Gear Ratio Comparasions

[Workshop Help]

Aahh, this is much better.

Here we go, with all our variances in transmission ratios and differential ratios, it will be necessary for each of us to state what we have AND the RPM readings on our tachometers at 60mph in 5th gear. I'll start the comparasion off with the required information:

60MPH - 1976 TR7, late 1981 5th gear with a 3:63/1 differential = 2600RPM.

The next person will please state his/hers/its data just like this.

Please state whether you have a TR7 or a TR8.

Mildred Hargis

[Benzo]

Are you running a 4 speed back axle with a 5speed box?





Currently back to Basics with my BMW V8 Engined Build.

[Workshop Help]

Yes, Benzo, I am running the four speed differential with an early series LT77 that I installed the very late 5th gear in it.

Nick made a good point. My tires are the stock 185 x 13 on the stock steel wheels with 30lbs of domestic air pressure.

Just as a reference, the four speed cars with the stock 3.63/1 differential will be spinning about 3275RPM's at 60mph. Also, note 4th gear in a LT77 is the same direct drive ratio as top gear on the four speed gearbox. That's why we should limit this comparison to 5th gear at 60mph with stock height tires.

Mildred Hargis

[nick]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Mildred</i>

Yes, Benzo, I am running the four speed differential with an early series LT77 that I installed the very late 5th gear in it.

Nick made a good point. My tires are the stock 185 x 13 on the stock steel wheels with 30lbs of domestic air pressure.

Just as a reference, the four speed cars with the stock 3.63/1 differential will be spinning about 3275RPM's at 60mph. Also, note 4th gear in a LT77 is the same direct drive ratio as top gear on the four speed gearbox. That's why we should limit this comparison to 5th gear at 60mph with stock height tires.

Mildred Hargis
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OK. Here are the numbers on the FHC with the 5 speed and the 3.45:1 diff and tires at 195-60/14. 60 MPH shows 2500 RPM compared to 2620 RPM calculated. Here are the rest of the numbers: at 3000 rpm: 1st=18, 2nd=29, 3rd=42, 4th=60, 5th=71.
All of these numbers are within 1.7 MPH of the theoretical that I calculated.

If I change the tire size to 175-70/13 the calculated numbers are at 60 MPH are 3.9=2890, 3.63=2690, 3.45=2555.

Looks like the slightly taller tires I have make the highway cruise a little more quite.

[img][IMG]http://i615.photobucket.com/albums/tt234/nickmi/TR7%201975/Yellow.jpg[/img]
nick
'79 TR7 DHC
'76 TR7 FHC

[dursleyman]

Mildred, This information taken from the TR7 speedo reading, I'll check it against our everyday Merc speedo which I think is pretty accurate and alter the text if it comes up significantly different.

60mph, 1980 TR7, 5th gear with standard 3.9 diff = 3000rpm

Russ

1980 TR7 Sprint DHC
Dursley
UK

http://tr7russ.blogspot.co.uk/

[Hasbeen]

More or less as Russ says.

By the book the stock 7, speed per 1000 RPM is 33.5 Km/H or 100.5 Km/H at 3000. This equates to just under 62.5 MPH.

I have 205 65 R13 on the back of mine, basically the same rolling diameter as the stock 185 70 R13s & the satnav agrees with this 100 Km/H at 3000 RPM as closely as you can read the rev counter.

This speedo is wavy but reasonably accurate. Every other 7 & the 8, the speedo has been about 9% fast, reading 100 when rev counter & sat nav read 91. Helps to stop speeding fines at least.

Hasbeen

[Workshop Help]

Okay, with the first few numbers in, let's start some analysis.

Nick with his supposedly 1980 LT77 and a 3.45 differential is at 2500rpm@60mph.

Russ with his 1980 LT77 and a 3.90 differential is at 3000rpm@60mph.

Hasbeen checks in with near identical numbers to Russ. We thank you.

That's a 500 rpm difference.

Hmmm, I suspect our boy Nick has, in fact, a 1981 gearbox with it's higher 5th gear and the 1981 3.45 differential. I opine this because only the fuel injected TR7's had this gearing setup. This was done because of the second phony oil shortage brought on by Iran and it's Khomeini with the kidnapping of all those U.S. embassy people in 1979. This second oil crisis prompted car makers world wide to get more fuel economy from their designs and altering gear ratios was a cheap and fast way to get it done.

It would helpful for others out there to provide their numbers to further sort this issue. Does anyone have the 3.08 differential installed?

