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Old 10-08-2011, 09:46 AM   #1 (permalink)
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Question 94 Accord EX, 5 speed - Help w/ RPM/MGP

Hello all - -- i've searched some, but haven't had much luck finding a thread to help, so I'm hoping someone has done this and can point me in the right direction. I travel a LOT. Mainly interstates where I like to cruise at 80-85mph, but my Accord is screaming at me and I'm honestly a little worried about longevity of the car as much as I am about my mpg taking a beating. I can average 33 mpg while driving 60-65, but it drops to 27 or less at 80-85 due to turning about 3300 rpms in 5th gear. Is there any hope for this?? I've researched tire sizes and such and really it only seems like I can gain one inch in diameter which won't affect things a whole lot I don't think.

I'd love to change the final gearing, but have no clue in a FWD car -- I can change gears all day long in a RWD car as I used to race dirt track and that was a normal occurance for changing tracks and setups, but the similarities are very few

Any help or advice would be GREATLY appreciated! She has 218k miles and I'd like to keep her running for another 100k or so at least!

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Old 10-09-2011, 10:13 AM   #2 (permalink)
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Well - - - -

I commute 50-55 miles (depending on the route) each way. So I've worked through a few things re. frequent long trips.

I think there's potential in "faking" a higher gear ratio by going to a slightly larger tire size. My wife drives a '96 Accord DX with 5-speed. Owner's manual says 195/60-15 but it has 195/65-15's mounted. If you're running 14's the numbers are different but the idea is the same. We haven't had any issues with tire rubbing with those 15's.

If you're going to change tires you DEFINITELY should change to LRR tires: Low Rolling Resistance. And plan to run them at the sidewall imprinted pressure ("max sidewall" as we say here), and that's measured cold in the morning before driving. The softer the tire, the larger the "flat spot" on the road, and the more flexing the rubber has with each rotation. That's more work for your engine. I run my tires at pressures that I won't want to post here but I can say they roll very well and handle superbly on our twisty New England roads.

To find LRR tires, go to tirerack.com, and once you get to the page for your size tire, use the filtering on the left side panel to get the LRR tires. It's all the way at the bottom of the filter-options column.

For winter, I got Michelin X-Ice 2's. They are the only LRR snow tire I found available and they only cost me 1 or maybe 2 mpg compared with my regular LRR summer tires. I found them to be good on snow, and superb on any kind of ice or hard packed snow. Yeah studs are better on ice but forget about fuel economy with studs.

Don't worry too much about super low weight wheels but avoid any that are truly on the heavy side. Wheel weight is more of an issue for drivers with lots of acceleration, aka city driving.

I did some cursory reading on regearing Honda trannies and I'm pretty sure that their final drive is kinda buried down in there, and not easy to change. But, you should research here: Honda Accord (1990 - 2002) - Honda-Tech for better information.

The other thing that's hurting your mpg is the aerodynamic losses that increase dramatically at higher speeds. This is probably costing you more than what you lose by not having somewhat taller gearing. The power needed to overcome drag goes up with the either the square or the CUBE of the speed increase, depending on which way you interpret the formula. So doubling your speed multiplies the power needed by 4 or 8. You pay for that at the pump.

The easy way to reduce aero losses is to slow down but that's not so practical with long road trips, if you care what time you arrive. The other way is to reduce the drag. Start hanging out at the Aerodynamics subforum: Aerodynamics - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com to learn what you can do. You'll find several threads on different approaches to my suggestions below:

Flat wheel covers
Rear wheel skirts (I recommend extending down only part way so you don't have to bulge it out sideways to clear the tire)
Belly pan, especially from the front bumper lip to the firewall (that's where I got the most benefit)

Rear aero spoiler. I and California98Civic have both worked to build something that reduces drag at the rear. We don't have this solved yet but there's potential for gain there. My own project is currently in progress: http://ecomodder.com/forum/showthrea...upe-18906.html This not complete by any means but I do hope to gain from some version of this project.
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Old 10-09-2011, 10:20 AM   #3 (permalink)
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Oh yeah.

