Differentials, Ratios, Speeds and RPM's
 

I've been doing a lot of research on this forum and around the internet for the past few months learning about the diffs that came in these cars and trying to figure out what ratio would be best for my project.

The car I have the most experience driving when I was younger was the 745 w/ ZF Auto that I'm currently working on, but I don't plan on using the ZF when I put it back on the road. I would like to retain the driving/cruising characteristics that transmission provided, just with the ability to row gears.

The other two diesel powered Volvos that were in my family growing up wer a 84 244 w/ 3 speed auto? (that car sucked) and a 245 w/ M46. It wasn't until I started reading the comments on this forum did I remember how short 1st gear actually was on that trans.

I picked up my 83 760 will intentions of doing a full drivetrain swap into the 745, but I really don't think Ill be happy with the M46 and after seeing all the T-5 swaps, that got my gears turning on my own trans swap.

It's really been bugging me not knowing what gear ratios were in the 3 cars I have so today I set out to answer that question and here's what I found.

http://i1153.photobucket.com/albums/...321_120619.jpg

1983 760 D24T/M46
1031 Housing - 3.54 Ratio - Non-Locking

1989 745 B230FT/AW71
1030 Housing - 3.73 Ratio - Non-Locking

1985 745 D24T/ZF22
1031 Housing - 3.91 Ratio - Non-Locking

I already knew none of the cars I had were locking diff equipped, but I will hopefully remedy that in the future, the ratios were the important thing for the day, now I could calculate gear ratios, final drive ratios, speeds and RPMs based on OE tire sizes.

The ZF/3.91 combination provides the following results;
ZF Ratios - 1st: 2.48 - 2nd: 1.48 - 3rd: 1.00 - 4th: 0.73
Final Drive Ratios - 1st: 9.70 - 2nd: 5.79 - 3rd: 3.91 - 4th: 2.85
Speed @ 3000 RPM - 1st: 22 MPH - 2nd: 37 MPH - 3rd: 55 MPH - 4th: 76 MPH

The M46/3.54 combination provides the following results;
ZF Ratios - 1st: 4.06 - 2nd: 2.16 - 3rd: 1.37 - 4th: 1.00 - 5th: 0.79
Final Drive Ratios - 1st: 14.27 - 2nd: 7.65 - 3rd: 4.85 - 4th: 3.54 - 5th: 2.80
Speed @ 3000 RPM - 1st: 15 MPH - 2nd: 28 MPH - 3rd: 45 MPH - 4th: 61 MPH - 5th: 77 MPH

Now that I have some numbers to work with, I can figure out what diff will work best for me. Just figured I'd post this now because this was the information I was looking for when brainstorming the project, hopefully someone else can find it useful in the future.

The stick shift D24Ts got the 3.31 set, no?

No, as far as I can tell they all got the 3.54. I have a 3.31 that I want to put in mine because I do a lot of highway driving.

Nope. Some may exist, but clearly it cannot be assumed that they were all 3.31's. I've seen others comment on here that the factory M46 cars were 3.54's, my car supports that claim, but I haven't been back in the Volvo game to comment otherwise, just sharing my findings.

just the same, I found conflicting info on the ZF D24T's, but a majority did support the 3.91 which is what I found in my car and I know its the original rear.

ive been running a 215/55 16 on the rear for a while now. it helps for about 2 more mph at the same revs.

I have the greenbook which gives all specs for 84-86 cars. It's at the shop but I believe only the NA m46 cars got the 3.31 rear end (which seems counter-intuitive) and the ZFs got the 3.73 or 3.91. I'll double check this evening.

This may help i made a little spreadsheet to calculate speed vs rpm with different gearbox ratios and final drive options. It factors in tyre size will plot speed at a given rpm (but default every 1k rpm up to 6k rpm) and a separate bit for max speed

https://docs.google.com/spreadsheets...it?usp=sharing

its open to anyone to edit so make a copy if its any use.

anything in red is to be edited as needed and i think i have the maths correct if someone doesn't mind checking it over and if its useful take a copy.


