TheGreenZ28

Hey all!

I was looking at LT4 conversion kits as something for a future project, and while searching I decided to look up how much mileage I could look forward to losing after the swap. Turns out, after looking at a lot of forum posts on other camaro/LT1 sites, people barely lost mileage, some even gained mileage.

Now, that's a good thing. More mileage and power? Gimmegimmegimme. I have a problem with one minor thing though:

Everyone says their highway mileage decreased to 25 or 24 or some gained mileage to 24 or 25, which made me wonder why my car seems to only get about 15 mpg.

Yes, 15 mpg. And my car is basically stock, minus the 3.42 gears and the new air filter. I get about 240 miles to a tank (even if I top it off), and 90% of the time I am driving on the highway. 240 / 15.5 (fuel tank size) = 15.4mpg. And no, before you ask, I do not drive crazy. I drive very normally, even conservatively sometimes to try to save gas (not going over 2.5k RPM when accelerating, etc).

My question is this: why is my gas mileage so bad for a basically stock Z28 when others who have hotcams in their cars are getting mid 20s? Do I need to send my PCM in for a retune? What else could be affecting this? The O2 sensors have been replaced, the emissions is passing with flying colors, so why is my gas mileage so significantly low compared to others who have more performance mods than I do?

Thanks!

~Nick

Injuneer

My suggestion would be to figure the actual mileage you are getting more accurately. Miles/tank is far from accurate.

At what point do you refill the tank? It can't be 15.5 gallons, which would mean you only refilled it when it was empty. So you can't divide miles driven by 15.5 unless you ran it dry. In your example, if you drove 240 miles and only filled it with 12 gallons (typical amount used when the needle is approaching 1/4 tank)... you are at 20 MPG, not 15 MPG.

Fill the tank, record the odometer miles. When you refill, record the new odometer reading, and subtract to find the miles driven. Then divide the resulting miles driven by the actual gallons pumped. Do this for several tanks.

When people claim 24-25 MPG for highway driving, that is typically ALL highway. With my M6 (3.42 stock gears) I got up to 30 MPG driving totally at 70+ MPH in 6th gear on CA freeways outside the city areas. But, driving on near-city freeways, with some driving to and from the freeways, 21-22 MPG. In ridiculously heavy NJ bumper to bumper commuting traffic, it was 17 MPG.

In your example of 90% highway, you have to use a weighted average of the "city" driving number and the "highway" driving number. The EPA ratings for a 96 A4 were 23 highway/15 city. You'll be off a small amount for the 3.42 gears, since the EPA number does not appear to differentiate between the 2.73 and 3.23 gears available with the A4.

TheGreenZ28

I will fill up before going to work tomorrow and I will also fill up after my entire day of driving and I will update with what I got!

Not Dave

If you had stock gearing and reset your trip meter after filling you take the trip miles divided by how much gas it took to fill back up again. Being modified your odometer/speedometer will be off unless it's corrected. A GPS will give you exactly what you drove. Get a smart phone app or use a GPS to track actual miles driven.

I don't know where it is on these cars but every motorized vehicle has an optimal sweet spot between speed, RPM and throttle where it gets its best MPG. Now that my skip shift is eliminated I don't wait to shift at higher RPM's, not that 3k is less efficient than a 2.5k shifting point.