Effect of weight on MPG

The rule of thumb I remember from an engineering book is this:

Half of the percentage change, in the opposite direction.

So e.g. if you add 10% more weight, fuel economy will drop by 5%

 

I'm not sure where the 1mpg per 50lbs came from but that cannot be right. It sounds too high. I would be more likely to believe 1mpg per 100lbs but even that may be too high.

Here is a neat paper showing simulations with weight reduction effects on fuel efficiency and acceleration.

Pg. 23 and 35
http://aluminumintransportation.org/downloads/AluminumNow/Ricardo%20Study_with%20cover.pdf

The paper supports Sagbrush's general rule of thumb but it's not quite as generous.

Something else to think about before you go tearing your car apart.

"Reducing vehicle weight (mass) results in less tractive effort required to accelerate the vehicle and less rolling resistance from the tires. Drive cycles with more acceleration events (EPA city and European) show greater fuel economy benefits from weight reduction than highway or steady state conditions. Also, at higher vehicle speeds the engine is typically at higher throttle (better BSFC) operating points and provides less opportunity for improvement. Since the tire losses are a greater percentage of total tractive effort at lower speeds (aerodynamic losses increase by velocity squared) the potential for fuel economy gain from weight reduction is greater at lower vehicle speeds."

 

Weight matters more when most of your MPG is due to coasting at speeds under 40 mph on a smooth road and when your going uphill (up a side of a mountain). To counter the effect of gravity (weight) of alternating hills, hypermiling conserves gravitational energy with a technique called Driving with Load. How weight is distributed matters - more cargo in the rear underneath the rear axle means more traction on the rear axle but also higher rolling resistance on the rear tires. The more surface road tire contact the higher the rolling resistance and the lower the fuel efficiency.

 

Lowering the car due to weight could indeed change the aerodynamic effect on your mpg but it may not be beneficial. There are too many unanswered questions regarding how lowering would affect mpg on a Prius because it has a fairly clean underbody. Studies has shown that reducing ground to body clearance on a vehicle with a smooth underbody can increase drag whereas lowering a vehicle with an aerodynamically dirty u derbody can decrease drag. Which is the Prius considered and what is considered the breaking point where lowering is actually detrimental?

I say take any extra junk that you don't need out of the car, continue to lose weight and workout so you can get this Tiffany Yep girl you are crushing on.

 

if you are carrying over 500 pounds ( driver+ passenger+ cargo) - I would recommend limiting the tire pressure to 40/38 psi (+5 psi over the recommended setting ) to prevent putting too much stress on the shock absorbers and suspension.

Added weight increases traction which is sometimes useful when driving over the snow and over ice - however, in this scenario - your MPG will drop because it takes more power to move from A to B because due to the lousy road surface traction..

Increase road traction is good to have in high speed racing especially when cornering at high speed. However, high speeds and cornering at high speeds will lower a Prius' fuel efficiency so you won't notice any improvement in MPG with the added weight. At higher speed -- aerodynamic drag lowers a vehicle's fuel efficiency. When cornering at high speed -a vehicle loses some of its momentum (stored mechanical energy) via centrifigal force. The higher the turning speed the higher you'll notice a drop in MPG.

Hypermiling can mitigate some of the effects of gravity but not all - road conditions often don't allow for a full implementation of DWL but its enough to give you a 10% boost at times on a smooth dry road at speeds under 40 mph. For myself - aggressively applying DWL during the summer time (in hilly terrain roads) can often increase my overall MPG from 64 to 70 mpg ( driving temperatures must be >65F, trips > 15 miles/40 minutes long, top speed limit is 50mph - median top speed is 35 mph, tire pressure set to 44/42 psi ).

The best and most popular Prius mods for better fuel efficiency is a scangaugeII (gph, rpm, avg, fwt) and low rolling resistant tires.

Lesser known 3rd gen Prius MPG mods are light aluminum wheel rim, rear wheel skirts, electric block heater, Plug-in conversion(Enginer).

 

How much weight does a full tank of gas add? I use this an an excuse for why i let my PiP go down to the last pip before I fill up. It's not the real reason. Just an excuse.

 

See:
How much does a gallon of gasoline weigh
and multiply by the number of gallons you pump at a typical refill.

 

Everything being the same, I find it sometimes easier to get slightly better fuel efficiency (by 1 or 3 mpg) on the last 2 gallon of gas than the first 2 gallons of gas right after I fill up. If I'm very attentive and lucky - I can sometimes get the Prius to glide a bit longer on a near empty tank... I want to avoid driving until I run out of gas so my challenge is to estimate/know what is left in the gas tank after the Prius fuel gauge gives me the beep warning and the last bar starts blinking - which signals the Prius has about 2 gallons of gas left. Depending on whether Prius on an uphill or downhill or level road - my ScangaugeII Xgauge fuel gauge (Flv) readings will swings back and forth with +/- range/error of about 0.6 gallon. Last time when the Flv drop to 0.9 gallons - I gave up and refilled the tank(10.1 gallons of gas/681 miles , I had drive about 30 miles after Prius's MFD estimated remaining range left drop to zero). I am hopping to squeeze +700 miles out of my current tank ..However, I'm not expecting the FE bump from the last 2 gallons to make much of an impact on my mpgs.