Gear Ratio Comparasions

Orange Rag · Postby **Orange Rag** » 13 Aug 2013 17:25

Not to confuse, but with a Tremec T5 and now a 390 rear end, I'm 60 at 2000 rpm was an ideal 1400 with 3:45 (before it broke).
I'm surprised the TR8's are running 3000, with torque they have I would have thought down around 1500.

RJS · Postby **RJS** » 13 Aug 2013 18:57

I will throw in my numbers.

1980 TR8 w/3.45 (original would have been 3.08, had a 3.90 when I got it, and I found and put in a 3.45). Tires are 205-55 15".

60 mph in 5th = 2400 rpm. 80 mph is around 3200 rpm.

The question is, what would my rpm for 80 mph be if I had the tall 5th gear from an 81?

Rob

whitenviro · Postby **whitenviro** » 13 Aug 2013 19:28

Hasbeen: No wonder I feel like I have to drive fast in the car; because I'm really not.? Actually, the GPS mileage from my recent trip was 1,385 miles, but the odometer only showed 1,325. So about 4% difference, but in the wrong direction; I would think it showing higher than actual speed, the odometer would be reading higher than actual mileage. Is that correct?

[/img]
1980 Pageant Blue DHC with removable hardtop.

Workshop Help · Postby **Workshop Help** » 13 Aug 2013 20:19

Whitenviro, you indicate about a 60 mile variance twixt the odometer and your GPS toy.

Hmmm. Would any of your local constabulary have one of those radar devices on trailer in your neck of the woods? Like say around elementary schools or other traffic spots where they're trying slow folks down? If so, try driving by it at exactly the speed limit and observe your speedometer reading. My TR7 is spot on but my 1997 F150 will read 3MPH slower at 30MPH due to it's slightly smaller tires, 235-16 vs the original 255-16.

If there is significant variance in the radar reading and your speedometer, you may wish to fit a different speedo gear in the back side of the gearbox. It's easy to do once you figure out which gear you want, and can find it.

I am glad to see the TR8 folks show up. Their lower readings are to be expected due to the greater torque of the engines being able to pull the car at a lower RPM setting. I believe it was Odd who mentioned the availability of much lower differential gearing in the sub-3.00/1 regions. My F-150 automatic with the 3.55/1 differential pulls about 1750RPM at 60MPH on level ground.

Mildred Hargis

jeffremj · Postby **jeffremj** » 13 Aug 2013 21:59

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by RJS

I will throw in my numbers.

1980 TR8 w/3.45 (original would have been 3.08, had a 3.90 when I got it, and I found and put in a 3.45). Tires are 205-55 15".

60 mph in 5th = 2400 rpm. 80 mph is around 3200 rpm.<hr height="1" noshade id="quote"></blockquote id="quote">I have just been out for a drive and I get 70mph @ 2500 rpm in 5th (3.08 axle) which is around 2140rpm @ 60mph. My TR7V8 used to have the 2.83 axle, but unfortunately it started getting VERY noisy and so was replaced with a readily available 3.08. I must have been getting under 2000rpm @ 60 mph - although the speedo would have been out as only with the 3.08 is it correct!!

Hasbeen · Postby **Hasbeen** » 14 Aug 2013 00:24

The 8 is similar to jeffremj's showing 100 Km/H, [62 MPH] at around 2000 RPM. When tested against the sat nav 100 indicated was found to be 91Km/H true speed. That is with a 3.08 diff ratio, which was stock.

On one occasion, my son blew up yet another diff in the 8 on Christmas eve. It was too late to get any bits for it before the new year, with everything shut. As he only had a short leave, we fitted the complete diff out of my spares car, a stock 3.9 TR7 unit. We found it rather funny when it was indicating 146 Km/H when the sat nav said 100Km/H.

Don't think it worked too well either, too much wheel spin, but at least he enjoyed that 4.6L rumble for a few days. Some of his old school mates were left believing the 8 was a damn sight quicker than it actually is. [}:)] He can be a bit evil that boy.

Whitenviro the needle of your speedo is driven by a potentiometer, magnetically, where as the odometer is driven mechanically. In the only moderate quality instrument Triumph chose to fit, such a difference could be down to old dried out grease, or lousy calibration. It might also be correct, & my suggestion completely wrong. Hell I have been wrong once or twice before, [:D] but I can't imagine what ratio is available that would give that speed per thousand RPM.

Hasbeen

silverseven · Postby **silverseven** » 14 Aug 2013 01:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by nick

-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.

-By scribed I mean written with a sharp objected as opposed to raised as part of the casting. The former gets covered by dirt and grime while the latter does not.

<hr height="1" noshade id="quote"></blockquote id="quote">

Ok so I took a good look and unfortunatley see nothing scribed into the diff (guess the undercoat is a tad thick .....)
There is some raised lettering cast into the lower casing "RKC 0100"
So is there somewhere specific that I should be looking?
Could somebody post a pic of where I ther location is ?

thanks ,

Ron.

Odd · Postby **Odd** » 14 Aug 2013 06:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mildred
I believe it was Odd who mentioned the availability of much lower differential gearing in the sub-3.00/1 regions. <hr height="1" noshade id="quote"></blockquote id="quote"> Yes. Ordinary TR8 fitment were the 3.08:1 CWP [Crown Wheel with 37 teeth and Pinion with 12 teeth = 3.083333] and
then there's the elusive 2.84:1 CWP [Crown Wheel with 37 teeth and Pinion with 13 teeth = 2.846158]
(1113 of these units were fitted to the SD1 VandenPlas EFI automatic) Hard to find, expensive if found
- but well worth it for relaxed highway cruising!

