• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Reducing Revs at Higher Speeds

Share this page

BoyRacer said:
Nowhere yet in this discussion has anyone mentioned tire size. ...
Click to expand...

See above:

"There are gearing calculators online that will give pretty accurate RPM/speed numbers; some even include actual wheel/tire diameters."

Begs the question: Measure tires standing still or use manufacturers' numbers? I think the latter are probably most accurate, as diameter will increase at speed due to centrifugal force; anyone seen the slo-mos of the top fuel dragsters--we used to call them 'rails'--getting off the line? The sidewalls twist something crazy and the wheels squat, then the tires grow vertically big time--like 15-20% or more--as the centrifugal force slings them outward.

 
Last edited:
I am now looking for a reliable supplier of the ā€špumpkinā€˜ with ratio 3,54. I want to change the whole unit and not only crown pinion and crown wheel as I want to shelf the original differential and keep it as is.

Can anyone recommend a good supplier for a remanufactured rear differential In the UK? Any hint is welcome.

best regards, Martin
 
And a larger-in-diameter tire, such as the XAS 180R, will effectively reduce revs by producing slightly higher speeds, etc.
 
Kinda surprising they don't require a core; they must have a stack of them. LSD installed labor-free is tempting (though arguably not necessary on a street Healey).
 
Might be cheaper to buy a used pumpkin and put the 3.54 gear in it.
jus a thought.
 
Bob McElwee said:
I put 3.54 gears in a 67 BJ8 with a 4 speed w/OD and a 62 BT7 with a Toyota 5 speed (synchro 1st gear is heaven).
Initially on the BJ8 I ran 195/70 X 15 until I could no longer get them. They look much, much better IMHO. I went to 185/70 X 15 on both cars eventually.
we did a lot of long distance trips to regional meets and Conclaves. Needless to say my speedo was just for approximate MPH. GPS have me close to 75 MPH at 2500 RPM.
the 3.54 also made both cars so much more bearable around town.
Click to expand...
While wider, 185/70 -15 tires have less sidewall height than 165 - 15. 185 x .7 = 129.5; 165 x .8 = 132. With new springs and rebuilt shocks at all four corners I found that I can run 185 -15 with no rubbing issues. 185 x .8 =148. They fill the wheel arches very nicely. Also gives me an extra measure of clearance, which is always a good thing.
 
Jack T said:
While wider, 185/70 -15 tires have less sidewall height than 165 - 15. 185 x .7 = 129.5; 165 x .8 = 132. With new springs and rebuilt shocks at all four corners I found that I can run 185 -15 with no rubbing issues. 185 x .8 =148. They fill the wheel arches very nicely. Also gives me an extra measure of clearance, which is always a good thing.
Click to expand...
I don't know what any of that means. My Michelin XAS 180 HR 15s have an overall diameter of 26.8". That 's how I compare the height of tires.
 
Back
Top