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One of my dad's Mustangs, his 67 fastback GT, has been getting decidedly worse gas mileage for the last couple weeks.We've tried all the conventional methods - Complete tuneup, and I mean COMPLETE. Also adjusted the timing (and restabbed the distributor AND made sure the timing marks weren't slipping)

I was just wondering one day and thought it might have something to do with the transmission? It's an older C4, rebuilt about 15 years ago. Can an aging transmission cause an increase in gas consumption? If so, how?


Thanks for your help guys!
 

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One way that a C4 can increase gas consumption is if the shift points chage. Have you noticed an increase in the amount of slippage in the tranny or a higher rpm at the shift points? My guess though is that it's not the TX. Although you've had her tuned up to the max, a loss in mileage is usually associated with poor engine performance...ie. carb adjustment, timing, plugs/wires, ignition etc. Another remote possibility is a fuel leak! Have you looked at your lines? Does your pony leave a puddle when you park her? Can you smell fuel?
 

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Vacuum leaks cans cause poor MPG. Install a vacuum gauge and check the results. What ever your doing, the gauge needle should be steady. Depending on your motor, idle should be around 17 inches, light load (cursing on the freeway) should be around 12 inches. Of course, your right foot will determine MPG more than anything else.
 

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Thought of some other things to check.
clean/rebuild the carb (sticky needle or flot can kill MPG)
Tire PSI, should be around 30 PSI on all 4
Is the A/C always on?
Check the trans fluid, if it is dark and smells burnt then your bands are sliping.
compression/leak down test (a cylinder going bad?)
there is a lot of other things to test, hard to think of it sitting at my desk.
 

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The carburetor is a brand new Road Demon 750 4 barrel (it was on before my dad noticed the drop in MPG though, so that's not the cause alone). Vacuum tests show 16 PSI at idle and 13 PSI at 2,800 RPM. A cylinder compression check shows 185 PSI across all 8 cylinders with the only deviation being cylinder 7 rating 179 PSI, not enough to cause a problem.

He hardly ever runs his A/C, only when mom is in the car... I just checked the trans fluid at your suggestion and it's red, no air bubbles, and it smells fine. Tire PSI is something else I checked this morning.. His pony is shod with 235/50/17 Kumho tires and they're at 32 PSI on all of them.

We've also made sure there is no fuel leak as suggested. I put a piece of cardboard under his GT last night but no leaks arose (after a night of cruising too...)

I just don't know. All my mechanic friends are stumped too. My dad drives the car slowly... We hardly ever "get on it" when we drive that car, it's in too nice of condition. I understand the relationship between foot and gas consumption so that's the first thing we tested. Even if we accelerate slowly and decelerate slowly, no jack rabbit starts or anything like that, the MPG is still disgusting. I think last week my dad logged 8 miles per gallon on 91 octane. 3 weeks ago, his GT was getting 16 or 17.

??
 
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