I wonder if your carb needs tuning, not your gear ratio. As someone mentioned, it might be that the power valve is opening.



I have an 86 Mustang GT with the 5 speed and its rear gear is 2.8 or so. It gets over 25 MPG if I cruise at 65 mph with the back window down in 5th gear. It's turning right around 2000 IIRC.

Best regards,



Paul Menten

Meridian, Idaho

The carb could very well need a different step up spring for cruising.  I'll find that out with the vacuum gauge.  I have  a feeling it has to do mostly with gearing.  I have an 88 firebird 5.0 which will get 30mpg on a good day.  It will cruise at about 1800rpm.  The two engines are built considerably differently, though (besides the obvious difference in the manufacturer).  My firebird is stock and torquey right off idle.  My Y is more performance oriented and doesn't really start pulling until around 1500 - 2000 rpm or so.  This means that in overdrive I may not be in the power band which isn't really ideal for fuel economy.


Lawrenceville, GA

Charlie,

      if you do know what the number, way to I.D. the cam and the specs on it, cam card,  maybe John in CA could tell you what the sweet spot or power range would be on it to gear it accordingly. Also too, you can go to Richmond Gears website use their strret calculator and figure what your RPM's would be at 70 mph, as an example. A friend of mine used here in Vegas and had great reults when regearing his truck, gained about 2-3 mpg when he did this.

                                                        Michael

Michael

Sandy Valley, NV

Tip..

check rear end gear ratio by lifting the rear end

mark the a rear tire/wheel  and drive shaft

rotate the tire/wheel one full turn

and count the rotation turns  of the drive shaft

cheers

I'll do that.  Just have to wait for a relatively warm day.


Lawrenceville, GA

Charlie,

Sorry, here comes the math teacher.

Using the one tire rotation method is a good one.  It does require you to estimate the last rotation of the drive shaft, but would probably get you close enough to determine if you have one of the standard ratios.

If you are anyone reading would like to be exactly sure, it would take two people and perhaps 10 minutes.  Turn the tire 100 times and the number of drive shaft turns would be the exact numbers in the ring and pinion ratio.  So, 350 drive shaft turns would be a 3.50 ratio.  If 100 seems to many, turn the tire 50 times and multiply the drive shaft turns by 2 (25 tire turns, multiply by 4, 10 turns, multiply by 10).

This is the method I use in selecting the closest ratio transmission possible for the tiny-Y.  I put vice grips on the input and output shafts and turn the input shaft 100 times in each gear while my wife (crew chief) counts the number of tail shaft turns. (Using math is too tough!)

By the way, when you get around to doing the math with different tire diameters, don't forget the overdrive in calculating the final drive ratio.  It will require you to know the exact ratio in the overdrive.  I put together an Excell program to calculate speed with given rpm, rear ratios, and tire diameters.  I would do the math on it for you, but it is specific for a 1:1 drive through the trans. 

I am changing to a T-5 myself this winter.  If 4th gear is not 1:1, I will have to change my little program and then I can do the math for you.  If the transmission ID tag is still on the trans I have a reference chart to tell you the exact overdrive ratio.

Hope this helps.....................

Charlie Burns Laton, Ca (South of Fresno)



BurnsRacing981@gmail.com

Charlie.  Don’t recall which carb you are running but that in itself can be a big player when dealing with fuel economy issues.  On some carb models, I find it necessary to enlarge the PVCR (power valve channel restriction) while decreasing the primary jet size.  This allows the same amount of fuel at full throttle but permits the engine to use less fuel under cruise situations.  With a leaner fuel mixture, the engine will prefer an increase in ignition timing though.  And while on the subject of ignition timing, this is also another area worth exploring as the Y’s do tend to like more timing at cruise than is normally being supplied.

 

Fuel quality from station to station also varies.  I find it increasingly difficult as time goes on to maintain a consistency in fuel mileage even when getting the fuel at the same station.  As an additional note, gasoline with an actual 10% added ethanol will get a reduced mileage over pure gasoline (no ethanol) but without testing each load of fuel beforehand, it’s difficult to know exactly what is going into the fuel tank.  I sometimes wonder if the ethanol itself stays adequately mixed in the gasoline or if some separation takes place over time in the storage tanks which in turn changes the amounts getting to a vehicles fuel tank.

 

As far as rear end gearing changes go, it’s typically the most cost effective change that can be made when working with the mechanicals.  If a ready to go additional third member is already set up, then the actual third member change in the vehicle only takes an hour.

Lorena, Texas (South of Waco)

It is an Edelbrock carb.  If it is the cause, it seems almost like I'll need two tunes.  The cruise and power modes are tuned well for city driving.  Timing hasn't been optimized, though.  I'm not going to get too caught up on the carb.  I plan to get rid of it this year, anyway.

I was actually thinking about taking a piece of masking tape, marking it every 10 degrees for the circumference of the driveshaft, using a magnet to hold a pointer and use that to figure out the ratio.  That way I can translate the number of degrees of rotation into the precise number of turns it made for one turn of the wheel.  For instance, the difference between 3.70 and 3.89 is 68.4 degrees.  That will be easy to distinguish with 10 degree graduations.

Lawrenceville, GA

Charlie,

I like your idea, and I agree with the math.  Perhaps the most gratifying is that it is creative and possibly an original thought.  Keep in mind that for every one degree error in one rotation of the tire, it will effect the drive shaft rotation approximately 4 degrees.

Good luck..........................

Charlie Burns Laton, Ca (South of Fresno)



BurnsRacing981@gmail.com

Thank you.  Good point, so then I will make a mark and put a pointer on the tire as well.

Well, I got some more data on the way back.  For the first tank of gas I did not use overdrive.  I got 13.33mpg.  I was trying to keep it at 3500rpm, doing about 80 keeping up with traffic.

For the second tank I did use overdrive and tried to keep it around 2200rpm which was also about 80.  I got 14.28mpg.

I forgot to mark mileage and gallons used for the rest of the trip.

A friend suggested that I get on the interstate, use 4th gear and drive at 2000rpm, check mileage, then 2500rpm, check mileage, then 3000rpm and so on and so forth, to find the most efficient rpm.  Then I know where my cruising rpm will have to be.  It sounds logical to me.  What do y'all think?

Lawrenceville, GA