What did I do to my mpg!?

[LaGrasta]

I got my car back from the Todd Fields 3 1/2 month adventure and my mpg has taken a major dump. I thought it was going to increase from nearly 20mpg to ???
Instead, I'm getting 12-15mpg. I went to 14" wheels and a 3:00 rear end (among other changes that I don't think would make a differance). What gives?

 

[66Sprinter]

Tony...

Can you describe briefly your before and after situation with the engine and carb..and also the before and after for gearing from the tranny to the rear and wheels.

....are you running more RPMS for the same MPH??

 

[Mike]

How much weight did you add with the suspenion changes?

Mike

 

[BIGREDRASA]

What size/circumference tires did you have before? What size/circumference do you have now? What gear ratio did you have before? Did you change speedo gears to compensate for the changes?

 

[LaGrasta]

I had 175-80,13 before, 185-75, 14 now. Same tranny and gearing. Went from 3:20 to 3:00. I was told the speed gear didn't need changed for such a minor amount. I looked into that and it seems this is the truth. As far as weight, it's probably a wash. I gained some with the 8" rear end, but proabably lost some with the drums vs. disc.

 

[Anonymous]

I think you dropped 259 rpm @ 60 mph and your speedo is 3.8% slow based just on tire diameter. http://www.bgsoflex.com/auto.html
http://www.miata.net/garage/tirecalc.html

@ 2430rpm you are now going 60 mph.

 

[LaGrasta]

600 rpm! You think I would have gained some mpg. I have a 2:79 pumpkin in the garage, hmmm. What would that do for me? More of a slug for sure. Any better mpg?

 

[Anonymous]

What does your speedometer indicate at 2430 rpm? Your speedometer and odometer are no longer accurate.

 

[LaGrasta]

I have no tach.

 

[Anonymous]

Then you have to figure another way. Maybe you have someone you trust enough to pull you at 50 and 60. That would give you a place to start. Or mark 10 miles off with a good vehicle, then drive it with yours noting the position of the odometer.

 

[Anonymous]

i gave up on figuring actual milage...I am just trying to get more miles per tank....whatever the odometer says PLUS some next tank

my speedo is way off due to tires....

 

[CZLN6]

Howdy All:

I have no doubt that your odometer is off enough to not give you an accurate before & after comparison. The one true and easy way to determine your speedo accuracy is a stopwatch and a section of highway with mile markers.

60 seconds is a true 60 mph. Start by timing a measured mile with the speedometer at 60. If the stopwatch is less than 60 seconds, you are traveling at a speed greater than 60 mph, and your odometer will be off a similar %. Given your changes that would be my guess.

If the measured miles times in at say 50 sec. You will be traveling at a true 70 mph- 16.66% difference- and your odometer is off by 16.66% too. Multiply your current mileage readings by 1.1666 and you will have a fairer before-and-after comparison.

Sometimes higher gearing, whether by taller tires or a lower number R & P, causes an engine to lug more, which uses more gas. You might try cruising with a vacuum guage hooked up. As more is being asked of the engine, vacuum and mileage drop.

Check it out.

Adios, David

Too bad you didn't establish some objective baselines before the changes so you would have accurate comparisons.