If your in the 18 to 20 range your getting the most out of your set up.

thanks for the info
will get a vacuum and start checking

I am running a 350 with a 700R4 and 3:73 gears. I get an easy 20 MPG on the highway. Took a few long trips with her last summer and she seemed to stay in that range on both trips!

56srw, my street rod is a 350 sbc powered '32 Ford roadster with a mild Crane cam 218*/230* @.050 and a 2004R trans (.67 OD). It was a dog in OD even at highway speeds with 3.40 gears. With 3.80 gears it pulls all the gears well and gets a best of 20 MPG on the highway, much better than with 3.40 gears.

After 15,000 miles of test and tune I don't think I will be able to get 21 MPG out of it with my skills.

while rear end is apart doing gaskets and bearings i will go ahead with the higher gears, hard to deny 15000 miles of test and tune, after 20 years of 12 mpg average anything will be an improvement, especially since it does not run on regular gas, 305 out of a 84 z28, slight mods, on reg gas it diesels, on super reg it has slight pinging, on premium it kicks butt

Right on,with the 700R4 tranny a 3;90 rearend gear is just nice.Mine is a built 350 engine goinginto a 700R4 tranny and 3:90 rearend.Great performance and 20 MPG all day long.You can even go to a 4:11 rear and still be very economical.

thats a good looking 55

Your s.90 set up is what i am going with but i am running a 383 with vortech heads and a 625 cfm Barry Grant Carb i hope i can get 20 or better becuse of the extra tourqe the 383 makes

Way too many of you guys are looking at a particular engine to get good MPG and performance.My friends,its all in the gearing,with my 350 and 700R4 setup the 700R4 has a very low first gear that will kick [censored] on take off.Then with the overdrive 4th gear it is very economical.No reason why a 383 should not run economical too.Course with any setup ya gotta drive with a little common sense.At 73 yrs. of age I drive my truck at a very reasonable pace.Then there are the other days when some young jerkoff wants to race it from a stoplight.Sometimes I accomidate them and guess what.Yeah Old yeller leaves them sitting,and they wonder what in the hell happened,youi know like how in the Hell did that old pickup beat me back there.Those are the fun days.So for performance and economy look to gearing first,then worry about horsepower.

Very good point. It takes the same amount of energy to get and keep the truck down the highway regardless of engine displacement. Pick the engine you want. Then, set cruise rpm to accoumplish this task with the best vacuum and least throttle imput to maintain the desired speed in the conditions you drive in most. With carbuteted engins keep out of the power valve our your licked from the beginning. Electronic Fuel Injection is why modern LS1 vettes can cruise 70 at almost idle and get away with it. The combination of EFI and low cruise RPM with close ratio gearing in the higher tranny gears is the key to best mileage.

Someone mentioned on another site that the optimum compression ratio for best fuel economy was not the same as that for best power but they didn't elaborate. I'm not a member of that board so I thought I'd ask you guys what you recommend. I'm primarily interested in a 250 and a 283 because those are the two engines I have to work with. Any insight will be appreciated.

google stoichiometric ratio...that would be the definition of a perfect burn to achieve best gas mileage but may not be the best power output. wrap your brain bowl around this.....

A stoichiometric mixture is the working point that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. For gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7; i.e. the approximate mass of air is 14.7 mass of fuel. Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture - given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles using an oxygen sensor(s) or other feedback-loop to control fuel to air ratios (usually by controlling fuel volume) will usually compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition, while vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain boutique blends of fuels (esp. winter fuels used in some areas) and may need to be rejetted (or otherwise have the fueling ratios altered) to compensate for special boutique fuel mixes. Vehicles using oxygen sensors enable the air-fuel ratio to be monitored by means of an air fuel ratio meter.

Not sure I follow all that but how does air / fuel ratio affect compression ratio? To clarify my original question, I'm looking for the best compression ratio for gas milage on a carbureted engine burning regular gas. I suspect the stock CR gave the best milage on the fuel of that day but might not be the best on today's gas. Don't plan to use any fuel additives, just plain regular gas. Thanks.

I would try for 9.0 to 1. Thats a good number that will run on 87 octane with killing the pistons or bearings. You can get away with more with a good timing curve, and carb calibration, but you will give up some performance for fuel mileage. At 9.0 to 1 you can achieve mileage with out giving up much.
Some of the new cars are in the 11 to 12 to one range, but the computers are able to control all the inputs and compensate as needed, your carb won't!

For the 250, a simple piston swap will get you there, for the 283, I have no idea.

Joe

Thanks Joe, that's what I'm looking for. I'll do some research on heads for the 283, possibly from a 305.

jfnar, the compression ratio you pick depends on whether you are using iron or aluminum heads. Before you laugh, a "valve job" that puts used heads in like new condition (bronze guide liners, 3-angle valve seat regrind, centerless grind valve stems to restore roundness, grind valve faces, new springs, new valve seals, resurface gasket surface) can actually cost more than new heads. And with performance heads you also get nice bowl blending and screw in studs.

