Help with gear ratios

348tripower · December 29, 2012, 12:59:33 PM

My COE is turning around 2600 rpm at 70 mph. I have a gear vendors O.D. unit that is .78. I would like to re gear this and change to a 700r4. From my calculations it looks to me that I have a 3:73 gear set in this. I don't really know. This truck weighs 4500lbs. Has 29.5 tall tires. I would like to see this turn about 1800 to 2000 at 70mph. I don't want to lug this for too long out of the hole either. The reason for the 700r4. Seems about 50 to 55 mph I get great gas mileage after not!

350TPI
400th with gear vendors O.D.
29.5 rear tires
4500 lbs

Thanks,
Don

unklian · December 29, 2012, 02:10:11 PM

http://www.et-studios.com/motorsports/gears/gears.html

unklian · December 29, 2012, 02:17:41 PM

http://www.blackradon939.itgo.com/gearratio.html

wayne petty · December 29, 2012, 03:54:12 PM

My COE is turning around 2600 rpm at 70 mph. I don't want to lug this for too long out of the hole either. The reason for the 700r4.

Seems about 50 to 55 mph I get great gas mileage after not!

350TPI

still got your OBD1 scan tool...

what is the block learn and the intergator numbers at 55... and again at 70... how close to 128 are they at either speed.

how about the injector pulse width at both speeds.

did you replace the injectors? with the bosch III units . i am worried that if you have stock injectors they are NOT flowing enough at 70MPH and you might be calling for more fuel on the other injectors...

and yes.. i know what a PAIN in the .. to get the injectors out is on a TPI motor...

i think we went over getting it into closed loop fuel control... with all the sensors working properly..

you are pushing a lot of air out of the way at 55 and significantly more at 70... that truck has a higher Cd than a corvette or a camaro..

before you swap the gear set.... can you try some TALLER tires ... is there room for them??? see if that leaves you with the better fuel economy and still the seat of the pants feel..
this is a temporary test only... something in the 31 to 33" tall range.. just for a day or two.... even if they are narrower.. this is just to see the gear ratio change..

just a rough calculation..
you have 570 turns per mile at 29.5" 2126 RPMs at 60.
you will have 524 turns per mile at 32" 1924rpm at 60mph

tire rack has a bunch of tire size comparisons and also turns per mile.. i am not saying to buy a set...

this again is to just let you know if it will help or hurt...

Crosley.In.AZ · December 30, 2012, 08:37:57 AM

You will need to keep in mind that from a .78 OD ratio in a GV unit, to a .70 OD ratio in a 700r4 trans ..... the engine will need to over come the added OD ratio to keep that round brick COE going down the road at a given speed.

I believe tomslik mentioned about pushing lots of air with the COE when this build was started. Tom had to deal with the altitude in Colorado and the large - tall cab pushing air when he built his COE.

You may end up with lower RPM , but the throttle pushed down further to maintain speed

Crosley.In.AZ · December 30, 2012, 10:26:13 AM

I forgot to add...

it may be better to stick with a 3 speed trans that is 1 - 1 in high gear and reduce the rearend ratio as suggested via tire size as Wayne mentions

348tripower · December 30, 2012, 02:11:45 PM

Well I have a lot of good suggestions. One of the problems I have is it is winter. So I can't get it out to do the tests. I will save all this info and go after this in the spring. The feed back is what is was looking for. guess i'll go make a snowman.
Thanks,
Don

39deluxe · January 01, 2013, 05:21:56 AM

It doesn't seem to me like there is a big enough difference between the .78 Gear Vendors and the .70 in the 700R4 to warrant the cost and work involved in the swap. I don't think you will get where you want with tire diameter either considering the amount of rpm you want to drop at 70 mph. I think that rear gears are going to be the answer. Use the calculators above to be sure you are not getting too far out of the engine's sweet spot or you will have to down shift every time you want to accelerate. That would become a pain.

Is there some other reason you want to get away from the Gear Vendors? Is it on a THM400?

Tom

enjenjo · January 01, 2013, 11:31:53 AM

One advantage of the the 700R4, it has a much lower first gear, which give better acceleration. 2.52 versus 3.06.

39deluxe · January 02, 2013, 06:19:29 AM

I forgot about that.

Crosley.In.AZ · January 03, 2013, 10:00:29 PM

Quote from: "348tripower"My COE is turning around 2600 rpm at 70 mph. I have a gear vendors O.D. unit that is .78. I would like to re gear this and change to a 700r4. From my calculations it looks to me that I have a 3:73 gear set in this. I don't really know. This truck weighs 4500lbs. Has 29.5 tall tires. I would like to see this turn about 1800 to 2000 at 70mph. I don't want to lug this for too long out of the hole either. The reason for the 700r4. Seems about 50 to 55 mph I get great gas mileage after not!

350TPI
400th with gear vendors O.D.
29.5 rear tires
4500 lbs

Thanks,
Don

just some additional thoughts

My calcs on the 3.73 gear, 29.5 inch tire, OD at .78 indicate the RPM at 70 is at 2400. More close number I have is 2370 RPM

T-400 first gear is 2.48 , 700r4 : the 3.06 first gear would help a little at take off.

700r4 has lock up converter to reduce RPM a few more, T-400 trans does not have converter clutch

:!:

348tripower · January 04, 2013, 06:52:54 AM

I am still debating what I am going to do. I just got done spending about $1600.00 on a new furnace and T8 lighting for my shop. Seems the more time I spend working out there it warrants the upgrades. Now I need to get to work to pay for this stuff.

enjenjo · January 05, 2013, 10:02:20 PM

Quote from: "348tripower"I am still debating what I am going to do. I just got done spending about $1600.00 on a new furnace and T8 lighting for my shop. Seems the more time I spend working out there it warrants the upgrades. Now I need to get to work to pay for this stuff.

I sent you a PM.