Bad highway MPG, Rebuilt stock 7a-fe Options

[PaukST]

So I just took a trip a few weeks ago from South Carolina to visit a friend in Virginia. Everything went well, nothing broke. But when I did the math for how much gas I used, apparently I only got 25 miles per gallon on the trip. Re-filled, calculated again after the return trip, 25 mpg again. This seems like way worse than I should be getting, even considering how many hills were involved in the trip. I was getting 28 mpg at the time delivering pizzas, lots of lights, traffic, stop signs, everything. After the trip I replaced my plugs, wires, dist. cap and rotor. I'm still getting about the same mileage as I was in my daily driving, so I'm not assuming my highway mpg has changed. So, here are the only other culprits I can think of:

I do have an exhaust leak, from a decent sized crack in the manifold before the first O2 sensor.
Aerodynamics? I haven't gotten my front end fixed since that accident in september, and the bumper cover is at least an inch or two off where it should be. Also, I don't have the fender well plastics, nor do I have the plastics under the engine bay on the car.
Gearing? Tires? I have the 16" 7gc 5-spoke wheels with 215/55 tires, and I'm not sure where those put me in relation to stock final gearing. My speedometer seems to be pretty accurate, so it doesn't seem likely that they changed my final drive a lot, but they are pretty fat, so I don't know about weight or higher drag.

So does anyone have any suggestions as to where my problem is coming from? I'd really like to get better than 25 mpg on the 600 mile drive I'll be taking to visit my family for christmas. I'm probably going to get an ebay header and magnaflow cat (and hope the new header doesn't bring any leaks of its own) to fix my exhaust leak, and I'm hoping to get the front end damage fixed soon too.

[Box]

Exhaust leaks can cause quite a loss in mpg. Increase in rotational mass can decrease mpg as well. I'd be more willing to bet it's from the leak, than the heavier wheels though. Could use some exhaust putty to fix the leak for now.

[mkernz22]

what was your average speed on the way there and back? I notice that if I stay right between 62 and 68mph, I get the best gas mileage.

[Galcobar]

A leak before the O2 sensor will likely screw up your AFR as the ECU gets inaccurate readings, particularly if fresh air is getting in through that leak -- the ECU will see too lean a mix and add fuel. Tada, poor mileage.

However, if you're just seeing higher gas consumption on the highway I'd be prone to suspect the lack of fender well and under-engine plastics are a significant cause of drag at high speeds. Plus it's better for engine and suspension durability to have them in place.

If the stock tire size is 205/55-15, your tire and wheel setup would put you 5.7% over the stock size, putting your speedometre 6% low (reads 56.5 mph when travelling at 60 mph). That means you'd travel 1.06 miles for every mile you recorded -- one gallon would take you 25 miles by your odometre, but 26.5 miles in reality.

[PaukST]

Given that the speed limits were typically 60-70mph, that would put my speed at about 70-80. I've gotta see if I can get a set of fender well plastics (they were gone before I bought the car) and I guess I'll fit the under-engine plastics back on. They weren't off for any reason beyond ease of access for working down there. I'm kinda wondering if it'd be wise to invest in some smaller, skinnier tires in the near future. Less weight, less drag, and lower final drive might make a difference (hopefully not having to work as hard per revolution would pay off despite running at higher revs for the same speed). I was pretty suspicious that drag may have been a big issue, and I'm going out on a limb and guessing that exhaust leak makes things worse at high rpm's than low.

Thanks for the input! Hopefully I'll have a better number to share after my next trip!

[mkernz22]

Given that the speed limits were typically 60-70mph, that would put my speed at about 70-80. I've gotta see if I can get a set of fender well plastics (they were gone before I bought the car) and I guess I'll fit the under-engine plastics back on. They weren't off for any reason beyond ease of access for working down there. I'm kinda wondering if it'd be wise to invest in some smaller, skinnier tires in the near future. Less weight, less drag, and lower final drive might make a difference (hopefully not having to work as hard per revolution would pay off despite running at higher revs for the same speed). I was pretty suspicious that drag may have been a big issue, and I'm going out on a limb and guessing that exhaust leak makes things worse at high rpm's than low.

Thanks for the input! Hopefully I'll have a better number to share after my next trip!

If you cruise at 60-70, then you'll also save a lot more gas cause out mpg just tanks downward after 65mph

[CAcelica29]

you could put the stock 205 50 16 size tire back on the 7gc rims. it makes almost no difference in anyway on my car running that size. i also have no fender plastics and still get around 28mpg.

[Box]

Well, going from 70 down to 55 is a 15% increase in mpg.