I recommend an engine heater. I use one of the glue-on pads that heats the oil pan. I think the ones that heat the engine coolant by an element that inserts into a freeze plug opening are better - but installation of those is beyond my skills and I didn't want to pay a mechanic.

The cost of the electricity to run the heater is less than the cost of the gas that would do the same job. If you preheat the engine electrically, the car will go into closed loop operation sooner. That is, regular fuel mixture vs. the rich mixture that it runs when the engine is cold. Get a simple light or appliance timer to turn it on a few hours before you plan to start up. Just check the wattage ratings of the heater and the timer. Mostly they are about 300-500 watts.
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Old 10-09-2011, 12:35 PM   #4 (permalink)
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Quote:
Originally Posted by 97txaggie View Post
Hello all - -- i've searched some, but haven't had much luck finding a thread to help, so I'm hoping someone has done this and can point me in the right direction. I travel a LOT. Mainly interstates where I like to cruise at 80-85mph, but my Accord is screaming at me and I'm honestly a little worried about longevity of the car as much as I am about my mpg taking a beating. I can average 33 mpg while driving 60-65, but it drops to 27 or less at 80-85 due to turning about 3300 rpms in 5th gear. Is there any hope for this?? I've researched tire sizes and such and really it only seems like I can gain one inch in diameter which won't affect things a whole lot I don't think.

I'd love to change the final gearing, but have no clue in a FWD car -- I can change gears all day long in a RWD car as I used to race dirt track and that was a normal occurance for changing tracks and setups, but the similarities are very few
Any help or advice would be GREATLY appreciated! She has 218k miles and I'd like to keep her running for another 100k or so at least!
You can get better mileage and your car will last longer if you do one thing. SLOW DOWN!! What's your hurry? I get passed by people like you all the time. I'd really like to know why you think you need to drive so fast? What's so important that it can't wait a few extra minutes? Slow down and enjoy the drive. Take time to look at the scenery.Tomorrow isn't guaranteed to anyone.

Last edited by Quasimoto; 10-09-2011 at 03:11 PM..
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Old 10-09-2011, 08:40 PM   #5 (permalink)
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Come to think of it - if you want overall taller gearing, research to find out if the DX version came with that. Worth taking a look. If so, you'd swap the tranny. Also find out if the lower rpms will give you a check engine light. In some newer cars that's the case, but I don't know about your year's computer on that question.

Slowing down is definitely a valuable option. But maybe your time is worth $100/hour, or whatever the amount would be so that the time saved has more value than the cost of the gas used. Slowing down saves a lot because it gets you out of the problem where the fuel amount used goes up geometrically with the speed increase. You need lots more fuel when you get there faster.
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Old 10-13-2011, 05:09 PM   #6 (permalink)
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As is implied by the above posts, it is not as straight-forward to change out gearsets in a transaxle as in a transmission and separate diff. Particularly, the RWD setup lets you swap rear-end ("final drive") ratios pretty easily, as that is a completely separate part that is quite accessible.

With a transaxle, everything is in one housing and there is almost never room to disassemble that housing without removing it from the vehicle.

The Accords also don't have sixty-odd years of gearing to choose from with possibly billions of units produced of all kinds, as (for instance) GM transmissions have. So you're more likely to be stuck swapping in a whole transmission from another version of the same model year range of Accord. It would be worthwhile to figure out what years and which cars used the same type of transmission as yours, and see what gear ratios those came with. Those would be good swap candidates.

I don't know how sensitive Honda transmissions are to measurement tolerances, but I know that in some older transmissions the ring and pinion are a matched pair and always must stay together. And there is a bunch of non-trivial work to be done in order to swap an R&P set into another transaxle case. Hopefully the Accord is built with much tighter tolerances or just plain doesn't care as much about them, but I really don't know.

Good luck!

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