Help with some final drive math

[Ecky]

For what it's worth, my Insight has a 1.0L engine. It has a 3.208 final drive and 0.710 5th (top) gear. With its default tire size the engine is turning at 2217rpm @ 65mph, and 1875rpm @ 55mph. That works out to approximately 4.5% shorter than your truck's gearing. Pretty close indeed, and my new engine/transmission will be even closer.

Another important factor, it weighs 1850lbs, and has 66ft-lbs of torque. That works out to around 71ft-lbs of torque per ton.

With the IMA in place (which I've removed) it had 90ft-lbs of torque. That works out to 97ft-lbs per ton as the target Honda was aiming for.

With the IMA, the car was spunky and fun. Without, it's pretty slow, but accelerates fine if you wring it out.

Your truck weighs in at ~3100lbs (?) and 135ft-lbs of torque (?), best I could find. That works out to ~87ft-lbs of torque.

Or, in other words, you have about 25% better power to weight than I do right now but I'd consider your gearing to still be quite tall.

EDIT: If you have a 3.45 right now, to have similar gearing to my Insight (in terms of power to weight, ability to accelerate in each gear) you'd need a 2.76 final drive.

[hat_man]

My current rear gear is a 3.73 giving me a tire adjusted final of 2.80

Oh Deer weighs in at 3100# and the 2.3l Duratech had 143 hp and 154 ft/lbs of torque according to the internet.

[Ecky]

What's your tire size? I'll run some numbers.

[hat_man]

Tire size and all my math are in the first few posts but they are 235/75/15

Just want you to have all the numbers I came up with.

[Ecky]

So, I'm going to duplicate some math here, and it's to help me wrap my head around everything.

My Insight:
FD - 3.208
1st gear - 3.266
5th gear - 0.710
Tires - 165/65r14
Weight - 1850#
Torque - 67ft-lbs
Power to weight - 72ft-lbs/ton

Your truck:
Current FD - 3.73
Proposed FD - 3.45 and 3.27
1st gear - 3.72
5th gear - 0.79
Tires - 235/75r15
Weight - 3100#
Torque - 154ft-lbs
Power to weight - 99ft-lbs/ton


Comparatively speaking,

With your 3.73 FD, your RPM in 1st and 5th gear are almost identical to my Insight's. You have 42% better power to weight in 1st gear, and 37.5% in 5th gear, taking tire size into account.

With a 3.45 FD, you'd have 31% better power to weight in 1st gear, and 26% better power to weight in 5th.

With a 3.27 FD, you'd have 25% better power to weight in 1st gear, and 20% better power to weight in 5th.

Taking tire size into account, to have the same power to weight in 1st gear (and thus effectively equally "tall" gearing) you'd need 2.63 final drive. To have the same power to weight in 5th gear, you'd need a 2.71 final drive.

Or, in other words, even with a 3.27 FD and bigger tires you'd still have considerably more power in every gear than I do. I'll admit that I could do with a slightly shorter first gear without the IMA to help, but it's very drivable.

Totally up to you just how tall you go.

[hat_man]

Thanks for making my decision hard again.

[19bonestock88]

Based on Ecky’s work, you’d about be able to get away with a 3.08 FD and still accelerate as well as a G1 Insight, but at much much lower RPMs...

[Ecky]

Originally Posted by 19bonestock88

Based on Ecky’s work, you’d about be able to get away with a 3.08 FD and still accelerate as well as a G1 Insight, but at much much lower RPMs...

Which is to say not very fast at all.

[hat_man]

I just don't want to be too slow. The little towns I pass through lead directly into two lane blacktop with few if any safe passing zones. Slowing down other drivers also on their way to work could result in someone trying to pass in a sketchy area. I can't control other drivers but I don't want to make a bad situation worse if you know what I mean.

Any idea on how slow I might be from a dead stop with the 3.27? After reading all this great information, I am again leaning towards that. Unless it's still going to be a turtle.

I can probably improve acceleration by freeing up some usable hp with an e-fan and maybe an aluminum drive shaft. Building a hybrid M5OD-R1/HD transmission would definitely help but be pretty expensive.

[19bonestock88]

A hybrid R1/HD trans? The only different variation of the M5OD-R1 that I know of is the one behind the 4.0V6, which has a taller 1-3 gear...

You got any more info?