Dan

Apologies for bumping up an older thread, just wondered if anyone had any thoughts on how accurate the figures above are?

I've got a 740 with M46 gearbox, and it does approx 3000rpm at 70mph according to the rev counter and sat nav. It definitely has the 3.54 rear end.

My old 940 td auto had a similar reading - approx 3000rpm at 70mph.

Yet everything I read suggests that 3000rpm should be closer to 77mph.

Are the rev counters just slightly optimistic, or is it possible the diesel versions of gearboxes had different ratios? My friend had a petrol (gas) 240 wagon which was only doing 2500rpm at 70mph, made for more relaxed cruising... I'd love to get that if I could.

Apparently the M46 had only these ratios:
Gear ratios: (1) 4.03:1 (2) 2.16:1 (3) 1.37:1 (4) 1.00:1 (5) 0.79:1

Source:
http://www.nuceng.ca/bill/volvo/database/tranny.htm

Note that the OD is a relatively high (numerically low) ratio.

I do know the M46 did get different OD pressure relief valve shim/spring packs for higher PSI for higher torque engines (D24T included).

What tires are you running?

Thanks for your reply - both cars were/are running standard 15" wheels and tyres.

https://i.imgur.com/dmj1liQ.jpg?1

All the figures I've seen agree with your numbers - but by my calculations the car should be doing 77mph in OD/5th at 3000rpm.

Whereas it's only doing 70mph at 3000rpm if you believe my sat-nav (which seems accurate) and rev counter...

The question about tires was because lower-profile tires are smaller diameter and circumference. Causing more RPMs for actual travelled distance. Not all brands/models are the same (measured) size for a given listing, worn-out tires are obviously smaller, etc.

A little hard to tell from the photo but the tires certainly do look like standard profile size.

Sorry, I wasn't by the car earlier to double check. They're Kumho (not sure if you get those in the US?) 195/65R15s, which I think are correct for the wheels. The car definitely has the correct 3.54 diff too, as I led underneath and checked by rotating the prop with one wheel off the ground. :)

Although it may seem counterintuitive that the gassers, with less torque and a more peaky power band, got a taller rear end ratio than the diesels, there are usability factors that help make sense of it. Keep in mind that the right choice for final drive ratio needs to not just optimize cruising speed but also take rev limit and the upper end of the power curve into account. The diesel makes its power in the midrange while the gasser engines need to use higher revs, BUT, the diesel's maximum speed is around 1000 rpm lower than the gas engines' (~5000 vs 6000), and in stock form the diesel's power falls off well before that 5000 rpm limit.

So, taking a hypothetical example, if the gasser and the diesel had identical gearing and you drove them both up a steep mountain grade, at a certain point, both cars would need to downshift to maintain speed. If you were trying to keep the speed at, say, 65 mph, you would probably need to drop down two gears in the transmission. But, in the diesel, third gear might be inaccessible because of the rev limit, while in the gas car, it could reach the lower gear due to the wider rev range. As a result, the diesel car (in stock form) gets a driveability benefit from using a slightly shorter rear end ratio and a faster cruising RPM, since it has less RPM flexibility and fewer ratio options at any given speed.

One final relevant note to keep in mind -- gas engines benefit much more than diesels do from keeping cruising RPM low and engine load relatively high, in terms of fuel efficiency, because of the reduction in throttling losses. Diesel engines, without a throttle, typically do not see a significant impact on MPG from incremental reductions in cruising RPM. Using the tallest viable gearing to lower cruising RPM and increase economy in the gas cars may have been an engineering priority. For the diesels where the efficiency benefit of taller gearing was minimal or none, the performance benefit of a shorter axle ratio might have been more important.

Thanks for the interesting post! I'd not considered the effect that not having a throttle would have on the diesel engine efficiency...

Because my car only gets used on long runs at motorway speeds, and is almost never laden, never tows, it never seems like the engine needs to be revving so high. But I'm currently averaging 37.5MPG (US)/45MPG (UK), so maybe I shouldn't complain too much.