DRAG-REDUCTION vs BSFC

[aerohead]

Originally Posted by Christ

Let's not forget that tire sizing is a factor in overall drive gearing, and might be a cheaper alternative than actually swapping gears in your vehicle, especially as a "test" setup, where the result might not be optimal.

Right! The issue of clutch-slipping at dead starts with manual transmission cars might defeat"life with taller tires",although it is sure an economical way to investigate gearing.

[MetroMPG]

The challenge in going to significantly taller tires is that they also raise the vehicle height, so you're changing 2 potentially important variables, even if you manage to hold width and rolling resistance constant.

[aerohead]

Originally Posted by MetroMPG

The challenge in going to significantly taller tires is that they also raise the vehicle height, so you're changing 2 potentially important variables, even if you manage to hold width and rolling resistance constant.

Yep,I don't want to even go there.I mentioned elsewhere to some member that I'd have to change the shackles or re-arc the leaf-springs to restore original ride-height,and then I'd still lose all kinds of clearance inside the wheel house.I wouldn't be able to climb grades without down-shifting,acceleration on freeway on-ramps would suffer,"passing" would be an issue,it just goes on and on!------------------ The Tremec 6-spd trans is something I'll think about.-------------- An engine swap is intellectually a delicious thing to think about also,although intimidating, considering the truck is daily-driver and used in construction/work/income.

[jime57]

Great thread. Sure helped get my thinking back on track after being led well into the wilderness by another well meaning soul. Thanks Aerohead for the great summary in post #19 - ironic since it was post #19 from another site which led me off track in the first place ;-)

[wwkayaker]

When adding larger tires, remember to change the gears for the speedometer and odometer. I got three speeding tickets in 2000 within two months after going to one size larger tires because it through my speedometer off by 5 mph (e.g. 74 mph on speedometer =79mph shown on last ticket). I assume this would also throw off the odometer (1 mile could = 1.1 miles).

[wyatt]

Originally Posted by wwkayaker

When adding larger tires, remember to change the gears for the speedometer and odometer. I got three speeding tickets in 2000 within two months after going to one size larger tires because it through my speedometer off by 5 mph (e.g. 74 mph on speedometer =79mph shown on last ticket). I assume this would also throw off the odometer (1 mile could = 1.1 miles).

Yes, at least it does on mine. I am running 155/80/R13 (instead of the 155/70/R13 recommended since they are no longer easy to find) and my speedometer and odometer are both off. On my drive into work this morning the GPS said I traveled 14.08 miles, the odometer said 13.3. Over a 350+ mile trip the GPS and ODO were off by about 5.6%.

[Big Dave]

MazdaMatt noted:
“…Lower TP at the same RPM puts your engine in a less efficient point on the BSFC chart?”

Big Dave comments:
Absolutely true for an Otto-cycle engine. Maybe not true for a diesel. Diesels do not have throttles and hence have flat torque curves and very large BSFC ”islands.”

Even with a gas engine, sometimes the loss of BSFC is less than the overall fuel burn rate so you still get an improvement in MPG, just not as much as the proportional reduction in drag would indicate.

This increase in BSFC is the nugget of truth in the old urban legend of driving at higher RPMs gives better MPG.

If you can achieve a reduction in CdA, then the correct follow-up is to re-gear to keep yourself in an efficient range at the desired road speed.

As I keep saying, gearing is (like aerodynamic improvements) a powerful tool for the real seeker of MPG.

Larger diameter wheels/tires are counter-productive and by no means a substitute for changing gearing. Larger diameter wheels add heights and frontal area and worse yet, add rotational inertia. If bigger wheels improved MPG, then all the "Bigfoot-Wannabe" lifted trucks should get terrific MPG.

[thndriia]

Originally Posted by Big Dave

Larger diameter wheels/tires are counter-productive and by no means a substitute for changing gearing. Larger diameter wheels add heights and frontal area and worse yet, add rotational inertia. If bigger wheels improved MPG, then all the "Bigfoot-Wannabe" lifted trucks should get terrific MPG.

I agree that increased rotational inertia and "in general" larger diameter tires will be counter-productive (for fuel economy), but I wonder if the effect is reduced on a vehicle with a taller ride height.

For example, a 4wd pick up truck has a tall ride height. So I speculate that on a truck with aero mods, increasing the tire diameter slightly (and possibly reducing the tread width), may bring the BSFC back into an efficient island; without significantly increasing the aero due to ride height increase, as there is already so much air traveling under the vehicle in its stock configuration.

Getting down to this level of detail, the amount of rolling resistance a particular set of tires produced would also have an impact. So, some educated tire shopping would be required.

[MazdaMatt]

thndriia... you goofed your quote. I never said that.

[thndriia]

My apologies. That was my first post on this forum, so I'm still working out the details.