At 80 MPH in 5th gear, my 08 Fit MT is at about 4K RPM. I want it closer to 3K, or 2.5.
my stock tires are 175/65R14. I just ordered 70 profile tires (the old ones were worn out anyway) which will add about 2 inches to the outside diameter, (yay ground clearance) but won't get me all the way there.
I searched for aftermarket final drive gears, but only found shorter ones. does anyone know of LONGER gears for the MT? if not, I'm considering having a speed shop around here make either a 5th or a final drive that will drop my RPM on the freeway.
right now, the car has phenomenal top gear acceleration on the freeway, but im not scared of downshifting to pass, etc.
and i know i'd get better mileage going slower. I just want good mileage while going fast, too.
does anyone know what RPM the engine is most efficient at? peak torque looks like between 4K and 5K.

 

First, assuming less rpm will yield better mpg is incorrect. All engines have a 'sweet' hphr/fuel range that provides the most mpg; on the Honda that appears to be about 2800 rpm. Specific fuel consumption rises both above and below that 'valley'. Going faster at that rpm simply requires more power than 2800 provides so the engine strains and uses more power - and more fuel. Porsche used to publish theirs in their handbooks but is the only one I know. Generally speaking Detroit tries to maximize mpg at the average speed in the EPA highway mileage tests for obvious reasons. Adding hills naturally really dumps the lower rpm vs mpg theory.
All cars have a set power required to move horizontally against tire friction and wind resistance and that power is required no matter what rpm you have. Manufacturers try to specify gears that maximize distance traveled at that rpm. And they spent a lot more effort finding that out than you ever will. Reducing rpm thinking it will reduce mpg will more likely decrease your mpg because the engine will strain to meet the power required or require downshifting, both of which increase fuel required.
More often than not increasing tire diameter also increases tire weight and that alone requires more power and more fuel. Its why most tire width increases at the same diameter do reduce mpg. If you want to increase mpg try narrower tires of the same diameter to reduce both weight and rolling friction; and a little higher tire pressure. Of course that will reduce cornering and braking performance. There's no free lunch.

 

mahout, are you claiming that it is more efficient to drive at high rpms? why, then, do you later claim that 2800 rpm is the "sweet spot" of specific fuel consumption?
If you think that the manufacturer is always right, why mod your car at all? the fact is, Honda geared the Fit low to trick people into thinking that it is a powerful car. "wow, this thing accelerates great in top gear. I don't even have to downshift!" yeah, cause it's already in the powerband. I'm not fooled, in fact, i like that it is weak. I'm totally okay with it. In fact, I'm going to make it even weaker by gearing it higher.
your argument that higher gearing will "decrease your mpg because the engine will strain to meet the power required or require downshifting, both of which increase fuel required" puts forth 2 different scenarios: one in which the engine can push the car in the higher gear, and one in which it can't. if it can, then great, it is more efficient. and if it can't, then fine, i just won't make it that high.
the reason that burn-and-coast is more efficient than cruise control is that the engine is more efficient at wide open throttle. making the engine work harder will not result in less mpgs.
as for weight, my skinny, 2 inch taller tires mounted on 14 inch rims will weigh much less than everyone else's super wide 16 and 17 inch rims. if i don't like them, I'll go back to 65 profile tires when these ones wear out.
a dyno will tell you the specific fuel consumption, which will let you identify the engine speed that is most efficient. i've seen several Fit dyno charts, but none that show specific fuel consumption, so i'll use your figure of 2800 RPM.
the stock 5th gear is .76. on stock tires, i'd want a ratio of about .55 to cruise at 80 MPH at close to ideal RPM, but I'll wait till I get the taller tires to decide whether to get the gears made or not, and what gear ratio.
it looks like it makes 91 lb ft (at the wheels) at 4K. at 2.8 K, it does about 89 lb ft.
91 lb ft at stock 5th gear means the wheels feel about 69 lb ft (i'm skipping final drive, the wheels don't really feel that, this is just for comparison)
89 lb ft in my "notional" .55 gear ratio puts wheel torque at about 49 lb ft (for comparison only. to get actual force figures i'd need to take into account the tire diameter and final drive ratio.) to decide if that is too low, i'd need to calculate the actual force requirements to move the Fit at 80 MPH.
The FitFreak Established cD seems to be .35 (unconfirmed) and frontal area seems to be 23.8 sq ft (a rough estimate, with extra fudge), and I'll be going into uncharted territory as far as rolling resistance is concerned because of my unconventional tire choice. it looks the math will get a little fudgy here, because all these numbers are pretty much made up.
The tires won't be here for another month, and I can't calculate rolling resistance without them, so it will take a while to determine the actual power needs.
it is also worth noting that I don't have a scangauge (i can't afford one with all these tires and custom gear castings lol) so we will never know for sure whether this all makes any difference, or how much.
also, my original question was whether anyone on the board had seen or heard of alternate gear ratios, and can anyone point me to a dyno graph that shows specific fuel consumption. i'd love to buy a "ready made" one vice a custom one.
industrial engine manufacturers always provide specific fuel consumption data on their engines, and when my buddy got his M3 dyno'd his chart showed specific fuel consumption. i just want this info for the fit.