Personally I'm curious as to exactly what amount of teeth the various '7 CWP rear ends had? I just love decimal diahorrea...[;)]

claypole1360 · Postby **claypole1360** » 14 Aug 2013 07:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by whitenviro

I don't know what ratio have, so I was hoping this discussion would help me figure it out. With standard 185/13" wheels I noticed on my recent trip that in 5th gear I was at 3000 rpm at 70 mph. I don't recall ever traveling at 60 mph....

[/img]
1980 Pageant Blue DHC with removable hardtop.

<hr height="1" noshade id="quote"></blockquote id="quote">

Glad someone in America is law abiding[8D]. Surely you should be measuring this at 55mph.
Clayps

1978 TR7V8 FHC
1968 Bond Equipe Sprint Car
1962 Herald Coupe 1200
2003 Peugeot 206 GTI
2009 VW Passat

FI Spyder · Postby **FI Spyder** » 14 Aug 2013 14:23

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by claypole1360

Glad someone in America is law abiding[8D]. Surely you should be measuring this at 55mph.
<hr height="1" noshade id="quote"></blockquote id="quote">

Actually that is a misconception. Back in the day there was a national speed limit of 55 mph and speedometers were pegged at 80 mph (like mine) to discourage excessive speeding in order to bring down traffic deaths and help alleviate the gas shortage with better milage at lower speeds. Many Americans didn't/don't like the federal interference plus the effect it had on timely inter state commerce, so many states raised the speed limit on major highways. To keep this under control the federal government devised a formula whereby the higher/more roads you (a state) have over 55mph the less federal money you have for highway infrastructure. It's a balancing act for the states and the major roads/interstate highways only have higher speeds of 65mph and I think higher in some spots. Even the major highways are not designed to support super high speeds like in Europe which are designed differently and cost something like tens times as much although they last longer. There was an interesting documentary a few years ago comparing the design, safety and cost of highways in US vs Europe.

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

nick · Postby **nick** » 14 Aug 2013 18:33

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by whitenviro

I don't know what ratio have, so I was hoping this discussion would help me figure it out. With standard 185/13" wheels I noticed on my recent trip that in 5th gear I was at 3000 rpm at 70 mph. I don't recall ever traveling at 60 mph....

[/img]
1980 Pageant Blue DHC with removable hardtop.

<hr height="1" noshade id="quote"></blockquote id="quote">

Dan, You must have the same set up I have which is the 3.45:1 diff fitted to the 5speed with the .83:1 5th gear. The other day when I checked, I was getting 71 MPH at 3,000 RPM.

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

saabfast · Postby **saabfast** » 18 Aug 2013 14:10

AS far as I know my 'box and diff are stock '81, but anything could have happened in the past. I have not found the 'scribing' on the diff - yet.
Tested it today against the satnav. The speedo is spot on (will need to watch that with the cameras as they normally overread a little and I don't want any more points). Results were:

60 mph = 2650 rpm
70 mph = 3150 rpm
Rpm is a bit of a guess as there are no intermediate graduations between the 500 rpm markers.

Should have added, wheels are standard alloys with 185/70x13 tyres.

Alan
Saab 9000 Stg 1
Saab 9000 2.3 FPT Auto
Saab 9000 2.3 LPT Auto
'81 TR7 DHC

nick · Postby **nick** » 19 Aug 2013 00:04

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by saabfast

AS far as I know my 'box and diff are stock '81, but anything could have happened in the past. I have not found the 'scribing' on the diff - yet.
Tested it today against the satnav. The speedo is spot on (will need to watch that with the cameras as they normally overread a little and I don't want any more points). Results were:

60 mph = 2650 rpm
70 mph = 3150 rpm
Rpm is a bit of a guess as there are no intermediate graduations between the 500 rpm markers.

Should have added, wheels are standard alloys with 185/70x13 tyres.

Alan
Saab 9000 Stg 1
Saab 9000 2.3 FPT Auto
Saab 9000 2.3 LPT Auto
'81 TR7 DHC

<hr height="1" noshade id="quote"></blockquote id="quote">

CL diff which is 3.45:1. Consistent with 1980 and later 5speeds.

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

silverseven · Postby **silverseven** » 19 Aug 2013 01:10

as promised, I submit my readings and judging by the numbers figure pretty hard that I've got 3:45's
(for the record I'm running 205-60-r13's)

At 60mph I'm a slight tick over 2500 rpms (guess I lost 1mph taking the pic [B)])

Ron.

Workshop Help · Postby **Workshop Help** » 19 Aug 2013 11:46

Kind souls, I thank you for your contributions to our RPM comparison survey.

It appears from analysis those cars with the early LT77 and the 3.90/1 differential will be trundling along at 3000 RPM @ 60 MPH. The later LT77's with the 3.45/1 differential will shave the engine speed to about 2500 RPM. Then come the hybrids such as mine with the later LT77 5th gear running thru the 4 speed differential, 3,63/1, cranking out 2650 RPM.

What we have not seen reported are anyone with a 3.27/1 differential or the 3.08/1 differential in a TR7. The TR8 folks indicate with the 3.08/1 gearing a 2000 RPM reading. Mention was made of other ratios but no actual field tests were reported.

In conclusion, the old adage of the slower the crank spins, the less your gasoline receipt reads, holds true. It also illustrates why the later models had highway ratings of the mid 30's MPG in the last years of production. The early four speed cars were rated in the upper 20's MPG while spinning at 3275 RPM @ 60 MPH.

Our next suggestion for a TR7 comparison comes from our friends in Australia and New Zealand. They propose we see how many plus size supermodels can be stuffed into the cabin of a FHC TR7 versus how many anorexic supermodels can likewise be fitted. For the record, my TR7 will not be subjected to such a test.

Mildred Hargis

Anonymous

Gear Ratio Comparasions

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