Installing Bigger Wheels
 

I have a 1994 Honda Civic DX 4 door. Right now it currently has 13 inch wheels. By installing 14 or 15 inch rims/tires would it improve my fuel economy?

You need a tire/wheel combo with a larger outside diameter than what you have now. Your car will be slower through all the gears, but on the highway, your engine speed should drop and efficiency will increase by a little bit.

My little brother had a 1992 Lexus SC400 and he upgraded from 17 inch rims to 20 inch and he said it killed his FC. Maybe this might be because the 20 inch combo he got was alot heavier?

Also I heard, I'm not sure if it's true but the speedo and odometer gets an inaccurate reading. IS this true?

Yes, the speedo/odo can change if the OD changed, if that size is a stock option, you can generally get the right speedo/odo components so the readings will be correct. When people go from big rims to really really big rims, especially on an auto, it can increase the amount of time spent downshifting and keep the tc unlocked which can hurt mileage. In any event, there's only so much you can fit in wheels wells, so the really tiny tires that go on really big rims probably don't have the best rolling resistance. What was the difference in OD going from 17s to 20s?

going up in wheel size may well decrease the torque at the wheels, making it harder for the engine to turn the wheels, meaning an increase in more gas burnt, lower MPG, etc. However once at highway speed, the engine turns less times for a given speed, so this issue is not linear, to be sure. Perhaps in city stop and go you'll get less MPG, but perhaps more on the highway.

There is a relative maxima in the MPG curve: as the tire size increases, MPG increase but to a certain point in the curve, then increasing the tire more will lower MPG...there is a sweet spot in tire size.

I think 90% or more of the time, people that put the over sized tires on their vehicles will actually experience less torque at the wheels (power) and less MPG....it's just for show...just like most of those wings you often see the backs of cars....these give no benefit...just for show...I would suggest sticking with the stock factory sized wheels, but I could be wrong on this.

I tried putting larger tires on my 87 Acura Integra and was surprised to see that it decreased my mpg. It may have something to do with the car riding higher and allowing more air to travel under the car.

I would guess that each vehicle is different and that it may help some and hurt others depending on aerodynamics, torque etc.

Bigger tires mean less torque at the wheels, which means more engine effort to turn those tires, which can translate into less MPG.

Are the bigger tires are wider or taller, or both? Narrow tires will tend to give better mileage. Wide tires will give worse mileage:

Wide tires have more tread on the road. Wider tread means more tread deflection, which increases rolling resistance.

Wide tires have more frontal area, increasing wind resistance.

Wide tires and some rims have terrible drag coefficients.

If you want better mileage, install SKINNY tires on stock rims: I put skinny 235/85R16s replaced fat 265/75R16s. I'm now getting close to 20 mpg out of a 1 ton pickup.

The best of all worlds is to install tall & skinny tires. Taller tires would decrease engine rpms and use less gas. However, tall / skinny tires and suitable rims can be very hard to find, since car tuners all want the pimped look.

If you make a tire / wheel change don't forget to adjust your speedometer for the different tire diameter.

It seems that manual transmission swaps (switching to a trans with different gearing to lower cruising rpms) help most cars' mpg. I believe that my swap helped my little 87 Civic (I switched to an HF trans from an 84 CRX- the one that was rated for 50 mpg on the highway).

In a way, the trans swap also lowers torque at the wheels (by having a "higher" gear). So it seems like using bigger tires is simular to a mini trans swap that also messes with the aerodynamics.

I know several of us have done a trans swap and all the news I have heard has been good- although in theory a person could be hurt if their engine didn't have a high enough torque peak at the rpm that they would use on the highway. From what I have read here, it seems like there have been mixed reviews when it comes to increasing tire size.

AND, I think that for a given tire diameter, a smaller wheel diameter gives better mileage. But I can't prove it.

For example, if the max that your car will allow is a certain 215/80R15, then that will have lower rr than the same diameter 215/??R18.

I think that it has to do with the sidewall stiffness...