Iron heads at 9:1 will not tolerate 87 octane gas on a hot day if you are running a carburetor unless it is jetted way too rich for clean running and good power. My street roadster has 9:1 aluminum heads and it took some very careful carburetor and distributor calibration to avoid pinging on 89 octane.

There is a HUGE variety of small block Chevy heads out there with combustion chamber volumes from under 60cc up to 76cc. That range, in addition to the similar HUGE variety of pistons, will let you pick any compression ratio you want.

Supposedly one of the biggest bargains out there is vortec heads but there are some good versions and some ones to avoid.

If you're building an engine from scratch you can make it be anything you want it to be. Have fun!

i talked to the shop my truck is at and was told 273 was the gear ratio in the rear end he installed, he has driven it on the highways and said it was running good and smooth, i get off the boat in 9 more days and i will see what it is like, i am sure it wont take my 2 porsches off the line anymore, wife and son will like that, i was just wanting to get it to cruise the highways a little better, will see soon

Point of clarification...

Are you rear gears 2.73:1?
Are you using a TH350 or 700R4?

If using the 700R4, the 2.73 gears will be absolutely worthless.
Well maybe not worthless, but you won't be able to use the OD unless you are going down hill with a tail wind.

Each engine has a torque/fuel curve. There will be a "sweet spot" where the engine delivers the most torque per unit fuel.

Maximum fuel economy will be realized by gearing the vehicle such that the power produced at this RPM will exactly equal the power required to move the vehicle at the given speed in the highest gear.

Remember that the power required is the sum of all the power losses to include: heat losses in the power train past the engine, rolling resistance (tires) and aerodynamic losses.

If one will simply look at the heat loss figures (often difficult to locate) for transmissions furnished by their manufacturers, one can EASILY determine the largest gain in fuel economy is virtually always obtained by junking the automatic transmission in favor of a standard!

And my the way, my iron head V-8 with 10.3 compression and dual 4 barrel carbs went from occasionally pinging on 93 octane to running with no pinging under normal load with 87 octane and pulling a 16 foot trailer with 2 L & G tractors with no pinging at 91 octane JUST by junking the automatic. No other changes. Fuel economy also increased by approximately 30 percent at normal highway speeds without the trailer.

Yes, the total gearing is very important, but less so than being lazy with an automatic to shift for you.

My opinion, yes, but it has proved to be true with every transmission swap we have done in the last 40 plus years.

Jon.

I can find HP and torque curves for most engines in various locations on the web but I've never seen a torque/fuel curve. Any suggestions where I can find those?

273 rear gears with 700r4, i will find out next week whats its all about

Look for "brake specific fuel consumption"

Jon

I found some info on brake specific fuel consumption but no charts. I did find a statement that's very helpful though. "The best BSFC will always be at the same engine speed as the peak torque." So if you want the most efficient setup you should have your cruise speed rpm at or near the peak torque speed. I think it might be good to be slightly higher so you'll have some backup torque when you hit a hill or some other increase in load. Some of the info I've gathered for engines I have: 235(6) peak torque is at 2000 rpm, 250(6) - 1600 rpm, 283 2bbl - 2100 rpm, 283 4 bbl - 2800 rpm. These numbers are pulled off of charts that sometimes are blurred a little so they may be off a little. I'm somewhat disappointed in the 283 4bbl peak torque. I don't really like the idea of running at 2800 - 3000 rpm cruising down the highway. Of course these charts are plotted at WOT. Maybe the shape of the torque curve changes at partial load. It would be interesting to see fuel consumption numbers at partial load; something simulating a pickup at 60 mph.

"273 rear gears with 700r4, i will find out next week whats its all about"

Well, if you have read and understand the responces in this post... 2.73 gears with a 700R4 will not perform well. The stock 3.90 gears would have done much better. I am hoping for a typo in there somewhere. Perhaps your new rearend is a 3.73 ratio. If no typo was made, I suspect the 2.73:1 rear will come out in the near future as performance will be less than stellar.

Acceleration will be poor and overdrive will be best locked out.

Good luck to you

This has been an excellent thread.

Here are some charts from a university course on engine performance maps. They would be easier to understand if they had used real units.

As they point out, one cannot usually cruise at the peak rpm at the throttle setting for best BSFC as the engine will probably be putting out more power than the car or truck needs. The variable displacement engines attempt to allow the engine to run closer to peak efficiency at cruising speeds.

I've made a few attempts to find this information for engines of interest to stovebolters, vintage hot rodders, etc. IC engine textbooks also show examples, and SAE papers have some information, but nothing very interesting for a daily driver engine. Chevrolet must have generated this information for the 216-235-261 family but I have not stumbled upon it.

As they point out, one cannot usually cruise at the peak rpm at the throttle setting for best BSFC as the engine will probably be putting out more power than the car or truck needs.