 

I said driving at higher or lower rpm than the minimum specific fuel consumption curve will not be as economical. The specific horsepower curve measures the amount of fuel consumed at various horsepower in an hour. The curve usually looks like a valley between two mountaibns. The low place of the valley is the most efficient perating rpm for a given engine. Manufacturers try to get the average highway rpm of the EPA test to concentrate on that 'valley' for obvious reasons.


When you travel at a higher speed the added power required forces you up the specific horsepower-hr curve and higher fuel consumption occurs regardless of your rpm. If the engine can't deliver the needed power you're needing a higher gear. The problem with your thought is that if you gear a vehicle to travel at a higher speed at the same rpm you didn't take into account the horsepower required will increase the fuel required. (Drag alone increases with velocity by the eigth power). So mpg drops because the engine works harder producing less mpg..



Mahout, you big OLD dummy. That data was for carbureted vehichicles, not FI ones. The curve for FI vehicles is so close to linear that the difference in the valley is minute. While there is a limit how much you can decrease rpm without using more fuel, 5% increase in gearing (decrease numerically) is probably beneficial. And that you can get with a narrower but taller tire so long as it fits in the fender. Thats about 1" taller but at least 10 mm narrower and possibly 20 mm narrower tire. Any more than that and the ratio of drag and rolling resistance eats up the less horsepower available from the engine at that rpm.
Well, least you were half-right.

The specific fuel curve is the same shallow u-shaped shape for both carbureted and FI engines. The FI engines don't have the dip as much but then emissions weren't as much an issue with carb engines. Metering precision became necessary to contriol emissions.

 

Going 80MPH in a sub compact, light and shifty car will probably make getting high MPG's pretty difficult. They will still be decent compared to other cars, but still... Such a small car, so light, any wind plus the resistance from driving that speed, its just going to make getting high MPG's more and more difficult.

 

My friend has a junky old BMW 318ti (the hatchback, for those not into the beamer scene) and he dropped a taller final drive in there and increased his fuel economy to over 40mpg. Just throwing that out there.

Obviously, i'm a believer of Lower rpm at cruise = lower fuel consumption.

 

weight has nothing to do with mpgs at speed, almost all of the force that you are overcoming is air-drag related, with a fractional amount from tire and wheel bearing drag. the faster you go, the more infinitesimal this becomes. further, the lighter the car the less tire/wheel bearing loss, so it being light weight really has little to do with mpgs.

mahout you're pretty incomprehensible, i have a hard time reading your post, but a simple answer to the "lower gearing, better mpg" question is found in the sport/non-sport automatic comparison.

the only difference between those cars is the lower final drive, and with the non-sport getting better mpg in a controlled test (e.g. the EPA test) it's obvious that lowering the final drive (at least by a small amount) will increase fuel economy in some highway driving. now at what speeds it will make it better is another discussion, and one that honda had when they chose this final drive ratio. some consider 60mph highway while others consider 80mph highway.

me, i just wish i had a 6th gear haha.

 

Only gears I know of are lower ones for racing purposes. Shoulda bought an AT if you wanted lower rpm at higher speeds is all I can say N yea, the fit has low gearing cause it's a lil compact car made for getting around town managing 30+mpg in stop n go traffic.

 

putting custom gears in your fit will never pay off. youd be better off putting that $$$$ towards gas

 

haha yeah i wish you could just magically "add" a 6th gear

 

but the taller gearing on the AT doesnt even help it get better fuel economy haha.

 

lol. yup. which throws out the idea of lower rpm ='s better mpg all the time
n yea, the $1k+ you'd spend getting that higher gear...well that'd be a years worth of gas if not more...least for me it'd be more.
high five sol.

 

hahaha *high five*

 

Sorry about the technojargon. Sometimes I have a hard time too.
But the reason the auto doesn't get better gas mileage is the power losses thru the automatic; robs about 3-5 hp. So there is more than gearing differences.
Good point about weight having little to do with mpg when driving at constant speed and on level ground. Few of us can do that though.
Based on the working rpm range of a Fit I doubt a higher 6th gear would help tho' I suspect another intermediate 6th gear (with CVT) in the range like BMW, Merc, and others would improve mpg. Keeping the rpm within a narrower range in each gear should be big help.
Maybe next year we'll get that.

Mahout, you big OLD dummy. That data was for carbureted vehichicles, not FI ones. The curve for FI vehicles is so close to linear that the difference in the valley is minute. While there is a limit how much you can decrease rpm without using more fuel, 5% increase in gearing (decrease numerically) is probably beneficial. And that you can get with a narrower but taller tire so long as it fits in the fender. Thats about 1" taller but at least 10 mm narrower and possibly 20 mm narrower tire. Any more than that and the ratio of drag and rolling resistance eats up the less horsepower available from the engine at that rpm.
Well, least you were half-right. You don't need different gears, just different tires.

cheers.

 

The '09's have a larger wheel well opening, then older models. I'm surprised know one has installed a larger OD tire and enjoyed the expected FE gains on the hi way.

 

Actually they have. And both of them got worse gas mileage.

 

Just need to make some wheel skirts then. I couldn't bring my self to do that to the FIT.

 

Well how about some details or a link

 

taller tire would make for less mph shown on the speedo. "supposedly worse mileage"

 

yeah, I was hoping I could get get a new gear cut for a couple hundred bucks, but so far no one can do that. I guess I'll just keep the factory gearing
as for the automatic, I love clutching and shifting, and no amount of gear ratio optimization will change that.
I'll see how it does with the taller tires when they come.