INSTALLED 14" WHEELS - MPG IS WORSE??

[Housewiz]

Thanks to a previous post, found a set of Ford Escort 14" wheels and installed 205 tires which appear to have a 1" larger diameter and a few less revolutions per mile vs. the 13" stock wheel/tire set up. Was getting 30 MPG w/the 13 inchers, now getting 25MPG w/the 14 inchers.

Ideas??

[MR2Di4]

Remember that you're not driving a muscle car and if you've increased the diameter, rolling resistance, and possibly the weight at each wheel your engine is having to work harder to make revolutions. Harder work means more fuel burnt. The 3A-C isn't exactly a torque monster, so seemingly small changes can have a big effect.

MR2Di4

[Housewiz]

The 13" tires/wheels were visually unappealing - they made the little wagon look like a little toy. After installing the bigger skins, the motor slowed around 500 RPMs at 60 MPG so I was hoping the added inertia of the larger/heavier wheels/tires would be equaled out.

[4wdchico]

The 13" tires/wheels were visually unappealing - they made the little wagon look like a little toy. After installing the bigger skins, the motor slowed around 500 RPMs at 60 MPG so I was hoping the added inertia of the larger/heavier wheels/tires would be equaled out.

So, how are you measuring this 60 MPG, guessing you mean MPH? By the spedo, timing a known distance or GPS? I need more info before I can give you a relevant answer.

For example: If you are driving with, say 60 MPH indicating by the stock speedo, and reading 3k RPMs on the stock tach in 5th gear, this will not change with bigger or smaller diameter tires. Your actual speed would change a bunch with large changes in tire diameter. Matter of fact, you can put your car up on jack stands and take the wheels off and it will still have the same 60 MPH speedo reading to 3k shown by the tach in 5th gear relationship. To change that relationship you would have to change the gearing in the car's transmission, or diffs.

[Petros]

change in the engine rpms usually does not affect economy that much for the same speed. the weight of wheels and tires only matters on acceleration and deceleration. aT steady state speeds there will be no affect at power required. depending on the torque curve, usually the way it works out the lower rpm means you will have the throttle open further for the same steady state speed, and that usually results in higher fuel economy. but again that depends on the shape of the torque curve vs. throttle position.

In general however, that was the justification for an overdrive 5th gear, it lowered the engine rpms, and presumably improved economy. But the engine is putting out the same hp, it takes the same hp to push the car at the same steady state speed, but the throttle being open further usually reduces induction losses and improved engine efficiency slightly.

[deejay1272]

Don't forget about tire pressure. a 10 psi difference can cause a material loss of MPGs. Check it out and make sure you're running at just over 35 psi in all 4 tires...

[dlb]

housewiz, i had the same problem a few years ago when i tried running larger tires. remember that larger tires will complete less full rotations than a small tire to drive the same distance. this is what causes the speedo, odo, and trip meters to read incorrectly. you need to figure out the exact % of speedo, odo, and trip error to accurately gauge your mpg. you only mentioned that the new tires are 205's (which measures the width of the tire) but we also need the second # (the height of the tire from the wheel to the tread) to find the error percentage. hypothetically, if your stock tires were 175/70R13 and your new tires are 205/65R14, your speedo, odo, and trip should all read about 8% too low. so...

30 mpg - 8% will read as 27.6 mpg
60 mph - 8% will read as 55.2 mph

so remember to account for error due to tire size when you check your next tank and add whatever the error percentage is for your particular size of tires. that explains part of your apparent loss of economy. i would chalk the rest up to more city driving or different driving habits on that particular tank. when tracking fuel economy, look at trends as opposed to single tanks. and don't forget to account for error when driving. it's easier to speed when your speedo is reading low.

this is the online tire size calculator that i usually use:

http://www.1010tires.com/tiresizecalculator.asp

[Housewiz]

Tires are 205/70R14's

Decided to check the MPG again. Drove 200 miles back and forth to work - 95% highway. Drove 55 MPH constantly as I believe most MPG data is based on driving 55. Took a bit longer to get to work but what the heck - it's all for scientific evaluation, eh??

199 miles divided by 6.6 gallons = 30.15 MPG. Now those 14" wheels and tires don't look so bad. Motor ticks over a bit less, the cabin is a bit quieter, and the car looks way cooler.

How do I post a photo so you guys can see the little Road Rocket??

Thanks,

Steve

[dlb]

good to hear all is well again. just checked your actual tire size against stock for comparison:

30 mpg will read as 26.5 mpg
60 mph will read as 53.7 mph

i upload all my pics to my photobucket account and then copy/paste the IMG code.

[teranfirbt]

Engine friction also has a large part to play in efficiency, which is the primary reason for dropping engine RPM with an overdrive gear.

[4wdchico]

Engine friction also has a large part to play in efficiency, which is the primary reason for dropping engine RPM with an overdrive gear.

Yes engine friction is a significant factor. The biggest fuel econ gains from higher gearing are, more often than not, from a larger throttle opening and it's lower frictional pumping losses past the throttle plate as Petros said earlier.

The problem with a low powered carbed car like a t4wd is that running significantly higher overall gearing via larger tires can fairly easily cause lower fuel econ by dropping manifold vac to the point that the power valve in the carb opens and enriches the fuel/air mix out of an econ ratio into a richer acceleration mixture.

[Ace]

Has anyone figured on using larger 14" wheels and cruising in 4th gear instead of 5th to keep the engine revs near the same? Wondered if that would give similar fuel economy with the larger tires. I've debated trying larger tires for more ground clearance. How large can the tires be without scraping in the fenderwells? I realize that larger tires will affect steering, brakes, suspension ... enough to be a problem? I would be interested to hear about other folks experiences. So far I have kept my '83 Tercel all stock except for no muffler, just the cat.

[Housewiz]

We put on 205/14" w/very little rubbing - only at full lock. Never while driving.

[Petros]

I have run the larger 14" wheels for many years, with lower profile tires. the rolling dia of the wheel/tire combination is the same. do not assume that larger wheels means larger rolling dia at the road. you should be talking about overall rolling dia, which is independent of wheel size.
that being said, you need to adjust the speedo/odo error IF your rolling dia is different before you know if there was actually a change in fuel economy.

Larger rolling dia will affect both the speedo and odometer, to make it appear you are traveling slower and driving fewer miles, which could result in under reporting fuel economy. Also, how you measure fuel economy is important, it is not always obvious on how to do this accurately. No one here yet has said how they are measuring it, what driving conditions, etc. Also, wider tires and tires with more aggressive tread design (i.e. all-season tires) will also deliver less fuel economy. Skinny, thin and fine treaded tires pumped up to 50 psi will give the best fuel econmy, but would ride harsh, handle and stop poorly and be unsafe on wet roads. So tires are not really a good place to try and get better fuel economy, what is the point in saving a few $ in fuel if you wreck your car?

Without accurate information, we know nothing about how fuel economy was affected, if at all. I suspect if done in controlled conditions, with proper measurements and adjustments made, there will not be much of a difference.

[dlb]

anyone interested in getting the best mileage they can (aka hypermiling) should check out forums like this one:

http://ecomodder.com/

pretty impressive what some people have accomplished. from what i've read though, it sounds like while things like lower cruising RPM do increase mileage the biggest differences are made by simple driving habits. i'm actually looking into installing a vacuum gauge to help me improve my mileage.