Weight — and gearing — have a huge effect on fuel economy. Even to the extent of almost eliminating 40 years of “advanced” technology, including computer-controlled electronic fuel injection. To make the case, I offer the following comparison:
I own one relatively modern vehicle — a 2002 Nissan Frontier pickup — and one extremely not-modern vehicle — my infamous 1976 Pontiac Trans-Am, aka the Orange Barchetta (or Great Pumpkin).
These two are very different — and very similar.
One is a truck and has a fuel-injected, computer-controlled four-cylinder engine. The other is a muscle car with a huge V8, fed by a carburetor controlled by my right foot.
The V8 in the TA displaces 455 cubic inches, which amounts to 7.5 liters in today’s anodyne, metrosexualized metric-speak (how that took over is fodder for a future rant).
The Nissan’s four is a mere 2.4 liters, less than one-third the displacement.
Yet both vehicles get about the same gas mileage.
Part of the reason for the Nissan’s dismal mileage — 20 city, 23 highway, according to the EPA and less according to me, its owner, who has direct real-world experience of its actual mileage — is because the thing is heavy (about 3,300 lbs.) and underpowered (just 143 hp to pull all that around).
Plus, one other reason, which we’ll get to presently.
The TA is even heavier — about 3,700 lbs. — and that big V8 makes more than twice the horsepower.
Despite that, it hardly uses more gas — and sometimes, less. On the highway, the TA has achieved the EPA’s best-case advertised mileage for the Nissan (23 MPG) which in real-world actual driving is more like 18 or 19.
This disparity puzzled me . . . until I noticed the tach.
The Nissan’s cruise RPM — how fast the engine is spinning — is higher than the TA’s cruise RPM at greater road speed. At 60 or so MPH, the little truck’s four is buzzing along at about 2,500 RPM while the TA’s engine, at 70, is burbling along at barely 2,000 RPM — not much more than a fast idle.
The TA can achieve 75-80 without the RPMs rising much higher, too.
Then I remembered some other things — and looked up a few more.
About ten years ago, I replaced the TA’s factory non-overdrive transmission with a transmission that did have overdrive gearing — a very deep .067 overdrive in top gear. This transmission counteracted the fuel-sucking effect of all those liters (or cubic inches) connected to comparatively aggressive 3.90 gears in the TA’s rear axle. Without the deep overdrive gearing in the transmission, those 3.90s would have the TA’s big V8 screaming at around 3,500 RPM at 65-70 MPH and using more gas than the Nissan’s worst-case actual mileage.
Which brings me to why the Nissan uses as much gas as it does — which is as much as a ’70s-era V8 muscle car — notwithstanding its smaller engine, lower curb weight and all the technological advantages of the preceding several decades:
It hasn’t got overdrive gearing as advantageous as the TA’s.
The truck has a five-speed manual transmission — and fifth gear is an overdrive gear. But the ratio is .81 — which isn’t as low (or as high, depending on how you prefer to put it) as the .67 overdrive ratio in the TA’s transmission.
It also has an even more gas-hungry (and RPM-escalating) 4.63 ring and pinion, probably necessary because there’s so little engine and not much power and when that’s the case, you badly need leverage to get 3,300 lbs. of truck moving decently.
Nissan could have gotten maybe 25 or better — actual/real-world mileage — out of my little truck by using a less aggressive ratio in the rear axle, something like a 3.08 or even a 2.73. This would have reduced engine RPM in top gear at cruising speeds considerably. It also would have made my already lethargic truck — zero to 60 in 11.4 seconds, slightly off the pace of a Prius givin’ her all she’s got, capn’ — practically cataleptic.
But a six-speed transmission, with a deep overdrive in sixth — would solve both problems. The truck would be quicker, probably (tighter gear spacing, more advantageous leverage) and — once up to speed — it would be more fuel-efficient.
I looked up the specs of the current (2019) Frontier, which is a bigger (mid-sized) and much heavier (4,200 lbs.) and enormously more powerful truck (261 hp from its optional 4.0-liter V6) and discovered that its EPA-rated mileage — 16 city, 23 highway — is practically dead heat with my truck’s mileage.
The highway numbers are in fact identical — 23 MPG for both.
The bigger/heavier (and bigger-engined) 2019 truck uses more gas getting going (16 city) but once it’s rolling, the .76 overdrive in its six-speed transmission makes up for a lot.
And makes a much bigger real-world difference than annoying gimmicks such as ASS — Automated Start/Stop and quite possibly things like cylinder deactivation and direct injection, too.
If an old muscle car like my TA — with one of the biggest and thirstiest V8s ever put into any car — can approximate the mileage delivered by a modern vehicle with a small, fuel-injected and computer-regulated engine just by adding deep overdrive gearing to the mix — imagine what could be done with a more reasonably engined vehicle.
Without even needing to add fuel injection or computers to the mix.