Let me know if there are any questions. When i took an IC engine course back in 1965, we generated an engine performance map using a 265 SBG on an electric generator dyno. I seem to have lost the lab report. We generate similar types of plots for jet engines, but we use thrust instead of torque and horsepower.

Hoyt

This is very interesting, and if I understand what I'm seeing on these charts, the optimum rpm for fuel economy is slightly lower than peak torque rpm. The fact that the shapes of the curves change at different torque requirements is why modern engines have computers to control them. The computer can make compensations for all the variables. The published torque and power curves are at WOT, but I seldom drive at WOT and don't want to. If I had access to a dyno, I'd like to estimate the power that is required to cruise at 55-60 and then measure fuel consumption at that power level at various engine speeds from about 1800 to 2500 rpm. Once the best fuel economy speed is found for a particular engine at that power level, then find the correct gearing to stay near that speed on the highway. That info may be available but I don't think I've seen it.

One of the more interesting issues to come out of this subject is some of our government's attempts to artificially control fuel economy.

In seemingly another lifetime (the early 1970's), I had not yet learned the benefits of buy American, and still was driving small import vehicles. Some of the import manufacturers DID publish charts such as these, and I calculated approximate gearing necessary to maximize fuel economy at 70 MPH (then Missouri's speed limit). I special ordered the vehicle and then played a few tricks with the carburetor, exhaust, and distributor curve. After a break-in of 1000 miles, highway mileage was 39.3 (I corrected the odometer, and kept a complete record of all fuel entering the tank). Somewhat later that year, the national 55 MPH speed limit took effect. I wasn't really happy about the reduction in speed (more time on the road), but was curious about fuel economy. Imagine my chagrin when a 300 mile road trip DROPPED my fuel economy to 33.6! Worse, I had absolutely NO power. I found by driving in fourth gear rather than fifth, I gained back the power, but the mileage still stayed around 33~34. I ended up selling the car in disgust!

Very easy to put in O.D. tranny and numerically low final drive, and get both less power AND less economy.

Jon.

This is an interesting thread. My 57 has a 350 (1970 truck engine), 4 bbl square bore, MY6 trans, and 3.36 rear. I have been trying to get the most speed from the fewest rpms, paying no attention to vacuum measurements or power curves. Might explain why my fuel economy is less than it could be. I don't know what my engine has for internal parts, but if we assume it is stock, is there a site which shows a power curve/ fuel consumption for any given motor?

I would think there are to many variables to go off any chart. They were probably tested in a very scientific manner and real world results are going to be way different. Your best bet is a vacuum gauge and some highway miles. Something else to consider about old charts and data is they were using 100% gas, not 10% ethanol like we have to run.

If you can keep your speed with the least amount of throttle opening and highest vacuum, you will get the best mileage possible for your combination.

FWIW, this car has 3.06 rear gears and a Tremec 5 speed with a .68 overdrive.

http://smg.photobucket.com/albums/v228/Avette4me/video/?action=view&current=20091126133941.mp4

Never had a problem using 5th, even with the high rear end ratio. I’m at ~2050 at 70 MPH and ~2350 at 80. Gave it a gas mileage check driving a full tank at 80 on the interstate and got about 17.9 on that run. It was always about 11 before the swap.

Your 2.73 might be a little high, but since it’s in already you can swap it later if needed.

Lonnie,
I ran your numbers through an online calculator. If the numbers/calculator are correct, your tires would be about 24 inches in diameter.

The tires on my truck are 28 inches in diameter (275/60R15). To make an apples to apples comparrison,I ran a calculation to see what gear ratio would give the same rpm with the 28 inch tires, and came up with 3.59:1 rear gears.

Tire size will play a huge roll in performance. I think a 56 would look wierd with 24 inch diameter tires. I suspect more of the popular tires would be between 28 and 30 inches in diameter.

The tire size can change the effective rear end ratio a little bit, but it might not be practical.

i got the truck back a few days ago, running great and quiet, it has the 273 rear gear ratio with 700r4,tire size is 215x75 r15, still has plenty of pep and shifts into overdrive at about 45 and holds at highway speeds, even when i drop it to 3rd on highway still quieter than before,will find out gas mileage at next fillup, well satisfied, it was a great post for all the info it generated
thanks
stephen

I have a 216 with a set of 3:07's and a T-5. I have no power issues or starting problems. At 65 MPH my Tach says the engine is at 2960 RPMs in 5th.
Normbc9

I have a 216 with a set of 3:07's and a T-5. I have no power issues or starting problems. At 65 MPH my Tach says the engine is at 2960 RPMs in 5th.
Normbc9

that seems like a lot of rpm?

That's what I was thinking, too. At 2900 rpm, I'm pushing close to 100 mph with a 3.90 diff gear and a .70 overdrive.

faulty speedo, faulty tach, or really tiny tires

Bill