• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
  • When posting a classified ad, you MUST select a prefix from the drop-down next to the subject line. If you don't you will get an error and your ad will not be posted!
Tips
Tips

TR2/3/3A TR3 Diff Ratio for Highway Driving ?

Share this page

K

karls59tr

Obi Wan
Bronze
Country flag
Offline
I just noticed that BPNW is selling a 3.45:1 crown and pinion set for the TR3 Diff. My 3 has the stock 3.7:1 diff and a TR6 gearbox. It has a rebuilt engine,mild street cam and header and it is quite quick. I'm thinking of driving the car some summer approx 3000 miles in Canada from Winnipeg to Vancouver. My question is if I installed the 3.45:1 diff for better highway cruising how much would I lose on the acceleration end of things?I know its a tradeoff but would it be awfully slow off the line or barely noticeable? I realize of course that ideally I should put in a Laycock de Normandville OD or the Toyota conversion but these look too expensive at this time. Any thoughts on the downside of a 3.45:1 Diff? Cheers Karl
 
Got a 3:45 in the Wedge. I'm thinking of someday changing that to a lower #. I know the Wedge is a different animal but it is very quick off the line. Almost to quick. Don't know how guys with the shorter rears do it.
 
I was under the impression that if you had a certain gear ratio in the Diff it would either be,depending on the ratio,good for acceleration or highway cruising or somewhere in between. BPNW says the 3.45:1 is for highway driving so wouldnt that mean you lose at the other end ie. acceleration? And yet your car dosnt seem to with that ratio. Do you have an OD in your car?
 
The 3:45 is better for highway than stock as it lowers the final drive . Will be slower off the line. How much? That's for someone else to say.
 
All, had a 4:11 in the GT6 for autocross, it jumped out of the corners, HOWEVER, getting there and coming home was no treat. If I remember correctly 4200 RPM's got you 65 MPH. On the Roadster is a 4:55 and the best I can do is 65 MPH without a basket in the car for all the pieces I would have to gather, but 50 to 55 mph all day.
Unless you are looking for quick starts the 3:45 will be fine and a lot less stress on the engine and drive train.

Wayne
 
DNK said:
The 3:45 is better for highway than stock as it lowers the final drive . Will be slower off the line. How much? That's for someone else to say.
Click to expand...
FWIW, when Steve Hedke competed in The Great Race (a coast-to-coast TSD rally for antique cars) in his TR3; they had to very carefully calibrate acceleration times through almost every segment (0-10 mph, 10-20, 20-30, etc.). These were not "full throttle" times but fairly heavy throttle (to be more repeatable over different temperatures and elevations).

He reported that Scrappy (his TR3) would actually accelerate faster with the 3.45 gears! I can only theorize that was because less of the engine's torque went into accelerating the heavy flywheel and more into accelerating the car.

But I'm not sure that a conversion from 3.7 to 3.45 is worth the effort, as we're only talking a 7% difference. I'm planning to install 3.45 gears myself, but I've currently got 4.10 gears and the diff needs rebuilt (plus I bought a LSD that will go in at the same time). And with my shorter tires, the 3.45 will turn out just about the same overall ratio as the 3.70 with the factory tires.
 
Back
Top