crf70 mpg?

[natek]

couldn't find any info on this. want to build a street legal crf70. only thing engine related i want to do is a AHP 4 speed and a gear change. i read somewhere that a 140cc in a crf70 was getting 100mpg. i'm hoping the fuel saving from keeping the stocker would be that much greater to justify the cost of the build.

what gears should i run? top speed isn't all that important to me. just want to raise it up so 1st gear is actually useable. i hear about people start the bikes in 2nd cuz 1st is so short. something that will put 1st around the 2nd gear raitio so, it theory, i'll have 2 overdrive gears over what the stock 3speed had.

thoughts on mpg?

[PlanetMinis.com]

[minimotos95]

for MPG you want the tallest gears possible without bogging the engine, allowing lower cruising RPM(and a higher top speed if it can pull those gear ratios)
i can't speak for the crf70 but i did have a 72cc import with a 4spd on 10"s it got the same distance per tank as a lightly modded 50(exhaust, CDI and filter), a 70 should be better because the taller 12" drive wheel.

technically the AHP 4spd only has one overdrive gear

[natek]

yeah i know. my point was that with different sprockets you could make 1st as tall as 2nd without problems. that would make 2nd as tall as the stock 3rd and then having 2 more gears.

i didn't make that very clear. it made sense in my head lol

[dirtbkr188]

Just as an FYI, here are the trans ratios.

XR/CRF 50/70---------------- AHP

3.272------------1st-----------3.182
1.667------------2nd----------1.706
1.190------------3rd----------1.238
-----------------4th----------0.917

[minimotos95]

stock 3 speed with 15/36(what i think is stock on a 70)
5000rpm in top gear: 25.6
9000rpm in top gear: 46.1

AHP 4 speed with 15/36
5000rpm top gear: 33.2
9000rpm top gear: 59.8

AHP 4 speed with 15/30 gears
5000 rpm top gear: 39.9
9000rpm speed: 71.8


what speeds are you looking to cruise at? i assume the best cruising RPM for a 70 is in the 2-3k range

[zwieblekopf]

I'd have a hard time believing 100 mpg.
My mod 50 can get maybe 25 miles a tank (.8 gallons) on faster trails, not as good as you would on the road but I don't think you'd do almost 4 times better.

[minimotos95]

was able to get 50+ miles out of a tank in my old 72cc import(xr50 size tank) on semi-technical trails with a lot of WOT straits and hills... 100 mpg is possible if you keep your wrist out of it most of the time, street tires will help too(less rolling resistance)

edit:
if a cbr250 can average 87mpg i am sure a crf70 can average over 100
http://ecomodder.com/forum/em-fuel-l...vehicleid=5229
i will keep searching on that website to see if i can find something more like a crf70
edit2:
ct70s are averaging between 90-110 from what i see....

[natek]

stock 3 speed with 15/36(what i think is stock on a 70)
5000rpm in top gear: 25.6
9000rpm in top gear: 46.1

AHP 4 speed with 15/36
5000rpm top gear: 33.2
9000rpm top gear: 59.8

AHP 4 speed with 15/30 gears
5000 rpm top gear: 39.9
9000rpm speed: 71.8


what speeds are you looking to cruise at? i assume the best cruising RPM for a 70 is in the 2-3k range

very good info! thanks a LOT! i would like to comfortably cruise 40-45 (a typical 35mph road here). i'd go on faster roads but only short distances so i could deal with higher rpm's for that. is that realisitc or is that too tall of gearing?

[natek]

I'd have a hard time believing 100 mpg.

yeah... especially out of a 140. that sounded a little toooo optimistic to me.

[minimotos95]

with the tallest gears(17/31) i was able to find it's at.
5150rpm at 45mph
9000rpm at 78.7mph


it should be able to pull that gearing, acceleration can't be worse then riding 2 up... the bike shouldn't mind cruising at 5150 but it can't be to good on gas

[mr.skywalker]

yeah... especially out of a 140. that sounded a little toooo optimistic to me.

I have a DR200 that has been geared a little high and ive got as high as 93mpg and that was interstate driving(about 3/4 throttle), so with a 140 with tall gearing should easily hit 100mpg. Im sure if you had the power to pull 4th on the stock regeared crf70(doubt you could unless you weigh less than me at 120lbs lol) you could be in the 150mpg range(optimistic)

[natek]

i figured there would be a point where a taller gear would only cause more strain and actually hurt mpg. when you get to those extremes a bigger engine might actually prove beneficial since it could ride at a lower rpm a little more relaxed.

so what do you guys think would be a good middle ground for gearing on a stock 70 engine with a pod filter/tune?

[natek]

or with any other cheap/simple mods. the goal is to not spend a ton of money after all.