What is the best and cheapest way to improve mpg on my 2007 hyundai accent hatchback?

The problem is that the engine is not capable of putting out high enough horsepower unless the revs are high. Just like a motorcycle engine, no power at low rpm's but plenty at high rpm's. Your small engine is underpowered for the car, and being small it has low horsepower, and thus has to rev high just to maintain speed. Putting in a larger engine makes more sense than trying to swap transmissions. That's why a new Corvette turns @1600 rpm at seventy miles per hour. Big power and big displacement means low rpm's. That's why the Corvette gets 30+ mpg at 70.

displacement doesn't effect rpm's going down the road. that is effected by gearing. the corvette gets good mileage on the highway because the top 2 gears are both overdrives. now to your questions. you would be cheaper to pay for more gas than buy a whole new car or a new engine or transmission. you can buy a lot of gas for the $2,000 you will spend on the transmission or the $25,000 minimum you would spend on a hybrid. how fast are you driving on the highway? if you accelerate like a drag car and drive 90 on the highway you aren't going to get good mileage no matter what you drive. I picked up 2mpg in my truck just by dropping my highway cruising speed to 75 from 85.

does the same work for a manual? i forgot to mention that.

so if its possible to increase the engines horsepower via modifications that support higher hp, i can decrease the rpm's and increase fuel economy?

In short no. You want to increase torque (a function of displacement), horse power is generally a function or rpm's. You don't want to increase your horsepower. Adding a turbo will increase horse power but will not decrease your rpm's, if anything the rpm's will go up to get the benefit of the added horsepower. You want torque. The only way to get increased torque in your small engine is to increase the stroke, not the rpm's. Not really feasible because of the cost. Might as well get a larger replacement engine. But still, your stuck with the wrong transmission for your replacement engine. See where this is going? If mileage is what you want, get a car like a new Focus, plenty of power and a six speed (auto) tranny to get about 40 mpg's highway. The Accent is cheap, but certainly not economical.

and there inlies the problem. new car is not an option.

You could do a few things to help your mileage. Maybe a K&N intake system or at least the filter. I'm using a system in my 911 and a filter in my Volvo, sounds good but I don't think they really help that much with mileage. Try using a high quality lower viscosity transmission fluid. I like Klotz, great product in any gearbox. Also go to a lower viscosity good quality synthetic oil such as a 0W-20. This may or may not meet Hyundai's specs, so it's up to you. The rest is the usual stuff, empty the trunk and inflate the tires to the maximum sidewall spec, etc. Also you can go with larger diameter tires. This effectively lowers the gear ratio. However tires aren't cheap and to really gain from this you'll probably encounter clearance problems. Other than keeping your foot gently on the gas pedal, there's not much you can do other than putting a much lower 5th gear in the transmission. Good luck

you said high quality lower viscosity transmission fluid, can you explain that a bit more? and i live in florida if that helps with the thinness of the oil at all. if you can actually give specifics on what size tires, what weight transmission fluid, and what weight oil would be best. i'd appreciate you a lot more than i already do. and whats your source for this information?

As far as transmission (gear lubricant) is concerned, there are a number of good synthetic brands out there. I have used BG Syncro Shift II, Klotz, and Mobil1. They are so good in fact you can run a lighter weight (and thus less drag) than what the manufacturer recommends. I like Klotz the best, it really quiets noisy transmissions and shifting is smoother. I use BG 75W-80 in my wife's Focus. I'd recommend Klotz "Freegear" 75W-60 for your car. Mobil1 is probably the easiest to find, but I think they are too heavy in viscosity for what you want to do. Check out the manufactures web sites. As far as oil is concerned, any high quality light weight multi-grade synthetic will work. I know cost is a concern of yours, these products are expensive but the quality is so high they are worth it. For your application I recommend Pennsoil Platinum 0W-20. If it's really hot outside, maybe a 5W-30 weight is a better choice. For the money, this is a great oil. As far as larger diameter tires are concerned, I need to know what size is on the car now. If you go to a larger diameter you could encounter clearance problems, particularly when turning, and your speedometer won't be as accurate. With larger diameter tires your tire revolutions will be less for each mile driven, effectively lowering your gear ratio and increasing the mpg. I'm 67, did some racing, worked in a Porsche race shop, and try to keep learning as much as I can. Glad to help. My '83 944 race car is pictured (has a '93 968 engine).

currently I have 14 inch tires. and thank you for helping me.

You should have 175/70 14's on your car. That size tire has a diameter of 23.7 inches and a width of 4.84 inches. The largest size you can PROBABLY fit to your car is a 195/70-14. This tire has a diameter of 24.7 inches and a width of 5.4 inches. This new diameter is a full inch over the stock tire, so it will be 1/2" higher off the ground, but also a 1/2" closer to your wheel well and your spring perch on your struts. Also being about 9/16" wider than your stock tire, it will be about 1/4" closer to your strut cylinder. So fit MAY be an issue, the only way to tell is have a tire this size mounted and check for fit issues. This change should give you about a 5% improvement in your gas mileage. Because you've effectively lowered the gear ratio by going to the larger diameter tire, the pick-up will slightly suffer, and the speedometer will read a little lower. Check out the TireRack web site for more tire size information. So if you've tried everything we've discussed, you MIGHT get a 10% improvement in gas mileage, if you're lucky. Is this added expense worth the effort? If it were me, and I was so set on getting better gas mileage, I'd have new 4th and 5th gears put in my transmission that would give better gas mileage. Only problem, we don't even know if these gears are available. Maybe call Hyundai and inquire about a gear change, or maybe call a good transmission shop. Speed shops are a good bet, they are always changing gear-sets for finicky drivers. I'm glad to help, but with a such a small engine, it's a tough situation to be in. Good Luck Good luck.

Thank you very much good sir. ^_^

Let me know how things work out. stephen.smith22@comcast.net Your lucky to be in Florida, I'm in freezing cold Massachusetts, try working on a car in the snow. Best of luck!

I did in Wyoming with a 280zx. I know your pain.

A few years late to chime in, but you absolutely would benefit from a 6spd manual that Hyundai started putting in the 2012 accents. I'm looking at the same upgrade. I have a 2011 with a 5spd manual, and I knew when I test drove it at the dealership that it was geared way too low for highway travel. Same thing: 3200 rpm @ 60; 3500 @ 65-70mph; 4000+ @ 75-80mph. I've driven my 2011 Accent on the highway for more than 7 years. It doesn't have a lot of torque, but it has enough where the higher 6th gear they make for the 2012 could keep it moving nicely at 65-70mph, definitely enough to push it along at 80mph. If it wasn't a good idea for a 6spd manual, Hyundai wouldn't have put it into their 2012. Granted, the 2012 1.6 makes a little more power, but not enough in my opinion to say the 2011 1.6 would struggle to push the car down the road doing 70 mph at 2800 to 3000 rpm instead of the nearly 4000 rpm it needs to rev at with the 5speed. Also, the 2012 w/6spd is rated about 12 mpg more than the 2011 w/5spd. Cheers.