Gas Mileage

Fourth Generation Honda Prelude topics

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wickwear
Lude Dude
Posts: 32
Joined: Wed Jan 27, 2010 1:12 pm
Prelude Model: Si
Location: Phoenix, Arizona

Gas Mileage

Post by wickwear »

I have a '92 SI. I've owned it for maybe two years and know it has been in an accident and I just got the front suspension all fixed. I also had some other work done to it. Before the work was done to fix the suspension I was getting on average 28 miles per gallon. Now after a new front suspension and new tires/got it alligned I am averaging only 26 MPG. I am an average driver and have my spurts of agressive driving but I'm not bad. I always get premium and calculate my MPG everytime I gas up. Oh yea I live in Arizona and for the last 2 months have only used the heater, so my AC isn't sucking the power/MPGs. I would also say that my commute is between "in town" and "highway" if that makes sense. What does everyone else get? Do you have any suggestions for me?
(I also got my timing belt change and I swear that I am idleing a little higher than before the work was done, is there some type of adjustment that they might have made? That I can un-do?)

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spiffyguido
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Prelude Model: 1991 SE-SR
Location: Alberta, Canada
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Re: Gas Mileage

Post by spiffyguido »

Any chance your tire pressures might have changed?

wickwear
Lude Dude
Posts: 32
Joined: Wed Jan 27, 2010 1:12 pm
Prelude Model: Si
Location: Phoenix, Arizona

Re: Gas Mileage

Post by wickwear »

I had some 195/50 R15 or 195/55 R 15 (can't remember) on it before and I wanted something with a little more tread (and plus the budget only allowed me to spend so much), so I got 195/60 (in hind-sight I am not sure if the 195/60 were the right choice but oh well). The 195/60 R15 are heavier and the tire diameter is larger but they are inflated to the recommend PSI. The fronts look a little low but when I check them they are at the right level. But my old tires had the same look especially on the front. So long story, but the tire presure is where the tire place said it should be. The odometer and speedometer should still be counting normally right? Maybe that is where I am loosing my MPGs? What do you think?

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spiffyguido
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Prelude Model: 1991 SE-SR
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Re: Gas Mileage

Post by spiffyguido »

If the diameter and weight are up, that's for sure where you are losing your MPG. Big tires require more force to spin up.

It's the reverse of the old trick of putting tiny rims on a car before going for a smog test.

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RedRacer
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Joined: Sat Nov 19, 2005 10:38 pm
Prelude Model: 1995 Si w/mods
Location: Alabama

Re: Gas Mileage

Post by RedRacer »

Like Spiffy said, if you increase the size and weight (although the weight is going to be negligible in today's tires) you are going to find some loss due to the larger contact patch. If you look at it from a mechanical perspective, an increase of tire/rim diameter might effect the efficiency of the transfer of energy or torque upon the axle. However, I would speculate that the change would be negligible.

From a purely geometric perspective, changing the tire diameter could effect the "perceived" mileage", but not the "actual" mileage. The odometer (and speedometer) tabulate the revolutions of the tires (RPMs for the speedometer) so the traveled distance would be greater than the measured distance if the tire size were increased. (Just as someone would run slightly further if they ran around a track in an outside lane versus an inside lane) This is why you have to recalibrate your odometer if you change your tire/rim size. The car's mechanics were engineered and computed based on the OEM specs for tire and rim revolution. You alter those figures when you put larger/smaller tires on than what the factory specs call for. So, calculating the MPG now, with the new, larger tires, would provide an error unless the odometer/speedometer were recalibrated for the different size of the new tires.

In your case, this "error" does not actually "change your mileage" per se, but rather gives you an erroneous value when you use your odometer to track your mileage. Try using known distances (distance between mile markers vs. speed) in your calculation instead of your odometer and you'll probably find that your mileage is the basically the same.

In summary, I would suspect that the change in tire size is having a minimal effect on your "actual mileage", but the "measured mileage" is easily varying as a result of the calibration error.

Outside of that, the only other thing I would consider would be that the shop failed to align the car properly and they have the toe set improperly, but to a really large degree. Or, they just didn't check their work and something is not as it should be. I feel a little leery about your saying the car sits different than it did before the accident and work. That shouldn't be happening.

Do you have the same size tires all around? You don't have mixed tire sizes on the front and rear do you?

If there is a component issue or a really bad toe issue, you will know after a few thousand miles because the tires will begin to show improper wear on the inside or outside edges. Keep your eyes on how those front tires are wearing.

Does the car drive the same? Does it "feel" like your car before the accident?

wickwear
Lude Dude
Posts: 32
Joined: Wed Jan 27, 2010 1:12 pm
Prelude Model: Si
Location: Phoenix, Arizona

Re: Gas Mileage

Post by wickwear »

Hey thanks guys, that really took care of my concern. Everything is good and the car feels normal like before but it now is raised a little more beacue of the larger tires on it but yea it feels good, I just wish now that I would have spent the extra money to get the original equipment. But you live and learn. Thanks again!

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RedRacer
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Prelude Model: 1995 Si w/mods
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Re: Gas Mileage

Post by RedRacer »

Ah, I misunderstood. I was thinking you were saying that it was sitting LOWER than before....and that kind of raised a red flag that they did something bad wrong when they put things back together, as I was thinking that with larger tires, it should actually sit higher.

If it drives and handles ok to you, then you're good to go.

My only other piece of advice would be, as with any kind of major front end suspension repair, keep your eye on the treadwear patterns and make sure the tires are wearing correctly. If you see any kind of weird wear, take it back to that shop ASAP and let them know it wasn't doing that when you took delivery of the car after the repair.

One other thing I do with my vehicles before and after any major repair work is take pictures of the condition of the parts worked on or replaced before and after. This way, you have proof if things go wrong. Most places are fast to deny that they're not liable for work that's over 30 days old just on general principle, but if you can actually hand them proof that the car or part wasn't in that condition before you took the car to them for repair, they can't deny something is wrong.

Of course a good shop or mechanic is going to admit an error on their part and own up to it and do the right thing.

I had a routine scheduled 90,000k maintenance done by a different shop once, and a day later I was hearing a high-pitched noise coming from the accessory belt area.... so I took the car back to ask them to look at it. The shop tech noticed when I was backing the car into the parking space to leave it, that there was oil trailing from under the car as it moved. So you know if it was that visible, it wasn't just a little leak..... they pulled it into the shop immediately and put it on the lift.....turned out the mechanic who did the valve work and rear main seal work had not paid attention to the o-ring he replaced, and when he put the seal on, he crimped it and it was folded in at the corner....and oil was pouring out of the hole. Had I continued to drive it, the engine would have been damaged.

The owner was very unhappy that this had happened. He took the car in, re-did the work, and refunded me half the 900 bucks that I was originally charged. He wanted to make sure his reputation was protected and that I would have faith in returning to his shop in the future. THAT is a good shop. Admitted a mistake, fixed what was their fault, and then still gave me money back.

Good luck!

Stick around. Let us know how things go with your car and of course, ask if you have any questions.



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