box fuel mileage, a nice surprise?

[gadget73]

i've personally gotten 22 in his car. My own car driven the same way gets about 18. Speedo is pretty darn close in both. Mine reads very slightly optimistic, at 70 on the speedo I'm doing about 68 on the gps. I've never logged the odometer to see if it responds differently but its never given me clear indication that its off enough to worry over.

[GM_Guy]

which equates to roughl7 27mpg (90% highway). Is that normal for these old stock boxes? I'm curious to see how it changes with

When conditions are right, yes it is normal. My '89 (3.08 gears) would regularily hit the high 20's when taking road trips. My best ever was 29mpg. Very little traffic and likely had the wind to my back that day. But I routinely hit 25/26, occasional 27mpg on that same trip. Around town though easily half that.

Alex.

[tjc78]

Both my boxes never got better than 24 all highway.

The 04 mimics Sly's 00 numbers. I did hit 25 on a long straight highway run where I kept It around
74 the whole way.

[DerekTheGreat]

The scientist/engineer in me has many factors to consider before believing #'s. Altitude, temperature, tires, tire psi, downhill duration, speed ect all that play into it. I remember when we took Ashley's 89 down to Virginia there were lots of hills where I didn't have to touch the gas much at all. It got around 25 or 26 that tank IIRC. That was also with an average speed of about 60. I just don't believe that these bricks on wheels will do much better, certainly all the data I've collected between my old '85, '88 and her '89 supports that. It's interesting to keep track of but like Thain said, I just put gas in it. It was worth a grin to fill the tank until gas was standing in the filler neck and then drive over 330 miles until the next fill-up. I didn't think these things were good for much over 250mi/tank. Aeros & whales however...

[turbo2256b]

Best for Creamy was 27 to 32.9 highway and 20 to 22 in town with 3.55 gears. Tire diameter and speedo gear were dead on as can be ie full tread off just a tad like 1 mph half tread right on and off about 1 mph with tread down to the point for replacement. I do believe the optimum gearing for economy might have been using 3.2 gearing. Running a 400+ mile trip from place to Detroit would be about 1/4 to 3/8 of a tank left. The 27 mpg was mostly in KY were there are no flat lands even the live stock have 2 legs shorter here. 30 to 32.9 was more common in OH and MI.
Stop and go driving even on highways (stopping to fill up and or empty the bladder) has more to do with economy in heavy vehicle. Aerodynamics has the most to do with highway economy at a steady speed.

The head light covers make a difference on Creamy at speed even tried running with one off and one on the differance could be felt as it would pull to one side. Wish I had had my air damn on for testing at that time as it seems to help quite a bit.

[turbo2256b]

So I did a little test today to see what my fuel mileage would be, given my stock 302, with the 2bbl, stock duals, and 3.55 gears for curiosity sake. I drove around 460 km's from a fresh full tank to the next gas station and put around 53 liters in which equates to roughl7 27mpg (90% highway). Is that normal for these old stock boxes? I'm curious to see how it changes with the new heads/intake/4bbl thats going on in a couple weeks.

3.55 gears and 26.7 ish dia tires with the correct speedo gear work out the most accurate cant remember the speedo gear color which can also depend on the sspeedo drive gear in the trany

[knucklehead0202]

My '98 p71 was phenomenal on the highway and decent around town, but I only drive the freeway a few times a year, lol. The '90 does around 25 on the highway, about half that in down. Haven't fixed my odometer yet but my freeway drive to palm springs is a pretty well-known amount of miles to me at this point. Can't wait to get the car back but i'm sure the mileage will not improve with all the shit that just got stuck on it.

[VicCrownVic]

My best was 25 MPG in my '89 with the most ideal conditions ever. Just over 5 hour trip at 55-60 MPH with any wind traveling the same direction I was and elevation at the end of the trip was lower than at the beginning with very few hills in between and stops countable on one hand.
If my '88 wagon had the chance to do that kind of trip it probably would have done better since that car consistently did better than any other box I have owned.
Typically I have seen 15-17 MPG city and 20-21 MPG highway with all my boxes.

[DerekTheGreat]

My best was 25 MPG in my '89 with the most ideal conditions ever. Just over 5 hour trip at 55-60 MPH with any wind traveling the same direction I was and elevation at the end of the trip was lower than at the beginning with very few hills in between and stops countable on one hand.
If my '88 wagon had the chance to do that kind of trip it probably would have done better since that car consistently did better than any other box I have owned.
Typically I have seen 15-17 MPG city and 20-21 MPG highway with all my boxes.

This is pretty much exactly what I see with our boxes. Ashley consistently averages 17.5-19.5 with her box TC. FWIW the GPS & cop radar things along the road indicate the same speed her speedometer does and AFAIK nothing in the driveline has been swapped out. These are the numbers I'd believe. Back when the Firebird was all stock with its 2.73's it had ~30ft-lbs more torque & ~20 hp more than a box, not to mention way better aerodynamics & far lighter. Best I remember pulling with that car was 25-26 on the expressway @70mph.