Depends on how much you abuse it, your normal highway cruise speed, tire size and so on, but for 'normal' driving in a stock '78 in good tune, figure on 15-16 around town and low 20s on the freeway. Putting a 5-speed in will help a bit on the superslab, as will driving at or even slightly below the speed limit — something not easy to do in a Z. I've seen claims as high as 26-28 mpg on this forum, but I've never been able to get that in my '76 or my '82 turbo.
hahaha...the best i've been able to get outa my 83 280zxt running 12psi and 225/50 tires AT tranny, has been about 12-14 city/18-21 freeway...its almost as bad as my moms 85 mercury grand marquis with a V8...hahaha
I am getting 26 to28 on short trips [ all 15mi. or less ] as longe as I keep my foot out of it.
The 4spd should be as good as 5spd in city driving.
The wifes 94 Merc. G--M got 30 pulling a car trailer 150 mi. & 28 coming back with a 83ZXT. All I/S driving. Am supprised at the mi/gl from the big V8, but it was bilt for I/S crusing, I think she is getting 15+ on short driving, don't know how ?
The best car I have had for mi/gl was a 85 Stanza, 4cl. 5spd had 2 plugs/cl it got mid 40s I/S driving. I would like to have that peppie little moter in a Z for a day driver.
I drive to Daytona in my automatic with air on and I make it there with fumes.
I make the same drive with t tops off, no air and I arrive with a quarter tank.
When in the 5 speed air on 1/3 tank, air off I make it in just under half.
It's all in that air conditioner, that thing sucks up gas.
Broward County Z Club
Monday the 22nd at 6pm
Sheridan St and 56th Av
Don't know about the 78ZX but my 78 Z gets 21mpg average...city and freeway combined...auto trans...average driving style (I flog it once in a while too)...mpg drops to 17mpg or so on winding/mountain roads...car is kept in top-notch tune...
and with A/C I still get 17-18mpg...most gas mileage (IMHO) depends more on
the driver than any other single factor... )
My 78 Z with 4 spd gets around 19-20 just going to work and bumming around (mostly country roads with some city driving). On a recent trip to a dragstrip that was 50 miles away from home, it got 21.5 mpg for the 105 mile round trip which included 3 trips down the 1/4 mile (16.05 best run totally stock). This figure isnt that great until you figure that I ran about 60 mph @ 3100 rpm in 4th gear. I dont think my rear tire circumferance is the same as stock because of the high rpm out on the highway. Just think how good this mileage figure will be when I put a ZX 5 speed with a .750- 1.0 5th gear rather than the Z 4th gear at 1.0 - 1.0 ratio. I CANT WAIT!!!
I had an 82zx N/A 2+2 with 5 spd and it got 20 city and 28 hwy on long trips to
Washington state from Las Vegas Nevada.
My 82zxt automatic gets 16-18 city and 20-23hwy. (these 3spd autos suck at 75mph you are at nearly 4000rpm)
I have a '77 280Z. Originally it had 101,000 miles on it and had the 3.54 rear end with the standard 4-spd. This would be the same as the way the stock '78 is equipped. My records show that initially I averaged 20-22 mpg with it but, as I got braver and more aggressive in my driving that it dropped to 17.5-19 mpg. On a highway trip it got 23.5 mpg. Since then I did a tranny and differential swap. It now has a 5-spd and 3.90 rear end out of a '83 280ZX. My aggressive driving continued and my mileage continued to be in the 18-19.5 mpg range. Pure highway netted me around 24-25 mpg. Recently I got my foot out of it and I've been averaging 20-22 mpg again. The car now has 195,000 miles on an original engine. Never had the head off. for what it is worth. I record my gallons of fuel at each fill-up and compute my gas mileage for each tank of gas. I have a steno book with 9 years of mileage and repair history on this one car so my numbers aren't guesses. Another item of interest - the owners manual recommends 91 octane fuel. I've been running mine for two years now on 87 octane and have had no pinging, knocking, or any other negative effect on the car. No change in fuel economy either.
Hope this helps on your decision.
That steno book for keeping records is a very good idea. I have kept one on all my motorized toys for several years now including 78 pickup, 89 conv. van, 78 Z, 93 Saturn, 2 racing go-karts, diesel JD utility tractor, 2 ATVs, and original Cub Cadet show tractor...... I can look at this book at any time and know when I need to change oil or lube, or what mileage I get from season to season.