Mildred Hargis

[silverseven]

I'll play , but it will have to wait until the weekend when I go for a 300km trip. (my wedge stays put during the week)

Btw how do we confirm which diff we have , is it on the vin tag and / or serial number??
I've never had the reason to question my ratio, which I simply took for granted was a 3.90 since my car was a Aug 81 build w/carbs.

Ron.

[FI Spyder]

My FI has 3.90 rear. Remember FI was in 1980 California cars and available in other states as a $465 option. 1981 cars got FI federally. Canadian cars all had carbs but FI was available as an option. The last TR8 sold in Calgary was FI (now residing in Victoria, last I heard) but I think they all would have 3:45 rear. Ron's (Orange Rag) early American 1980 car had a 3:45 so they may have had a few snuck in early. the mysteries of British Auto manufacturing.

The diff's had letters stamped in it (not seen them yet, keep forgetting to look). Mysteriously my speedo used to be dead on with 60 mph at 3,000. Now after trying different speedo cables/cleaning Ox sensor box to get speedo needle more steady it reads high as Hasbeen noticed so at 60 mph I'm around 2800 rpm (tach accurate, speedo isn't).

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[parrish]

Just to throw a spanner in the works!
The LT77 i believe came with slightly different ratios on 5th.
As you know i think their are approx 6 different designations of the LT77.
Steve

1983 Del lines TR8. FIAT 131 Abarth Replica.1977 FIAT 131 1600 TC, 1996 BMW Z3, 1960 Mini Hustler. 2009 Sport Ka, 2012 Ford Fusion.

[Odd]

And then there's the R380 to that...

Anyway, Mildred asked: "to further sort this issue. Does anyone have the 3.08 differential installed?"

Yes, of course - most all the TR8 have the 3.08:1 rear axle ratio. (Some have the 2.84:1 and a small
fraction, mainly cars built for race or rallye use the '7 ratios...)

<u>My</u> TR8 currently have a R380 gearbox (= 0.77:1 5th gear) and the 3.08:1 rear axle ratio.
[<font size="1">The 2.84 with Quaife is currently sitting in the garage again.</font id="size1">) Stock alloy rims with 185-70*13 tyres.
The speedo is off by some kph/mph, I think this was a deliberate oem design decision...

OK Mildred, with the above data (and as close as I could keep your wanted 60 mph with a metricated TR8
(the wonders of GPS!) I get roughly 2290 rpm in 5th gear.

Cheers,
/Odd

[Troy ODoherty]

If you want this to be real world accurate you will need to actually measure the tyre circumference. Tyres vary from one brand to another, even if they are all 185/70*13, they will all measure differently and often by surprising amounts


Cheers Troy

[Workshop Help]

AARGH!!

Last night, late, viewing FI Spyder's and Silverseven's query as to what gearing their car has, I started to pull books off the library shelf and promptly put them back again. Then our Parrish sounded off on variances while Odd pops off on a R380 tangent with all the TR8 differential options.

Good people, it must be up to you to know what you have under all that gorgeous sheet metal and paint. My telescopic and x-ray vision is not what it used to be and the cape with the big red 'S' on it has long since been cut up for use as doilies for the furniture.

There is also the slight matter of the factory installing different components based on an individual countries mandates, consumers driving habits, geopolitical concerns, and the left overs from last weeks deliveries from the gearbox and differential factories. This, of course, plays all kinds of living hell with the accuracy of the reference books gathering dust on my library shelf. So I'm not even going to try to sort out the mess. Please ascertain what you have.

So far, our Nick is winning the Low RPM contest with his 2500RPM @60MPH.

Thank you.

Mildred Hargis

[Odd]

And 5 speed 3.08 CM ...

[nick]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by silverseven</i>

I'll play , but it will have to wait until the weekend when I go for a 300km trip. (my wedge stays put during the week)

Btw how do we confirm which diff we have , is it on the vin tag and / or serial number??
I've never had the reason to question my ratio, which I simply took for granted was a 3.90 since my car was a Aug 81 build w/carbs.

Ron.




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I would say you have the 3.45:1. You have to jack the car and look at the bottom of the differential. Scribed (not stamped) will be a code. Here is the key:
5 speed 3.9:1 CH
5 speed 1980-81 3.45:1 CL
4 speed 3.63:1 CG
Auto 3.27:1 CT

You may have to scrape off some grime to find the code since it is scribed into the metal.

[img][IMG]http://i615.photobucket.com/albums/tt234/nickmi/TR7%201975/Yellow.jpg[/img]
nick
'79 TR7 DHC
'76 TR7 FHC