[forums123]

HERE'S my .02 cents on the tire size question. Take the difference in tire height divide by tire diameter, then multiply by 65mph or whatever highway speed you cruise at to get the difference in speed from your odometer reading after tire change.

EXAMPLE: old tire 21" tall, new tire 21.5" tall
Difference in height is 0.5"
Divide by old tire height 0.5" / 21" = .0238
Multiply by hwy cruise speed 0.0238 * 65 mph = 1.55 mph change in speedo

Changing to a Taller tire will cause speedo to read less than actual
Changing to a Shorter tire will cause speedo to read more than actual

in this example if your speed were reading 65mph after switching to the taller tire(21.5" diameter) your actual (true speed) would be 1.55 mph less than the speedo reading 65 - 1.55 = 63.45 mph (actual speed)

The effect also reduces your engine RPM's which could reduce fuel consumption assuming your engine has sufficient torque at the given speed and throttle setting. Your power/torque curve of your engine is not constant and generally as RPM decreases so does power and torque. So too tall of a tire could require more throttle to maintain the same speed thus offsetting any potential gains. A small change such as .5" in tire height is not bad though.

Changing to a taller tire also affects the final ratio in all your gears, so when you are accelerating your engine will have a little less mechanical advantage meaning your acceleration might be noticeably reduced. If you are tempted to push the throttle further down to accelerate as you are accustomed to, this may increase fuel consumption. Keep this in mind, as good/bad driving technique often nets the biggest change in fuel economy.

I noticed that California models tend to have lower gear ratios in the tranny, and some owners will swap to a federal tranny to gain the taller ratios for more fuel economy. I often wondered why Honda provided the lower ratios for California cars, and I think it may be due to California having more hills and or California drivers preferring better acceleration. I have noticed after switching to taller tires that my car requires more downshifting on tall and steep hills. The car is less capable of maintaining a good cruising speed going up long hills. If the engine RPM's drop below 2500 the power/torque drops off significantly provoking a downshift or more throttle (wasting more fuel)

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NOW TO BUMP MY QUESTION In hopes somebody might chime in their .02cents about it. (read below) I would like to know if swapping to a federal ECU on the 1988 CRX HF will improve my fuel economy on a California model? Would the tranny gear ratios have any impact on any potential benefit of a federal ECU? Perhaps the Federal ECU is also tuned to work better with the taller ratios on the Federal model HF Tranny?

Thanks
Dave

Originally Posted by forums123

Everyone seems to lean towards 14" Wheels for some reason. I guess there are better LRR tire options available in 14".

IMO I figure the taller 175/70R13 tires would give a taller ratio on the final drive and keep engine RPMs down. That should improve fuel economy right?

I am a california resident, so after seeing the Federal ECU (3-4mpg gain) comment from the Original Poster, I was wondering, why does the Federal ECU improve fuel economy on the CRX HF? Is that true for all years of CRX HF and Honda Civics?

Thanks
Dave